How to draw a TBitmap32 transparently on a TBitmap?

Question

I am trying to draw a source image (GR32 TBitmap32) that contains some fully transparent and also partially transparent pixels on a normal (TBitmap) image while preserving the transparency in the source image.

I use this:

BMP32.DrawTo(BMP.Canvas.Handle, 0, 0);

but the image is not drawn transparently.

---

Code:
Everything is pretty basic: I have a background bitmap (Bkg) in which I load, at application start up, an image. Then in procedure Apply I load a second image from disk (the one with transparency) and I draw the it over the background.

var Bkg: TBitmap;

procedure TfrmPhoto.FormCreate(Sender: TObject);
begin
 Bkg:= LoadGraphEx(GetAppDir+ 'bkg.bmp'); { Load bitmap from file }
 {
 Bkg.Transparent:= TRUE;
 Bkg.PixelFormat:= pf32bit;
 Bkg.TransparentColor:= clPink;
 }
end;


procedure TfrmPhoto.Apply2;
VAR
   Loader: TBitmap;
   BMP32: tbitmap32;
begin
 BMP32:= TBitmap32.Create;
 TRY

  Loader:= TBitmap.Create;
  TRY
   Loader.LoadFromFile('c:\Transparent.BMP');
   BMP32.Assign(Loader);
  FINALLY
   FreeAndNil(Loader);
  END;

  { Mix images }
  BMP32.DrawTo(Bkg.Canvas.Handle, 0, 0);    <----- The problem is here

  imgPreview.Picture.Assign(Bkg);    { This is a TImage where I see the result }
 FINALLY
  FreeAndNil(BMP32);
 END;
end;

Answer 1

It worked (but with 2 disadvantages) with TransparentBlt().

1. I am not happy with the way TransparentBlt handles the semitransparent pixels (the edge of the rotate image). After merging the src image into the background the edges look bad.

2. In order to use TransparentBlt, I had to define a color (clPink) in the source image as transparent. This means that if the source image has some pink in it, the result will look really nasty (it will be treated as transparent). Let's pray for non-pink images!

If you find a way to transfer the image (while preserving transparency) directly from Bitmap32 into the background please post and I will accept your answer!

A solution (still a hack) I see here is to process everything in a TBitmap32 and then 'export' the final result as TBitmap. I will try this tomorrow.

---

Update:
Solution:

This is how to merge two TBitmap32 images while preserving transparency:

 Dst.CombineMode:= cmBlend;
 Dst.DrawMode:= dmBlend;
 Src.Draw(0, 0, Dst);  

Dst and Src are TBitmap32 images. It doesn't work with TBitmap.