Delphi TGIFImage animation issue with some GIF viewers

Question

I have discovered that animated GIFs created using Delphi 2009\'s TGIFImage sometimes doesn\'t play correctly in some GIF viewers. The problem is that the

Answer 1

It's a bug in TGIFImage's LZW encoder.

In some very rare circumstances the LZW encoder will output an extra zero byte at the end of the LZW steam. Since the LZW end block marker is also a zero byte, a strict GIF reader might choke on this or interpret it as the end of the GIF (although the end of file marker is $3B).

The reason some GIF readers can handle this is probably that GIFs with this problem was common many years ago. Apparently TGIFImage wasn't the only library to make that particular mistake.

To fix the problem make the following modification to gifimg.pas (change marked with *):

procedure TGIFWriter.FlushBuffer;
begin
  if (FNeedsFlush) then
  begin
    FBuffer[0] := Byte(FBufferCount-1); // Block size excluding the count
    Stream.WriteBuffer(FBuffer, FBufferCount);
    FBufferCount := 1; // Reserve first byte of buffer for length
    FNeedsFlush := False; // *** Add this ***
  end;
end;