How to display a TBitmap with alpha channel on TImage correctly?

delphi, gdi, tbitmap, timage

Solution

Your shown code works fine on my system if I use Transparent=false for imgSource. I can reproduce the behavoiur with the black pixels if I load the bitmap from a file.

Different setting influence the displaying

procedure TForm3.SetAlphaFormatClick(Sender: TObject);
begin
  if SetAlphaFormat.Checked then
    ToggleImage.Picture.Bitmap.alphaformat := afDefined
  else
    ToggleImage.Picture.Bitmap.alphaformat := afIgnored;
end;

procedure TForm3.SetImageTransparentClick(Sender: TObject);
begin
  ToggleImage.Transparent := SetImageTransparent.Checked;
  Image1.Transparent := SetImageTransparent.Checked;
end;

procedure TForm3.LoadPngTransform2BitmapClick(Sender: TObject);
Const
  C_ThePNG = 'C:\temp\test1.png';
  C_TheBitMap = 'C:\temp\TestIn.bmp';
var
  SourceBitmap, TestBitmap: TBitmap;
  pngImage: TPngImage;
begin

  Image1.Transparent := SetImageTransparent.Checked;
  pngImage := TPngImage.Create;
  SourceBitmap := TBitmap.Create;
  TestBitmap := TBitmap.Create;
  try
    pngImage.LoadFromFile(C_ThePNG);
    SourceBitmap.Assign(pngImage);

    {v1 with this version without the marked* part, I get the behavoir you described
      SourceBitmap.SaveToFile(C_TheBitMap);
      TestBitmap.LoadFromFile(C_TheBitMap);
      TestBitmap.PixelFormat := pf32Bit;
      TestBitmap.HandleType := bmDIB;
      TestBitmap.alphaformat := afDefined;  //*
      Image1.Picture.Assign(TestBitmap);
    }
    //> v2
    SourceBitmap.SaveToFile(C_TheBitMap);
    SourceBitmap.PixelFormat := pf32Bit;
    SourceBitmap.HandleType :=  bmDIB;
    SourceBitmap.alphaformat := afDefined;
    Image1.Picture.Assign(SourceBitmap);
    //< v2
  finally
    pngImage.Free;
    SourceBitmap.Free;
    TestBitmap.Free;
  end;
end;

Problem

I have a TBitmap which contains semi-transparent image with alpha channel (in this example I got it from TPngImage). ``` var SourceBitmap: TBitmap; PngImage: TPngImage; begin PngImage := TPngImage.Create(); SourceBitmap := TBitmap.Create(); try PngImage.LoadFromFile('ImgSmallTransparent.png'); SourceBitmap.Assign(PngImage); SourceBitmap.SaveToFile('TestIn.bmp'); imgSource.Picture.Assign(SourceBitmap); finally PngImage.Free(); SourceBitmap.Free(); end; ``` When I save this TBitmap to a `TestIn.bmp` file and open it with any image viewer, I can see the transparency. But when I assign it to TImage, transparent pixels appear as black (TImage has `Transparent = True`). How to display TBitmap with transparency correctly on TImage?

Original source