Re: reading photoshop layers

From: Warren Galyen (wgalyen_at_modomail.com)
Date: 01/27/05 

Date: Thu, 27 Jan 2005 12:01:59 -0800

Mike D Sutton wrote:
>>I've actually found a graphics library that supports PSD files with
>>layers at http://www.soft-gems.net/Graphics.php#GraphicEx. Unfortunately
>>its written in Delphi and I have no experience whatsoever with this
>>langauge.
>
>
> Delphi isn't too bad at all to work with, I use it on a regular basis at work and find it to be somewhere in the middle
> between C#, C++ and VB, although probably most so to C# (as you would expect having been headed up by the same guy
> (Anders Hejlsberg)), but even without knowing either you can probably read the majority of Delphi code and made a
> reasonable guess as to it's meaning.
>
> Are you sure this library supports individual layer extraction though? I had a quick look and couldn't see anything in
> the docs or in the source code itself that suggested it could decode individual layers, although the colourspace
> conversion code would be handy to pull out..
> As for parsing the format yourself, the structure is simple enough (split into 5 segments), each segment can be read
> individually (or skipped entirely) and most have a sub-tagging system that breaks the internal data down into individual
> records. Non-understood tags within that can be ignored since each is stored in the same way with an ID and size
> prefixing the data itself, so writing a parser for the format knowing nothing about the data within it is a reasonably
> trivial task, going into more detail where required.
> The layers section is the most complex as it contains many sub-layers of data to get down to individual attributes and
> image data itself, but even then a few hours with a hex editor and some version of the format spec. (I was working from
> a 3.0 spec. to start with) and you'll have a pretty good understanding of the format.
> Having not worked with the format (at least in code, I probably use Photoshop itself almost every day) for a good few
> years it literally took a day to go from nothing to a working class that could extract the composite image including
> PackBits decompression, parse the structure of the layers section and extract the data blocks for each layer image (it
> doesn't expose them yet, but that's simply because I ran out of time and had to get on with real work instead ;)
> Really though, not a complex format at all since you can pick and choose which areas to go into detail to and simply
> ignore the rest.
> Hope this helps,
>
> Mike
>
>
> P.s. Here's a few of the utility functions from my class which should solve some of the more tedious stuff including
> endian swapping integers, and PackBits decompression:
>
> '***
> Private Declare Function RtlFillMemory Lib "Kernel32.dll" ( _
> ByRef Destination As Any, ByVal Length As Long, ByVal Fill As Byte) As Long
> Private Declare Sub RtlMoveMemory Lib "Kernel32.dll" ( _
> ByRef Destination As Any, ByRef Source As Any, ByVal Length As Long)
>
> ' DWord endian flip
> Private Function FlipDWord(ByVal inDWord As Long) As Long
> FlipDWord = _
> (((inDWord And &HFF000000) \ &H1000000) And &HFF&) Or _
> ((inDWord And &HFF0000) \ &H100&) Or _
> ((inDWord And &HFF00&) * &H100&) Or _
> ((inDWord And &H7F&) * &H1000000)
> If (inDWord And &H80&) Then FlipDWord = FlipDWord Or &H80000000
> End Function
>
> ' Word endian flip
> Private Function FlipWord(ByVal inWord As Integer) As Integer ' Endian swap on Word
> FlipWord = ((inWord And &H7F) * &H100) Or (((inWord And &HFF00) \ &H100) And &HFF)
> If (inWord And &H80) Then FlipWord = FlipWord Or &H8000
> End Function
>
> ' Signed to unsigned Word conversion
> Private Function SWordToUWord(ByVal inWord As Integer) As Long
> SWordToUWord = inWord And &H7FFF& ' Signed to unsigned Word
> If (inWord And &H8000) Then SWordToUWord = SWordToUWord Or &H8000&
> End Function
>
> ' PackBits decompression routine (No buffer overflow protection though..)
> Private Function UnpackBitsPtr(ByRef inRLE() As Byte, ByVal inRLESize As Long, _
> ByVal outRAWPtr As Long) As Boolean
> Dim RLEPos As Long, RunLength As Long
> Dim RAWPos As Long
>
> Do
> If (inRLE(RLEPos) = &H80) Then
> ' No-op, should never appear but ignore if it does
> ElseIf (inRLE(RLEPos) And &H80) Then
> RunLength = &H101 - inRLE(RLEPos) ' RLE run
> Call RtlFillMemory(ByVal (outRAWPtr + RAWPos), _
> RunLength, inRLE(RLEPos + 1))
> RLEPos = RLEPos + 2
> Else
> RunLength = inRLE(RLEPos) + 1 ' RAW run
> Call RtlMoveMemory(ByVal (outRAWPtr + RAWPos), _
> inRLE(RLEPos + 1), RunLength)
> RLEPos = RLEPos + RunLength + 1
> End If
>
> RAWPos = RAWPos + RunLength
> Loop While RLEPos < inRLESize
> End Function
> '***
>
>
> - Microsoft Visual Basic MVP -
> E-Mail: EDais@mvps.org
> WWW: http://EDais.mvps.org/
>
>

Thanks a lot Mike for the starting point!

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