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Stretch draw issue with TBitmap Canvas (Linux gtk2 only)

  • Lazarus/FPC Version: Lazarus 3.99 (rev main_3_99-2316-g00f3d3b397) FPC 3.3.1 x86_64-linux-gtk2
  • Operating System: Linux Ubuntu 22.04 64-bit
  • CPU / Bitness: x86_64

Preliminary note

This bug only occurs on Linux gtk2, not on qt5, not on Windows. The bug was reproduced by wp in https://forum.lazarus.freepascal.org/index.php/topic,68146.0.html Probably iLya2IK found the reason, why and where the bug occors in reply 7 of this Topic. The code shown by iLya2IK there was added in issue #8553 (closed).

What happens

The attached demo draws something on a TBitmap and then calls 'TPaintBox.Canvas.StretchDraw' to draw this TBitmap on a TPaintBox. If you use 'TBitmap.PixelFormat:=pf32bit' then depending on the Bitmap size this works or works not. Without 'TBitmap.PixelFormat:=pf32bit' this bug does not occur.

procedure TForm1.PaintBox1Paint(Sender: TObject);
var
  bmp: TBitmap;
  i: integer;
begin
  writeln('Event PaintBox1Paint fired');
  bmp := TBitmap.Create;
  try
    bmp.PixelFormat := pf32bit; {only with this line the bug occurs}
    i:=1;                       {'0' causes the bug, '+1' and '-1' work}
    bmp.SetSize(1000+i, 1000+i);

    bmp.Canvas.Brush.Color := clRed;
    bmp.Canvas.FillRect(0, 0, bmp.Width div 2, bmp.Height);

    bmp.Canvas.Brush.Color := clYellow;
    bmp.Canvas.FillRect(bmp.Width div 2, 0, bmp.Width, bmp.Height);

    Caption := FormatFloat('0.000', bmp.RawImage.DataSize/bmp.RawImage.MaskSize);
    Paintbox1.Canvas.StretchDraw(Rect(0, 0, Paintbox1.Width, Paintbox1.Height), bmp);
  finally
    bmp.Free;
  end;
end;

What did you expect

The attached demo should also work with TBitmap.PixelFormat:=pf32bit;

Steps to reproduce

  • compile and run the attached project
  • result: you see a TPaintBox which is half red and yellow (the bug did NOT occur)
  • edit line 41 (of the attached project) to be 'i:=0' instead of 'i:=1'
  • compile and run the project
  • result: you see a TPaintBox which is completely gray (the bug occured)
  • insert '//' in front of line 40 to deactivate 'bmp.PixelFormat:=pf32bit'
  • compile and run the project
  • result: you see the TPaintBox which is again half red and yellow (the bug disappeared)

project1.zip