PSD is not my favourite file format (2009)(github.com)
github.com
PSD is not my favourite file format (2009)
https://github.com/zepouet/Xee-xCode-4.5/blob/master/XeePhotoshopLoader.m#L108
108 comments
> Why, for instance, did it suddenly decide that these particular chunks should be aligned to four bytes, and that this alignement (sic) should not be included in the size?
Not saying this happened here, but I have seen this type of mistake before. It was because they "simply" cast a C/C++ struct to a binary blob https://en.wikipedia.org/wiki/Data_structure_alignment#Typic... and wrote that to disk (or sent over the network in my case). So that particular compiler version and architecture-specific struct field alignment became the "official" format. It just takes one goofy mistake like that and everyone has to deal with it for years to come.
Not saying this happened here, but I have seen this type of mistake before. It was because they "simply" cast a C/C++ struct to a binary blob https://en.wikipedia.org/wiki/Data_structure_alignment#Typic... and wrote that to disk (or sent over the network in my case). So that particular compiler version and architecture-specific struct field alignment became the "official" format. It just takes one goofy mistake like that and everyone has to deal with it for years to come.
Note that the issue here is less the part that you quoted and more the two sentences surrounding it, PSD has all of
* unaligned chunks
* aligned chunks with alignment included in size
* aligned chunks with alignment not included in size
The problem is not the specific choice, "Either one of these three behaviours would be fine", it's that "PSD, of course, uses all three, and more."
* unaligned chunks
* aligned chunks with alignment included in size
* aligned chunks with alignment not included in size
The problem is not the specific choice, "Either one of these three behaviours would be fine", it's that "PSD, of course, uses all three, and more."
As I understand it that's more or less what the older MS Office formats were, just a dump of the entire OLE object representing the document as-is.
I believe SimCity used a similar strategy as well, the .cty and I think SC2k's .s2k formats just dumped the in-memory representation to disk,.
I believe SimCity used a similar strategy as well, the .cty and I think SC2k's .s2k formats just dumped the in-memory representation to disk,.
For what it's worth, I think that's true of the pre-1997 version of the format. For the format from 1997 to 2007, it's a proper format, albeit a bizarre one (looking over the spec makes me think the person tasked with making it was yanked off the filesystem team against their will)
Nowadays, it's a XML/ZIP thing.
With things like Flatbuffers, I sometimes feel we've regressed to these old formats that are just memory dumps.
Nowadays, it's a XML/ZIP thing.
With things like Flatbuffers, I sometimes feel we've regressed to these old formats that are just memory dumps.
A story I heard at Sun, which may be apocryphal but was fucking hilarious enough to be a repeatable rumor, was that a release of an early operating system in BETA was determined to be solid and tested and ready to release and ship to customers, so they simply changed the version string from something like "SunOS2.1BETA" to "SunOS2.1FCS" (First Customer Ship), and recompiled. But the change from a 12 character version to an 11 character version threw off the alignment of some important data structures somewhere in the kernel, and the entire OS ran MUCH SLOWER because of 68k unaligned memory accesses!
> With things like Flatbuffers, I sometimes feel we've regressed to these old formats
Sorta, but they're expressed in IDLs that are independent of a particular compiler. The downside is the drift between internal structs and the IDL structs, but the upside is you can use the same "memory dump" on interpreted runtimes or entirely different platforms (even those with different endians). Plus impls like protobuf help guard against breaking backward-compatibility by numbering fields and not "allowing" you to remove fields in ways that would change the structs' ABIs.
Sorta, but they're expressed in IDLs that are independent of a particular compiler. The downside is the drift between internal structs and the IDL structs, but the upside is you can use the same "memory dump" on interpreted runtimes or entirely different platforms (even those with different endians). Plus impls like protobuf help guard against breaking backward-compatibility by numbering fields and not "allowing" you to remove fields in ways that would change the structs' ABIs.
> With things like Flatbuffers, I sometimes feel we've regressed to these old formats that are just memory dumps.
It depends on what your application requirements are, but there are compelling arguments that on-disk / on-wire representations should match in-memory representations. It’s not too hard to end up with in a scenario where encoding / decoding times are a significant contribution to overall performance.
It depends on what your application requirements are, but there are compelling arguments that on-disk / on-wire representations should match in-memory representations. It’s not too hard to end up with in a scenario where encoding / decoding times are a significant contribution to overall performance.
I worked at a company once where they, too, used raw memory dumps as their file format.
The compiler flags telling the compiler how to align structs were different for debug and non-debug builds. So, of course, the first thing I did, was to create non-debug build and try to open a file created by a debug-build-executable, the program crashed and burned without giving me a meaningful error message, it took me hours to understand my mistake.
Raw memory dumps are very neat efficiency-wise, but they are extremely fragile.
The compiler flags telling the compiler how to align structs were different for debug and non-debug builds. So, of course, the first thing I did, was to create non-debug build and try to open a file created by a debug-build-executable, the program crashed and burned without giving me a meaningful error message, it took me hours to understand my mistake.
Raw memory dumps are very neat efficiency-wise, but they are extremely fragile.
I'm curious, what was the reason for the difference? While the C language standard doesn't guarantee anything about alignment, I'm used to the behavior of compilers in practice being consistently 'align everything on its own size'; what changed between debug and release builds in that case?
In the non-debug build, the compiler was free to align structs as it saw fit, in the debug build, it was told to pack them really tight, no padding. Why? The only reason I can think of is to save a few precious bytes of disk space.
Primitives have not historically been aligned to their own size. There are plenty of older systems where double floats had 32-bit alignment instead of 64-bit, or 32-bit integers had 16-bit alignment.
They mentioned compiler flags, so my guess would be that they were doing something like tightly packing structs in the release build to lower memory usage or something similar.
Yes, I think that was the main reason.
It was kind if pointless, though, the biggest file I saw in my time there was around 10MB. How much bigger would that have been with padding? Even an overhead of 100% probably would not have had much of a performance impact and saving a few megabytes of disk space or RAM was not a very strong argument at the time any more (roughly 2007 - 2009)
On the other hand, this was a natural outgrowth of the other programmers' tendency to keep it simple and avoid bloat, which all in all I appreciated very much.
It was kind if pointless, though, the biggest file I saw in my time there was around 10MB. How much bigger would that have been with padding? Even an overhead of 100% probably would not have had much of a performance impact and saving a few megabytes of disk space or RAM was not a very strong argument at the time any more (roughly 2007 - 2009)
On the other hand, this was a natural outgrowth of the other programmers' tendency to keep it simple and avoid bloat, which all in all I appreciated very much.
Or the opposite, as packing usually requires slower, unaligned accesses.
That would depend on how much that struct is created in memory vs the total code that uses it. If you have one instance in memory then yeah the code is probably bigger and slower (depending on arch). But if you have thousands of the structs in memory the speed and code size trade off may be worth it on a memory constrained system.
Debug also sometimes turns on overrun buffers so you can check for over/under runs in your code at debug time. Some compilers have this others dont.
Debug also sometimes turns on overrun buffers so you can check for over/under runs in your code at debug time. Some compilers have this others dont.
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FWIW here's the original 68k SimCity "Classic" (open source Micropolis) save function, which I've long since cleaned up and put in byte swapping to make it portable to SPARC and x86, but yes it is just writing out some big buffers of memory with a function that now swaps bytes:
https://github.com/SimHacker/micropolis/blob/master/Micropol...
Here's some original SimCity 2000 Mac code that saves the city into Mac resources (not the flat part of the file, but the Mac resource fork) -- CompGameWrite actually does some simple run length compression of the raw memory:
https://github.com/SimHacker/micropolis/blob/master/Micropol...
Here's some original SimCity 2000 Mac code that saves the city into Mac resources (not the flat part of the file, but the Mac resource fork) -- CompGameWrite actually does some simple run length compression of the raw memory:
Boolean DoSave(short VolNum,Byte *name){
long count,CountTotal;
short i,j,filnum;
short x, y;
Byte *SaveText[] = {"\pGame Saved As:",NIL,NIL};
FInfo info;
Byte *NotCity[] = {
"\pSimCity 2000ª will not save over a non-city file.",
"\pTry to save again using a different name.",
NIL,
};
if (name[0]>20) name[0] = 20;
#ifndef DEBUG
if (GetFInfo(name,VolNum,&info)==0 && info.fdType!=CITYTYPE_ID) {
MessageDialog(NotCity);
return FALSE;
}
#endif
if (FSOpen(name,VolNum,&filnum)!=noErr) {
if (GameError(Create(name,VolNum,APPLICATION_ID,CITYTYPE_ID))) return FALSE;
if (GameError(FSOpen(name,VolNum,&filnum))) return FALSE;
}
if (!WriteHeader(filnum,0L)) return FALSE;
WriteLength = 4; // includes 'SCDH' but not header
//*** Write ***
if (!MiscWrite(filnum)) return FALSE;
if (!GameWrite(filnum,'ALTM',(Ptr)AltMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XTER',(Ptr)TerrainMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XBLD',(Ptr)BuildMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XZON',(Ptr)ZoneMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XUND',(Ptr)UnderMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XTXT',(Ptr)TextMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XLAB',(Ptr)LabelArray)) return FALSE;
if (!CompGameWrite(filnum,'XMIC',(Ptr)MicroRecord)) return FALSE;
if (!CompGameWrite(filnum,'XTHG',(Ptr)ThingList)) return FALSE;
if (!CompGameWrite(filnum,'XBIT',(Ptr)BitsMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XTRF',(Ptr)TrafficMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XPLT',(Ptr)PolluteMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XVAL',(Ptr)ValueMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XCRM',(Ptr)CrimeMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XPLC',(Ptr)PoliceMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XFIR',(Ptr)FireMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XPOP',(Ptr)PopMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XROG',(Ptr)ROGMap[0])) return FALSE;
if (!CompGameWrite(filnum,'XGRP',(Ptr)GraphData[0])) return FALSE;
//*** Close ***
WriteHeader(filnum,WriteLength);
GameError(SetEOF(filnum,WriteLength+8)); // 8 bytes for file header
GameError(FSClose(filnum));
GameError(FlushVol(NIL,VolNum));
BlockMove(name,CityStr,21);
DateCashTitle();
FilVolNum = VolNum;
//*** SUCCESS! ***
SaveText[1] = CityStr;
MessageDialog(SaveText);
return TRUE;
}
DoSave(VolNum,name)
int VolNum;
Str255 name;
{
long count,CountTotal;
int i,FilNum;
if (EqualString(name,"\pSimEarth",FALSE,TRUE)) {
MessageDialog( "\pERROR!!",
"\pThe name 'SimEarth' is reserved",
"\pfor this application.",1);
return;
}
if (FSOpen(name,VolNum,&FilNum)) {
if (GAIAError(Create(name,VolNum,'MYCR','SAVE'))) return;
if (GAIAError(FSOpen(name,VolNum,&FilNum))) return;
}
else if (GAIAError(SetFPos(FilNum,fsFromStart,0L))) {
GAIAError(FSClose(FilNum));
return;
}
/**Write**/
CountTotal = (count=GetPtrSize(Map[0]));
if (GAIAError(FSWrite(FilNum,&count,Map[0]))) return;
CountTotal += (count=GetPtrSize(Life[0]));
if (GAIAError(FSWrite(FilNum,&count,Life[0]))) return;
CountTotal += (count=GetPtrSize(OcTempMap[0]));
if (GAIAError(FSWrite(FilNum,&count,OcTempMap[0]))) return;
CountTotal += (count=GetPtrSize(OcCurrentMap[0]));
if (GAIAError(FSWrite(FilNum,&count,OcCurrentMap[0]))) return;
CountTotal += (count=GetPtrSize(DriftMap[0]));
if (GAIAError(FSWrite(FilNum,&count,DriftMap[0]))) return;
CountTotal += (count=GetPtrSize(EventMap[0]));
if (GAIAError(FSWrite(FilNum,&count,EventMap[0]))) return;
CountTotal += (count=GetPtrSize(SAirTempMap[0]));
if (GAIAError(FSWrite(FilNum,&count,SAirTempMap[0]))) return;
CountTotal += (count=GetPtrSize(SCloudDensity[0]));
if (GAIAError(FSWrite(FilNum,&count,SCloudDensity[0]))) return;
CountTotal += (count=GetPtrSize(SAirCurrentMap[0]));
if (GAIAError(FSWrite(FilNum,&count,SAirCurrentMap[0]))) return;
PutParameters();
CountTotal += (count=GetPtrSize(MiscHis));
if (GAIAError(FSWrite(FilNum,&count,MiscHis))) return;
/**Close**/
GAIAError(SetEOF(FilNum,CountTotal));
GAIAError(FSClose(FilNum));
GAIAError(FlushVol(NIL,VolNum));
BlockMove(name,EwinStr,256);
SetWTitle(editWindow,EwinStr);
MessageDialog("\pSave Complete:",EwinStr,"",1);
EnableItem(GetMHandle(301),3); /* Save */
FilVolNum = VolNum;
}That was very common not long ago. Even the JPEG/EXIF image file format is designed that way. So it's efficient for reading and writing, but introduces a lot of potential bugs with alignment and chunk size issues. Inserting an additional EXIF tag is a huge hassle because then you have to recalculate all the pointers, even those in other data chunks.
One form of true insanity is probably JPEG-encoded TIFF files. Apparently it's so bad that for the next version of the TIFF specification they are taking that insane approach out completely.
I can't do justice to the article on TIFF's problems with JPEG (http://www.simplesystems.org/libtiff/TIFFTechNote2.html), but my understanding is that:
1) Some JPEG-specific data is moved outside of the actual JPEG bitstream tag, and into separate TIFF tags, making editing JPEG-in-TIFF files non-trivial. 2) Size is not encoded in some of these fields, so the TIFF editor you're writing will have to partially implement a JPEG decoder just to know the size of some of those TIFF tags, and 3) Some tags/fields are pointers into other parts of the TIFF file, meaning if you edit the file, you'll have to update the file in many places.
(As a quick aside on insane formats, may I also mention the EPWING dictionary format?)
I can't do justice to the article on TIFF's problems with JPEG (http://www.simplesystems.org/libtiff/TIFFTechNote2.html), but my understanding is that:
1) Some JPEG-specific data is moved outside of the actual JPEG bitstream tag, and into separate TIFF tags, making editing JPEG-in-TIFF files non-trivial. 2) Size is not encoded in some of these fields, so the TIFF editor you're writing will have to partially implement a JPEG decoder just to know the size of some of those TIFF tags, and 3) Some tags/fields are pointers into other parts of the TIFF file, meaning if you edit the file, you'll have to update the file in many places.
(As a quick aside on insane formats, may I also mention the EPWING dictionary format?)
That's why you version your file formats and network protocols. Actually, designing file formats and network protocols is one of the few areas in software engineering where you do really need future-proof design and extreme extensibility, because once you release the thing, these are set in stone. Yet not many people seem to realize this. They instead "future-proof" their code with useless abstraction layers.
Anyway. Versioning helps you avoid ugly workarounds if you need to extend your format in the future in ways that its current version doesn't allow. You then keep the code for older versions as a backwards-compatibility-only kind of deal and move on to the new one.
Anyway. Versioning helps you avoid ugly workarounds if you need to extend your format in the future in ways that its current version doesn't allow. You then keep the code for older versions as a backwards-compatibility-only kind of deal and move on to the new one.
Versioning is one approach, but I favor making the format extensible in the first place. For example, if you pick an XML format you can add new attributes and tags, if you pick a Protobuf format you can add new fields. “Extensible” sounds like it can be a real mess but there are effective strategies to minimize the mess.
There are also various chunked formats like AIFC (AIFF) and PNG which can be extended by defining new chunk types, without needing to change versioning. AIFC includes the APPL/stoc chunk and PNG has various mechanisms.
There are a few problems with versioning file formats. One is that you often end up with 'if (version > 3)' scattered across your code base or other nonsense. Another problem is that it is easy to accidentally mark the wrong version, either a version which is too low (because you wrote something to the file and forgot to make your encoder bump the version properly) or a version which is too high (because you didn’t bother to use the minimum version your data requires).
There are also various chunked formats like AIFC (AIFF) and PNG which can be extended by defining new chunk types, without needing to change versioning. AIFC includes the APPL/stoc chunk and PNG has various mechanisms.
There are a few problems with versioning file formats. One is that you often end up with 'if (version > 3)' scattered across your code base or other nonsense. Another problem is that it is easy to accidentally mark the wrong version, either a version which is too low (because you wrote something to the file and forgot to make your encoder bump the version properly) or a version which is too high (because you didn’t bother to use the minimum version your data requires).
I'd say do both, actually. But versioning is more important IMO because it gives you the freedom to potentially start from scratch keeping only a small portion of the header.
On mainstream file formats... It's a mixed bag. Image formats — JPEG and PNG especially — are extensible and reasonably easy to parse. It's fairly trivial to get the image dimensions out of one of these without decoding the compressed data. I did as well write a JPEG decoder out of curiosity once to understand the compression algorithm better — it's an interesting exercise, really, every software developer should try it at some point.
But the worst format I've ever worked with is MP3. It's an absolute mess. First, there are two kinds of ID3 tags. These store metadata that your player displays. ID3v1 is a fixed-length, fixed-layout thing that goes on the end of the file. ID3v2 is an extensible, I'd say way too extensible, chunked thing capable of storing literally anything, including jpegs of cover arts, that goes on the beginning of the file. But none of them store the duration of the file. You're supposed to chop the tags off the ends of it, then find the first frame of encoded data by searching for the pattern 0xFFFx, read its header, and determine the byte length, bitrate, the sampling rate and ultimately the duration of a that frame using several lookup tables. Now that you know how much audio each frame contains, and how long it is, you take the size of the file (minus tags of course) and divide it by the frame length, then multiply by the frame duration. That's how you get the duration of an MP3. A constant-bitrate one. And to seek within an MP3, you calculate the offset into the file and round it to the nearest frame size and just start playing from there. It gets even worse with VBR, because now you can no longer rely on frames being the same byte length, but I don't really remember the details any more. The gist of it is that there's "header" encoded into the very first frame in the file, and there are two kinds of these headers, and there's a sort of lookup table in it, among other things, to help you seek into the right part of the file because the byte offsets don't linearly correspond to the playback time in a VBR file. After you seek, you have to go back and forth to find the 0xFFFx and play from there. Or not, because sometimes there's a 0xFFFx in the middle of a frame too, so you have to have some heuristics to detect that it's the real one.
On mainstream file formats... It's a mixed bag. Image formats — JPEG and PNG especially — are extensible and reasonably easy to parse. It's fairly trivial to get the image dimensions out of one of these without decoding the compressed data. I did as well write a JPEG decoder out of curiosity once to understand the compression algorithm better — it's an interesting exercise, really, every software developer should try it at some point.
But the worst format I've ever worked with is MP3. It's an absolute mess. First, there are two kinds of ID3 tags. These store metadata that your player displays. ID3v1 is a fixed-length, fixed-layout thing that goes on the end of the file. ID3v2 is an extensible, I'd say way too extensible, chunked thing capable of storing literally anything, including jpegs of cover arts, that goes on the beginning of the file. But none of them store the duration of the file. You're supposed to chop the tags off the ends of it, then find the first frame of encoded data by searching for the pattern 0xFFFx, read its header, and determine the byte length, bitrate, the sampling rate and ultimately the duration of a that frame using several lookup tables. Now that you know how much audio each frame contains, and how long it is, you take the size of the file (minus tags of course) and divide it by the frame length, then multiply by the frame duration. That's how you get the duration of an MP3. A constant-bitrate one. And to seek within an MP3, you calculate the offset into the file and round it to the nearest frame size and just start playing from there. It gets even worse with VBR, because now you can no longer rely on frames being the same byte length, but I don't really remember the details any more. The gist of it is that there's "header" encoded into the very first frame in the file, and there are two kinds of these headers, and there's a sort of lookup table in it, among other things, to help you seek into the right part of the file because the byte offsets don't linearly correspond to the playback time in a VBR file. After you seek, you have to go back and forth to find the 0xFFFx and play from there. Or not, because sometimes there's a 0xFFFx in the middle of a frame too, so you have to have some heuristics to detect that it's the real one.
That is because MP3 is not a file format in the first place. The file is originally simply an stream of MP3 frames written into a file without any structure and usually with technically invalid and undecodable frame at both ends (due to how MP3 compression works). Because it is originally designed for transmission across some kind of somewhat unreliable network, there are resynchronization structures in the stream and decoders tend to be able to ignore various kinds of totally invalid crap in the input data, this feature is exploited by all the ID3vX formats to essentially embed arbitrary data into the file. Several other commonly used "MPEG something" "file formats" are exactly the same thing.
This is somewhat ironic given the fact that quite large part of MPEG specification deals with various framing and metadata structures (on the other hand the overall architecture of all that is best described as "overengineered", so ignoring it makes some kind of sense).
This is somewhat ironic given the fact that quite large part of MPEG specification deals with various framing and metadata structures (on the other hand the overall architecture of all that is best described as "overengineered", so ignoring it makes some kind of sense).
This pretty much nails it.
I also do appreciate MP3's simplicity, in the sense that it's just a series of concatenated frames. It makes it really easy to just fling them over the network and get streaming audio working on a client. And there's also somewhat of an elegance (a very ugly and hacky elegance, mind you) to being able to exploit decoders ignoring malformed frames.
For example if you open an MP3 IceCast stream via HTTP in a media player like VLC, the server (if it realizes, via the HTTP request headers, that you're Icecast-aware) will occasionally barf the name of the current and following song into the MP3 stream. Meaning you don't need any higher-level streaming protocol to deal with, and can just send raw frames over the wire, where the MP3 decoder will ignore the song title as a malformed MP3 frame, but VLC will pick-up on the song title and display it for you as the server cycles from one song to the next. Kinda handy, because VLC will make use of this metadata, but at the same time, the MP3 stream's URL will also work in a web browser too, since the browser won't need to know how to deal with a higher-level protocol before being able to start receiving those frames.
Actually, nevermind all that. MP3 bad.
I also do appreciate MP3's simplicity, in the sense that it's just a series of concatenated frames. It makes it really easy to just fling them over the network and get streaming audio working on a client. And there's also somewhat of an elegance (a very ugly and hacky elegance, mind you) to being able to exploit decoders ignoring malformed frames.
For example if you open an MP3 IceCast stream via HTTP in a media player like VLC, the server (if it realizes, via the HTTP request headers, that you're Icecast-aware) will occasionally barf the name of the current and following song into the MP3 stream. Meaning you don't need any higher-level streaming protocol to deal with, and can just send raw frames over the wire, where the MP3 decoder will ignore the song title as a malformed MP3 frame, but VLC will pick-up on the song title and display it for you as the server cycles from one song to the next. Kinda handy, because VLC will make use of this metadata, but at the same time, the MP3 stream's URL will also work in a web browser too, since the browser won't need to know how to deal with a higher-level protocol before being able to start receiving those frames.
Actually, nevermind all that. MP3 bad.
IIRC IceCast essentially extends/abuses HTTP/1.1 chunked encoding to frame such metadata. The original Nullsoft's ShoutCast protocol simply multiplexes fixed size blocks of mp3 data with length prefixed metadata packets. In both cases you can pretty easily strip the metadata out.
Back when di.fm was an actual online radio and gave you a stream URL to play in a player of your choice, it was dead simple. I don't know what protocol it used, but there was no chunked encoding and no content-length, simply a 200 OK HTTP response and then an endless MP3 "file" with ID3 tags inserted every now and then. You could play it with Android's MediaPlayer in like 3 lines of code.
Oh and by the way. For this exact purpose, ID3v2 has a flag called "unsynchronized" that means all 0xFF bytes in that field are replaced with 0xFF00, so that players won't play the wrong thing. You're supposed to replace them back when parsing the tag. A very hacky solution.
Oh and by the way. For this exact purpose, ID3v2 has a flag called "unsynchronized" that means all 0xFF bytes in that field are replaced with 0xFF00, so that players won't play the wrong thing. You're supposed to replace them back when parsing the tag. A very hacky solution.
You should have a version, but given the (admittedly unrealistic) choice between versioning and extensibility I’ll take extensibility every time. There are plenty of formats where there’s a version tag and it’s never been bumped past “1”.
Early compressed audio / video formats were generally a total mess, with a couple exceptions like MOV, so I’m not surprised that MP3 is horrible.
Early compressed audio / video formats were generally a total mess, with a couple exceptions like MOV, so I’m not surprised that MP3 is horrible.
I second your observations on JPEG. It turns out that you can use the same code to skim through jpeg, lossless jpeg, Jpeg2000 and jpeg-ls headers. Impressive in it’s simplicity.
Every JPEG 2000 encoder I've seen encapsulates the JP2K data in an MOV/MP4-like format (or if one likes using many syllables, technically the ISO Base Media File Format). Even when using Motion JPEG 2000 image sequences embedded as frames in an MP4 video file, each separate JP2K frame includes the MP4-like container as you would see if the stand-alone JP2K file was sitting on-disk.
Anyway, it is indeed cool that you can encode JPEG-LS, Lossless JPEG and friends in the exact same format as you see baseline JPEG images in - but just in different application segments! (eg, baseline=SOF0/SOF1 segment, Lossless JPEG=SOF3, JPEG-LS=SOF48, etc.).
Anyway, it is indeed cool that you can encode JPEG-LS, Lossless JPEG and friends in the exact same format as you see baseline JPEG images in - but just in different application segments! (eg, baseline=SOF0/SOF1 segment, Lossless JPEG=SOF3, JPEG-LS=SOF48, etc.).
Data structures and object protocols are the analog to file formats and network protocols. They're not immutable but unless you want software changes to affect everything, you want something at least a bit more stable
In practice, it's the simplest and most efficient way, since the majority of the time you're not going to be dealing with any insanely weird architectures and stuff like 9-bit bytes has thankfully disappeared from common use.
I do wish they'd pack their structures, however.
I do wish they'd pack their structures, however.
This is one of my favourite rants.
The other one is the SO answer about X/HTML parsing with regex. https://stackoverflow.com/questions/1732348/regex-match-open...
The other one is the SO answer about X/HTML parsing with regex. https://stackoverflow.com/questions/1732348/regex-match-open...
My favourite is wm4's rant about C locales. It's quite impressive.
https://github.com/mpv-player/mpv/commit/1e70e82baa9193f6f02...
https://github.com/mpv-player/mpv/commit/1e70e82baa9193f6f02...
Ouch! Truth hurts! ;)
The worst part is that American programmers may never be hit by most of the horrors of LC_CTYPE et al, so will not fix it for those that do.
And the poor guy who installed Postgres and tries to sort Arabic can't figure out why in Heaven's name the order seems random.
The worst part is that American programmers may never be hit by most of the horrors of LC_CTYPE et al, so will not fix it for those that do.
And the poor guy who installed Postgres and tries to sort Arabic can't figure out why in Heaven's name the order seems random.
"Those not comfortable
with toxic language should pretend this is a religious text."
I love it.
I love it.
I prefer the end:
> All in all, I believe this proves that software developers as a whole and as a culture produce worse results than drug addicted butt fucked monkeys randomly hacking on typewriters while inhaling the fumes of a radioactive dumpster fire fueled by chinese platsic toys for children and Elton John/Justin Bieber crossover CDs for all eternity.
> All in all, I believe this proves that software developers as a whole and as a culture produce worse results than drug addicted butt fucked monkeys randomly hacking on typewriters while inhaling the fumes of a radioactive dumpster fire fueled by chinese platsic toys for children and Elton John/Justin Bieber crossover CDs for all eternity.
Unsurprisingly, this kind of attitude got him kicked of his own project.
"Please do not flag it for our attention." is the perfect finishing touch too.
I find it weird that the comment is locked rather than deleted.
Funny it be, it does not seem à propos for answering a quæstion.
Funny it be, it does not seem à propos for answering a quæstion.
It's from a time before StackOverflow (and StackExchange at large) got very ridged with all the rules. Back then, there were a lot of "fun" questions and answers, including the infamous "What is the best comment in source code you have ever encountered?"[1]
Some old timers might remember those days... SO was a much more fun place back then. Now it's rife with down-voters, close-voters and hostile-towards-newbie folks.
[1] https://stackoverflow.com/questions/184618/what-is-the-best-...
Some old timers might remember those days... SO was a much more fun place back then. Now it's rife with down-voters, close-voters and hostile-towards-newbie folks.
[1] https://stackoverflow.com/questions/184618/what-is-the-best-...
The post was locked because it had received hundreds of flags and comments complaining that the post was broken -- even with the moderator note at the bottom -- and several edits trying to "fix" the post. See https://meta.stackoverflow.com/questions/250099#comment637_2...
My point is that the comment should be deleted altogether as it's more of a personal rant than a serious answer.
You'll fit in extremely well as a moderator on today's StackExchange.
I'm not saying that these should be the rules.
Only that so long as they be the rules, they should be consistently enforced.
Only that so long as they be the rules, they should be consistently enforced.
To be fair, "law doesn't work backwards" is an estabilished legal principle.
At this point, it is a cultural artifact.
Joel on Software, "Why are Microsoft Office File Formats So Complicated"
https://www.joelonsoftware.com/2008/02/19/why-are-the-micros...
They were designed to be fast on very old computers.
They were designed to use libraries.
They were not designed with interoperability in mind.
They have to reflect the history of the applications,
https://www.joelonsoftware.com/2008/02/19/why-are-the-micros...
They were designed to be fast on very old computers.
They were designed to use libraries.
They were not designed with interoperability in mind.
They have to reflect the history of the applications,
PSD is horrible, that is certainly true. And underdocumented. And dumb... And we needed to implement read/write support for that in Krita.
But Painttool Sai's format is actively evil, with different kinds of encryption for different size layers and things like that: https://github.com/Wunkolo/libsai/issues/6 (We tried to use that library to implement sai file format support for Krita, but then ran into trouble...)
Manga Studio/Clip Studio Paint's file format is interesting as well: it's just an SQLite database, with raster data stored as blobs. Not going to implement that for Krita either.
But Painttool Sai's format is actively evil, with different kinds of encryption for different size layers and things like that: https://github.com/Wunkolo/libsai/issues/6 (We tried to use that library to implement sai file format support for Krita, but then ran into trouble...)
Manga Studio/Clip Studio Paint's file format is interesting as well: it's just an SQLite database, with raster data stored as blobs. Not going to implement that for Krita either.
> Manga Studio/Clip Studio Paint's file format is interesting as well: it's just an SQLite database, with raster data stored as blobs. Not going to implement that for Krita either.
I'm curious why? I'm not trying to start anything, but I remember this discussion from a few months ago:
https://news.ycombinator.com/item?id=23509914
> I wish my predecessor at Krita had made that choice (sqlite), instead of choosing to re-use the KOffice xml-based file format that's basically a zip file with stuff in it. It would have made adding the animation feature so much easier.
To summarize the benefits to something like sqlite over zip+xml (not all were made by that author) is; update in place means not having to overwrite the whole file, not inventing xml format for describing an image, not needing to keep the entire representation in-memory, a longer saving process is more fragile, incremental loading.
I'm curious why? I'm not trying to start anything, but I remember this discussion from a few months ago:
https://news.ycombinator.com/item?id=23509914
> I wish my predecessor at Krita had made that choice (sqlite), instead of choosing to re-use the KOffice xml-based file format that's basically a zip file with stuff in it. It would have made adding the animation feature so much easier.
To summarize the benefits to something like sqlite over zip+xml (not all were made by that author) is; update in place means not having to overwrite the whole file, not inventing xml format for describing an image, not needing to keep the entire representation in-memory, a longer saving process is more fragile, incremental loading.
Well, because the actual contents of the sqlite database are obfuscated, too.
That doesn't look like encryption or differences in algorithms, just the usual confusion and edge cases when you don't have a full mental model of the file format's fixed-size vfs entries.
Thank you for working on Krita.
I wrote a PSD v5 parser for BeOS back in 1999 or 2000 (it acted as a plug-in for the OS's image format subsystem, and was also usable by 3rd party apps to write PSD files).
I was one of the suckers who faxed a formal request to Adobe to get the file spec.
I was young, and foolish, and spent the better part of a month or so in a hex editor trying to understand why a single file format could have like 3 different string encodings.
I was one of the suckers who faxed a formal request to Adobe to get the file spec.
I was young, and foolish, and spent the better part of a month or so in a hex editor trying to understand why a single file format could have like 3 different string encodings.
Oh wow, did faxing actually work?
And I feel your pain, but you were only on v5. It just gets worse the more versions you add.
And I feel your pain, but you were only on v5. It just gets worse the more versions you add.
While I totally empathize with the poor author who had to suffer through this madness, I thoroughly enjoyed reading this rant. My favorite part was this:
PSD is not a good format. PSD is not even a bad format.
Calling it such would be an insult to other bad formats, such as ...I'd love to hear why they think JPEG is a worse format.
Not worse, that is the point. It is merely a bad format, which is better than what PSD is.
As for why JPEG is bad: Did you know there is no one defined way to store the width and height of a JPEG image?
As for why JPEG is bad: Did you know there is no one defined way to store the width and height of a JPEG image?
As someone who has written a JPEG decoder, I am compelled to say that you are wrong, and the width and height are stored in the SOF marker segment.
http://vip.sugovica.hu/Sardi/kepnezo/JPEG%20File%20Layout%20...
http://vip.sugovica.hu/Sardi/kepnezo/JPEG%20File%20Layout%20...
I might be misremembering, but at least, SOF does not tell you what is actually in the channels of the image, so it is not enough to figure out how to actually display an image.
Thank you, I totally misread that!
To my knowledge, I see most JPEG images encode their height in an "APP1" EXIF segment, and in their SOF1 segment (which is basically just a basic-info segment for the core JPEG bitstream). It's also possible to store the size in an APP1 XMP section, but that's mostly non-essential metadata for interchange purposes, and I usually don't see the size duplicated there for JPEGs anyway (PNG does though, if it has an XMP chunk).
It's possible that some of the other officially-defined segments (see https://exiftool.org/TagNames/JPEG.html) might also contain duplicate info, but then that'd be specific to the app that uses it, not for general-purpose use.
Storing the size in easily-parseable EXIF metadata and then once again in the JPEG bitstream seems pretty reasonable to me. Not to mention if you run a metadata/EXIF stripper on a JPEG file, then you remove all duplicate info and the only remaining image-dimensions will be stored in the JPEG bitstream anyway.
It's possible that some of the other officially-defined segments (see https://exiftool.org/TagNames/JPEG.html) might also contain duplicate info, but then that'd be specific to the app that uses it, not for general-purpose use.
Storing the size in easily-parseable EXIF metadata and then once again in the JPEG bitstream seems pretty reasonable to me. Not to mention if you run a metadata/EXIF stripper on a JPEG file, then you remove all duplicate info and the only remaining image-dimensions will be stored in the JPEG bitstream anyway.
Well for one thing it's not a file format. For that there's JFIF and EXIF.
I actually thought about that when I typed my reply, but decided the meaning of my question would still be clear the way it was phrased. Despite that, my question is more or less irrelevant since I misunderstood the author.
Personally, rather than printing things out and then setting them on fire, I've preferred printing things out then when I'm done with it donating them to be shredded and used as horse bedding.
Some code just deserves to end its life being shat on by a horse.
Some code just deserves to end its life being shat on by a horse.
So, what's this project about? There's no README nor description nor anything.
I'm curious as to why they'd find it so worthwhile to try to parse PSD despite those troubles. They seem to already have parsers for other image formats.
I'm curious as to why they'd find it so worthwhile to try to parse PSD despite those troubles. They seem to already have parsers for other image formats.
Xee is an image viewer. If you follow the link to the landing page of the repo, it says in the About section it is "Xee source code for xCode 4.5"
I've seen that rant a while ago. IIRC, Xee is an image viewer for macOS.
> So, what's this project about? There's no README nor description nor anything.
It's Xee, a lightweight (and excellent) image viewer for macOS.
That's not the original repo (or author), used to be on google code, moved to bitbucket, then abandoned / sold the project: it was open-source until version 2.2 or something, v3 is is closed source.
It's Xee, a lightweight (and excellent) image viewer for macOS.
That's not the original repo (or author), used to be on google code, moved to bitbucket, then abandoned / sold the project: it was open-source until version 2.2 or something, v3 is is closed source.
Unfortunately Xee3 is basically abandoned at this point. MacPaw bought it alongside The Unarchiver and they haven't really been doing anything with it. Last update was three years ago. I wonder if it's just going to stop working in some future macOS version.
> bought it alongside The Unarchiver and they haven't really been doing anything with it
Good, because IIRC The Unarchiver was caught gathering stats or sending something on the network, after an updated release.
Good, because IIRC The Unarchiver was caught gathering stats or sending something on the network, after an updated release.
I wrote the classic-Mac image-display app, "Shomi" (originally DePICT) which tried to display anything as fast as possible.
In modern times such an app would be just a bunch of API or library calls, but in the early days I had to write all the image format readers myself.
At the time TIFF was the worst, so complicated, so many options, and you even had to write your own LZW codec that matched theirs.
Next worse was BMP. It's upside-down and you get random zeroes where you expect sensible values.
I managed with a minimalist Photoshop parser (the 8BIM format then) that didn't support everything but coped with the real world files my designer friends tried. It's got more complicated since then.
In modern times such an app would be just a bunch of API or library calls, but in the early days I had to write all the image format readers myself.
At the time TIFF was the worst, so complicated, so many options, and you even had to write your own LZW codec that matched theirs.
Next worse was BMP. It's upside-down and you get random zeroes where you expect sensible values.
I managed with a minimalist Photoshop parser (the 8BIM format then) that didn't support everything but coped with the real world files my designer friends tried. It's got more complicated since then.
I would agree on BMP being slightly insane, since there's a number of things you have to magically know about it:
1) If using 8 bits per pixel or less, there's always a color palette between the header and bitmap. If more than 8bpp, then there's none.
2) If more or equal to 8BPP, you must know that you must always use Bitfields to specify which bytes correspond to which RGB(A) channels, and that there cannot be any overlap between the channel masks.
3) There's no intuitive way to tell which version of the Bitmap file you have (there's like 5 major versions).
4) You can specify, inline, the Chromaticity and Gamma (basically an inline color profile) somewhere in the headers.
5) You can also append an ICC profile directly into the file, OR, store a Windows-style filesystem path to the profile (in which case, you must know to ignore the in-header color profile, if you have one).
6) You must know that RLE compression only works if the RGB channels cleanly map to one-byte-each (no packing).
7) RLE compression doesn't work if you use a negative height (to indicate the image is encoded from top-to-bottom, rather than the standard bottom-to-top).
8) The (un)official documentation Microsoft hosts states that BMP files can also store PNG or JPEG bitstream data, but yet no application that I've seen has ever explicitly supported this.
9) You have to know that Windows only supports 2 or so variations of packed 16-bit BMP files (565, etc).
For the most part you can just assume you're dealing with a v5 BMP file, read or write the most pertinent parts like the dimensions (skipping dumb stuff like ICC profiles, pixels-per-meter and whatnot), and just have your way with the raw data. But there is still some dumb stuff in there that shouldn't be there, considering it should be a straight-forward format. What also baffles me is that even with the bloat Microsoft added to the format, BMP isn't even extensible. So it's kinda the worst of both: It's slightly complicated to the point of not being straight-foward to work with, and yet, you can't add extra info/metadata to it.
On that note, Targa is a dreamy image format. It's got a simple 18-byte or so header, and then the raw data. And, optionally, for Targa v2 you can append a few bytes in a footer that indicate an offset for an optional standard-targa-metadata area, as well as an option metadata area for your data you wish to add to the file. Followed by a magic string, that explicitly indicate you're using v2 of the Targa spec.
1) If using 8 bits per pixel or less, there's always a color palette between the header and bitmap. If more than 8bpp, then there's none.
2) If more or equal to 8BPP, you must know that you must always use Bitfields to specify which bytes correspond to which RGB(A) channels, and that there cannot be any overlap between the channel masks.
3) There's no intuitive way to tell which version of the Bitmap file you have (there's like 5 major versions).
4) You can specify, inline, the Chromaticity and Gamma (basically an inline color profile) somewhere in the headers.
5) You can also append an ICC profile directly into the file, OR, store a Windows-style filesystem path to the profile (in which case, you must know to ignore the in-header color profile, if you have one).
6) You must know that RLE compression only works if the RGB channels cleanly map to one-byte-each (no packing).
7) RLE compression doesn't work if you use a negative height (to indicate the image is encoded from top-to-bottom, rather than the standard bottom-to-top).
8) The (un)official documentation Microsoft hosts states that BMP files can also store PNG or JPEG bitstream data, but yet no application that I've seen has ever explicitly supported this.
9) You have to know that Windows only supports 2 or so variations of packed 16-bit BMP files (565, etc).
For the most part you can just assume you're dealing with a v5 BMP file, read or write the most pertinent parts like the dimensions (skipping dumb stuff like ICC profiles, pixels-per-meter and whatnot), and just have your way with the raw data. But there is still some dumb stuff in there that shouldn't be there, considering it should be a straight-forward format. What also baffles me is that even with the bloat Microsoft added to the format, BMP isn't even extensible. So it's kinda the worst of both: It's slightly complicated to the point of not being straight-foward to work with, and yet, you can't add extra info/metadata to it.
On that note, Targa is a dreamy image format. It's got a simple 18-byte or so header, and then the raw data. And, optionally, for Targa v2 you can append a few bytes in a footer that indicate an offset for an optional standard-targa-metadata area, as well as an option metadata area for your data you wish to add to the file. Followed by a magic string, that explicitly indicate you're using v2 of the Targa spec.
I'd forgotten about negative heights! Unexpectedly negative values in BMP files were another surprise that required extra code.
I had a good laugh reading this. Of course the irony is that the author, after having gone through the hellish process of learning the spec, didn't documented any of the code (in that file).
Here's a "cool" PSD quirk.
Take a PSD that has many layers. Look at its filesize (mine is ~70mb). Add one layer to the PSD, fill it with white, and make it the topmost layer. Save it as a new PSD and compare the filesizes.
The new PSD with the white layer is 55mb. Why?
Take a PSD that has many layers. Look at its filesize (mine is ~70mb). Add one layer to the PSD, fill it with white, and make it the topmost layer. Save it as a new PSD and compare the filesizes.
The new PSD with the white layer is 55mb. Why?
Most* PSD files contain a "preview" copy of the fully-flattened document (which is compressed.) Flat white image compresses far better, so that portion of the file doesn't take as much space.
Depending on what your layers look like (how many, how much they cover, etc.) it's not too surprising that the preview image could take a substantial fraction of the total file size (sounds like ~20% in this case!)
* I believe this behavior can be toggled off with an option.
Depending on what your layers look like (how many, how much they cover, etc.) it's not too surprising that the preview image could take a substantial fraction of the total file size (sounds like ~20% in this case!)
* I believe this behavior can be toggled off with an option.
Interesting. I want to believe this is the case, but I feel like it is still a bug/quirk.
In Photoshop's preferences there is an option under File Handling to disable image previews, and toggling this seems to have no effect on what I described above. Strange!
In Photoshop's preferences there is an option under File Handling to disable image previews, and toggling this seems to have no effect on what I described above. Strange!
Is the preview the full resolution? That seems very overkill.
Perhaps, but it's quite convenient for any software that wants to ingest PSD files without the (nigh-intractable) challenge of reimplementing the entirety of Photoshop's parser and rendering systems!
The "preview" is actually the main image data from the version of the file format that was used before Photoshop supported layers.
It remains there, I guess, so that you can display the image even if you don't support loading and compositing layers.
It remains there, I guess, so that you can display the image even if you don't support loading and compositing layers.
I remember reading this years ago, I think it was my first encounter with the "... fierce passion of a thousand suns".
It still made me chuckle even today, thanks for sharing it.
It still made me chuckle even today, thanks for sharing it.
You can see some other great code comments at: https://stackoverflow.com/questions/184618/what-is-the-best-...
It may be good that SO removed all similar questions (as they would otherwise have become Reddit) but we sure lost some great humoristic collections!
It may be good that SO removed all similar questions (as they would otherwise have become Reddit) but we sure lost some great humoristic collections!
For posterity's sake, the link should probably be changed to https://github.com/zepouet/Xee-xCode-4.5/blob/83394493f51991...
This came up a couple months ago. I really wish there was a standard open format that did most of the things PSD does. I had asked the folks at Procreate if they'd design something, since they are sort of a challenger to Photoshop, but they said it wasn't something they wanted to do.
Where this might get tricky is if you seriously want to support non-destructive editing as well, because in that case any filter that can be applied in non-destructive mode effectively needs to become part of the file format specification, too.
You could store filters generic enough that any software can skip over filters it doesn't recognize (e.g. you store filter type uuid, filter data length, then arbitrary filter parameters), have some open registry where you can get official filter uuids in return for an example implementation. Of course not everyone will register every filter, so the format should probably store the image once unfiltered and once with all filter applied, that way if a program doesn't recognize or implement some filter it can still fall back to the destructively filtered version. But of course that increases file size, which may or may not be a concern.
Procreate is 10$ for a one-time purchase.
PS is 13$/mo. Actually, if you got just PS it would be 30$/mo.
And even then I've seen people complain that procreate is too expensive.
I don't really have a point other than design tools and their pricing is a pet issue of mine and I like bringing up how insane it is at any given chance.
PS is 13$/mo. Actually, if you got just PS it would be 30$/mo.
And even then I've seen people complain that procreate is too expensive.
I don't really have a point other than design tools and their pricing is a pet issue of mine and I like bringing up how insane it is at any given chance.
We started working on OpenRaster for that: https://www.openraster.org/ -- but it's not really moving along very well.
> this Rube Goldberg of a file format
Concise, to the point, and a good enough insult as to keep it in file. I love it.
Concise, to the point, and a good enough insult as to keep it in file. I love it.
I have deep sympathy for this, after having done a ton of work with the SWF format and the (even more poorly documented) AEPX format.
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Plain text file is my favorite file format... because in the end, portability is what matters the most.
ASCII?
UTF-8?
UTF-16?
...
...
Anyone using the words "plain text" for a file had better mean either ASCII or UTF-8.
And for almost all purposes these days it should be the latter.
And for almost all purposes these days it should be the latter.
Shift-JIS? GB18030? KOI8-R?
"Plain text" does not really exist.
"Plain text" does not really exist.
By plain text, do you have in mind CRLF or LF, and for structured data, do you have in mind YAML, JSON or one of the umpty others?
more of these posts please 10/10
I would actually beg to disagree, as someone who was bored a few weekends ago and decided to go through a few reasonably-complex PSD files (eg, ones with different layer types, bezier curves, etc.) in a hex editor with the official specification in-hand in an attempt to understand the format. (The specification can be found here: https://www.adobe.com/devnet-apps/photoshop/fileformatashtml...).
After a few hours, I was able to grok it without too much difficulty, and found the format was reasonably well laid-out. Yes, it does suffer from some "let's take the in-memory representation and bake it into the on-disk format"-isms, and there were a few things were not covered by the specification (eg, a couple Resource IDs aren't mentioned at all, or they are, but there's no documentation on how to interpret their content) - but it's not anywhere close to insane.
To give an example, partially going off memory from what I remember seeing in real files, but mostly skimming the spec, a very basic gist of a typical PSD files might look something like this. (I can only assume HN is going to mangle the formatting of the following text, so expect to see multiple edits to this comment):
EDIT: I was right about the formatting getting messed up. I've posted it to Pastebin and will call it a day:
https://pastebin.com/raw/WZNrCSAP
So that (the Pastebin link above) is the basic and oversimplified gist of a PSD file. It's mostly just reading 4-byte tags and length fields. Now, rendering a complex PSD file (or worse: generating a complex one) is another matter, but the on-disk format is pretty understandable IMO. Creating a bitmap-to-PSD conversion program from scratch is also something I could see being doable in a weekend if you followed the spec, and, as long as the PSD only contained simple rasterized data (eg, no filters and other fanciness) and only the bare-minimum ResourceID/Layer tags.
Honestly, I really trust PSD as a general interchange format for images. It's quirky in some places, but it's pretty logically laid-out, is universally-recognized, and I can rest assured that a rasterized copy of my images will always be stapled to the end of the file if worse comes to worse. There really is no alternative, because while you can contain Photoshop data in a format like TIFF (Photoshop just staples the entire PSD file into a TIFF/EXIF tag anyway), and if an application unaware of that PSD data opens the TIFF files and saves it, the Photoshop data might go poof or have a mismatch with the other TIFF data. And then there's Gimp's XCF format, where the on-disk format is allowed to be changed willy-nilly because you are only supposed to use Gimp's official library for reading/writing to it (not to mention no application really supports XCF aside from Gimp and maybe a few open-source projects). And let's not even bother with $application-specific formats, because they are meant only for that specific application.
The moral of the story is PSD is an alright format.
After a few hours, I was able to grok it without too much difficulty, and found the format was reasonably well laid-out. Yes, it does suffer from some "let's take the in-memory representation and bake it into the on-disk format"-isms, and there were a few things were not covered by the specification (eg, a couple Resource IDs aren't mentioned at all, or they are, but there's no documentation on how to interpret their content) - but it's not anywhere close to insane.
To give an example, partially going off memory from what I remember seeing in real files, but mostly skimming the spec, a very basic gist of a typical PSD files might look something like this. (I can only assume HN is going to mangle the formatting of the following text, so expect to see multiple edits to this comment):
EDIT: I was right about the formatting getting messed up. I've posted it to Pastebin and will call it a day:
https://pastebin.com/raw/WZNrCSAP
So that (the Pastebin link above) is the basic and oversimplified gist of a PSD file. It's mostly just reading 4-byte tags and length fields. Now, rendering a complex PSD file (or worse: generating a complex one) is another matter, but the on-disk format is pretty understandable IMO. Creating a bitmap-to-PSD conversion program from scratch is also something I could see being doable in a weekend if you followed the spec, and, as long as the PSD only contained simple rasterized data (eg, no filters and other fanciness) and only the bare-minimum ResourceID/Layer tags.
Honestly, I really trust PSD as a general interchange format for images. It's quirky in some places, but it's pretty logically laid-out, is universally-recognized, and I can rest assured that a rasterized copy of my images will always be stapled to the end of the file if worse comes to worse. There really is no alternative, because while you can contain Photoshop data in a format like TIFF (Photoshop just staples the entire PSD file into a TIFF/EXIF tag anyway), and if an application unaware of that PSD data opens the TIFF files and saves it, the Photoshop data might go poof or have a mismatch with the other TIFF data. And then there's Gimp's XCF format, where the on-disk format is allowed to be changed willy-nilly because you are only supposed to use Gimp's official library for reading/writing to it (not to mention no application really supports XCF aside from Gimp and maybe a few open-source projects). And let's not even bother with $application-specific formats, because they are meant only for that specific application.
The moral of the story is PSD is an alright format.
No, it's nothing of the kind. It's not alright, it's not a general interchange format for images that you can trust, and the specification isn't complete or correct in any case.
But the problem is not in the "basic and oversimplified gist". If you look at it on that level, it looks reasonable.
The problem is in nearly every detail that is left out of such an overview. Every little detail is weird, unexpected or annoying.
The problem is in nearly every detail that is left out of such an overview. Every little detail is weird, unexpected or annoying.
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https://news.ycombinator.com/item?id=575122