How to draw a PNG transparently on a TMetaFileCanvas

Question

I need to draw a PNG transparently onto a TMetafileCanvas. This works fine when I draw to a bitmap (which I use when displaying on screen), but for printing I need a MetaFile. How can I achieve this?

Here's some code that demonstrates the problem. Put the attached PNG somewhere and update the paths in the code.

procedure TForm1.Test;
var
  vPNG : TPNGImage;

  vBMP : TBitmap;
  vMeta : TMetafile;
  vMetaCanvas : TMetafileCanvas;
  LDC : HDC;
begin
  vPNG := TPNGImage.Create;
  try
    vPNG.LoadFromFile('c:\temp\pic\pic.png'); 

    //1. Draw to bitmap. Draw stuff behind PNG to demonstrate transparency
    vBMP := TBitmap.Create;
    try
      vBMP.SetSize(vPNG.Width + 20,vPNG.Height + 20);
      vBMP.Canvas.Pen.Color := clLime;
      vBMP.Canvas.Pen.Width := 5;
      vBMP.Canvas.MoveTo(0,0);
      vBMP.Canvas.LineTo(vBMP.Width,vBMP.Height);
      vBMP.Canvas.MoveTo(vBMP.Width,0);
      vBMP.Canvas.LineTo(0,vBMP.Height);
      vBMP.Canvas.Draw(10,10,vPNG);  //This is drawn correctly and transparently

      Canvas.Draw(10,10,vBMP); //Draw to demo form
    finally
      vBMP.Free;
    end;

    //2. Draw to metafile
    vMeta := TMetaFile.Create;
    try
      LDC := GetDC(0);
      vMetaCanvas := TMetafileCanvas.Create(vMeta,LDC);
      try
        vMetaCanvas.Pen.Color := clLime;
        vMetaCanvas.Pen.Width := 5;
        vMetaCanvas.MoveTo(0,0);
        vMetaCanvas.LineTo(vPNG.Width+20,vPNG.Height+20);
        vMetaCanvas.MoveTo(vPNG.Width+20,0);
        vMetaCanvas.LineTo(0,vPNG.Height+20);

        vMetaCanvas.Draw(10,10,vPNG); //Not correct. Can't see the green line
      finally
        vMetaCanvas.Free;
      end;

      //Demonstrate that resizing works fine:
      Canvas.Draw(10,130,vMeta);
      vMeta.MMWidth := vMeta.MMWidth * 2;
      Canvas.Draw(150,130,vMeta);
      vMeta.MMHeight := vMeta.MMHeight * 2;
      Canvas.Draw(400,130,vMeta);
    finally
      vMeta.Free;
      ReleaseDC(0,LDC);
    end;
  finally
    vPNG.Free;
  end;
end;

Answer 1

Probably the only way to do this is using the AlphaBlend() Windows function. When you convert the PNG to a 32 bit RGBA file. What you need to do:

1. Load PNG into a 32bit BMP

2. Premultiply the RGB values with the A value in the bitmap (is an prerequisite of the AlphaBlend call)

   for y := 0 to FHeight - 1 do begin
     Src  := BM.ScanLine[ y ];
     Dest := FBitmap.ScanLine[ y ];
     for x := 0 to FWidth - 1 do begin
       A := Src^; Inc( Src );
       Dest^ := MulDiv( Dest^, A, 255 ); Inc( Dest );
       Dest^ := MulDiv( Dest^, A, 255 ); Inc( Dest );
       Dest^ := MulDiv( Dest^, A, 255 ); Inc( Dest );
       Dest^ := A;                       Inc( Dest );
     end;
   end;
   

3. Call AlphaBlend, with a source HDC as the Canvas.Handle of the premultiplied bitmap and a destination handle as the metafile canvas handle.

   BF.BlendOp             := AC_SRC_OVER;
   BF.BlendFlags          := 0;
   BF.SourceConstantAlpha := 255;
   BF.AlphaFormat         := AC_SRC_ALPHA;
   AlphaBlend( 
     Bitmap.Canvas.Handle, X, Y, BM.Width, BM.Height,
     BM.Canvas.Handle, 0, 0, BM.Width, BM.Height, BF );
   

Answer 2

I think you can simply use TransparentBlt function