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.

Answer 2

As already suggested by yourself, I would recommend performing blending operations entirely in the TBitmap32 domain! The reason for this is the fact that it offers more choices in regards of blending and it uses MMX/SSE/SSE2 where available (on modern machines always the case). With this one can typically get a notable performance boost in contrast to letting GDI perform the blending.

In particular this is not a good idea as GDI typically does not make use of any SIMD opcodes (MMX/SSE).

It also depends on how much each part changes. For example, typically the background is rather static and need to be transfered into a TBitmpa32 only occassionally on changes (e.g. on VCL style or UI theme changes). With each transfer (GR32 <-> TBitmap and alike) an implict conversion (DIB <-> DDB) might happen, which makes the entire blending an O(n²) operation. So you'd better transfer (with an implicit conversion) to TBitmap32 just once and then perform the blending entirely in the GR32 domain.

On the contrary, if the TBitmap32 is rather static and the TBitmap (or background) changes a lot, it might probably be better to leave GR32 alone and copy the TBitmap32 to TBitmap and perform the blending with GDI. Despite the fact that the GDI blending is slightly slower, you avoid the additional copy. Otherwise your bottleneck will likely be the memory throughput, while processing speed never can touch the limits.

Answer 3

It is dmBlend DrawMode of TBitmap32 to turn on the transparency, but it only works between two TBitmap32 bitmaps. Do not use TBitmap32 with transparent images to draw directly on HDC, Canvas or TBitmap. Use dmBlend and DrawTo or BlockTransfer() to draw on another TBitmap32. For example, to draw transparently on a TBitmap, create an intermediary cache TBitmap32:

1. Copy the image from TBitmap to the cache TBitmap32;
2. Apply your transparent image to the cache TBitmap32 using DrawTo or BlockTransfer(), avoid using Canvas or HDC to mix two images because they lose alpha layer information;
3. Copy the image back from the cache TBitmap32 to your TBitmap.