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 Loading 32 bit BI_BITFIELDS bitmap having non-8-bit channels
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Harald Lampke

Germany
3 Posts

Posted - Sep 15 2022 :  07:43:52  Show Profile  Reply
When loading a 32 bit bitmap with "biCompression = BI_BITFIELDS", and when the bitmask of a channel (including the alpha channel) has more or less than 8 bits set, the resulting colors respectively alpha values are wrong.

To fix this, in module bmpfilt.pas, procedure BMPReadStream, the lines

  rshift := IEGetFirstSetBit(BitFields[0]) - 1;
  gshift := IEGetFirstSetBit(BitFields[1]) - 1;
  bshift := IEGetFirstSetBit(BitFields[2]) - 1;
  ashift := IEGetFirstSetBit(BitFields[3]) - 1;


must be supplemented to

  rshift := IEGetFirstSetBit(BitFields[0]) - 1 + (_GetBitCount(BitFields[0]) - 8);
  gshift := IEGetFirstSetBit(BitFields[1]) - 1 + (_GetBitCount(BitFields[1]) - 8);
  bshift := IEGetFirstSetBit(BitFields[2]) - 1 + (_GetBitCount(BitFields[2]) - 8);
  ashift := IEGetFirstSetBit(BitFields[3]) - 1 + (_GetBitCount(BitFields[3]) - 8);


in order to scale the resulting channel values to the range of [0..255].

Additional, a 32 bit bitmap with "biCompression = BI_BITFIELDS" and a header of type BITMAPINFOHEADER only has three DWORD bitmasks (red, green and blue) and no alpha mask, not four bitmasks (see https://docs.microsoft.com/de-de/previous-versions//dd183376%28v=vs.85%29 ). So the code loading the bitmasks should be

  if xCompression = BI_BITFIELDS then
  begin
    case xBitCount of
      ...
      32: begin
        // red, green and blue masks
        fs.Read(BitFields[0], sizeof(dword) * 3);
        // no alpha mask 
        BitFields[3] := 0;
        IgnoreAlpha := true;
        end; // 32
    end; // case
  end; // if


and in line 1079

  if BitFields[3] = 0 then
    ashift := 0
  else
    ashift := IEGetFirstSetBit(BitFields[3]) - 1 + (_GetBitCount(BitFields[3]) - 8);

xequte

7215 Posts

Posted - Sep 15 2022 :  18:09:20  Show Profile  Reply
Thanks, I'll have my partner take a look at this. Can you upload/forward your test images?

Nigel
Xequte Software
www.imageen.com
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Harald Lampke

Germany
3 Posts

Posted - Sep 16 2022 :  01:52:02  Show Profile  Reply
Attached you'll find five 32 bit images: Genuine is 32 bit BI_RGB, and four BI_BITFIELDS encoded images in 7-7-7, 8-8-8, 9-9-9 and 12-11-9, all showing the same subject.

attach/Harald Lampke/202291614916_32 Bit BI_BITFIELDS.zip
985.15 KB
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xequte

7215 Posts

Posted - Sep 20 2022 :  21:30:50  Show Profile  Reply
Thank you for your code suggestions.

This will be implemented in the next update.

Nigel
Xequte Software
www.imageen.com
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