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

Question

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?

Answer 1

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;