How to Use Transparent Brush on Paint 3d

Problem Definition

I am trying to create a custom bitmap brush with transparency but it doesn't seem to be working as expected. If you look at this example. Add the code and hook up the paint, create and destroy events.

          type   TForm3 = class(TForm)     procedure FormDestroy(Sender: TObject);     procedure FormCreate(Sender: TObject);     procedure FormPaint(Sender: TObject);   private     FBitmap: TBitmap;   end;  // Implementation  function CreateBlockBitmap(const APenColor: TColor): TBitmap; begin   Result := TBitmap.Create;   Result.Transparent := True;    Result.Canvas.Brush.Color := clWhite;   Result.Canvas.Brush.Style := bsClear;   Result.PixelFormat := pf32bit;   Result.SetSize(20, 20);   Result.Canvas.Brush.Color := APenColor;   Result.Canvas.Brush.Style := bsSolid;   Result.Canvas.FillRect(Rect(0,0,10,10)); end;  procedure TForm3.FormDestroy(Sender: TObject); begin   FBitmap.Free; end;  procedure TForm3.FormCreate(Sender: TObject); begin   FBitmap := CreateBlockBitmap(clRed); end;  procedure TForm3.FormPaint(Sender: TObject); var   colNum: Integer;   rowNum: Integer; begin   // Paint the rectangle using the brush   Canvas.Pen.Color := clGreen;   Canvas.Brush.Bitmap := FBitmap; // This is using bitmap   Canvas.Rectangle(50, 50, 250, 250);   // Draw the block using Canvas.Draw   for rowNum := 0 to 9 do     for colNum := 0 to 9 do       Canvas.Draw(350 + rowNum * 20, 50 + colNum * 20, FBitmap); end;                  

This code produces two painted blocks. The left one is painted using a bitmap brush and the right hand side one is painted using a number of Canvas.Draw calls.

I need the brush to be painted with transparency similar to what would happen if you used a hatch brush. This SO answer seems to indicate that it's possible:

How can I draw a patternBrush with transparent backround (GDI)?

What I have tried

1) I tried using a solid background color instead of using bsClear. This just makes the background white.

                      Result.Canvas.Brush.Color := clWhite;   Result.Canvas.Brush.Style := bsSolid;                  

If I use clFuchsia then the color is Fuchsia. I also tried painting the background clFuchsia and then setting the TransparentColor to clFuchsia. The Canvas.Draw option paints with transparency and the brush doesn't.

2) I tried setting the alpha channel directly with the following code:

          procedure SetAlphaBitmap(const Dest: TBitmap;Color : TColor;Alpha:Byte); type   TRGB32 = record     B, G, R, A: byte;   end;   PRGBArray32 = ^TRGBArray32;   TRGBArray32 = array[0..0] of TRGB32; var   x, y:    integer;   Line, Delta: integer;   ColorRGB : TColor; begin   if Dest.PixelFormat<>pf32bit then  exit;    ColorRGB := ColorToRGB(Color);   Line  := integer(Dest.ScanLine[0]);   Delta := integer(Dest.ScanLine[1]) - Line;   for y := 0 to Dest.Height - 1 do   begin     for x := 0 to Dest.Width - 1 do       if TColor(RGB(PRGBArray32(Line)[x].R, PRGBArray32(Line)[x].G, PRGBArray32(Line)[x].B)) = ColorRGB then         PRGBArray32(Line)[x].A := Alpha;     Inc(Line, Delta);   end; end;                  

And then calling this routine immediately after the rectangle has been painted using the background color.

                      Result.Canvas.Brush.Style := bsSolid;   Result.Canvas.FillRect(Rect(0,0,10,10));   SetAlphaBitmap(Result, clBlack, 0); // Set the alpha channel end;                  

I know that the alpha channel is working because if I pass in an alpha value of 255 then it shows up in black in the Canvas.Draw too.

                      SetAlphaBitmap(Result, clBlack, 255);                  

3) I tried testing by creating a pattern brush and assigning that instead of the bitmap. That produces exactly the same results. FBrush is an HBRUSH.

                      FBrush := CreatePatternBrush(FBitmap.Handle);                  

And the setting the brush like this:

                      Canvas.Brush.Handle := FBrush;                  

4) I tried calling SetBkMode as indicated in the SO answer above. That made no difference at all.

                      Canvas.Pen.Color := clGreen;   Canvas.Brush.Bitmap := FBitmap;    SetBkMode(Canvas.Handle, TRANSPARENT); // This doesn't make a difference   Canvas.Rectangle(50, 50, 250, 250);                  

Edit

5) I just tested with a Monochrome bitmap and it has the same problem. The image is painted with a white background and black foreground for the brush and transparent for the Canvas.Draw.

          function CreateMonochromeBitmap: TBitmap; begin   Result := TBitmap.Create;   Result.Transparent := True;   Result.Canvas.Brush.Color := clWhite;   Result.Canvas.Brush.Style := bsSolid;   Result.PixelFormat := pf1bit;   Result.SetSize(20, 20);   Result.Canvas.Brush.Color := clBlack;   Result.Canvas.Brush.Style := bsSolid;   Result.Canvas.FillRect(Rect(0,0,10,10)); end;                  

And in the constructor:

          FBitmap := CreateMonochromeBitmap; FBrush := CreatePatternBrush(FBitmap.Handle);                  

In the paint we set the handle rather than the bitmap property.

          Canvas.Brush.Handle := FBrush;                  

How to Use Transparent Brush on Paint 3d

Source: https://stackoverflow.com/questions/31467048/creating-a-transparent-custom-bitmap-brush

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