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procedure AlphaImgs(Image1, Image2: TGraphic; ResultBitmap: TBitmap; AlphaValue: Byte);
type pixel = Array [0..2] of byte;
var bmpImg1, bmpImg2: TBitmap;
    x,y,h,w: integer;
    p1, p2: ^pixel;
begin
  bmpImg1:=TBitmap.Create;
  bmpImg2:=TBitmap.Create;
  try
    bmpImg1.Height:=Image1.Height;
    bmpImg2.Height:=Image2.Height;
    bmpImg1.Width:=Image1.Width;
    bmpImg2.Width:=Image2.Width;
    bmpImg1.PixelFormat:=pf24bit;
    bmpImg2.PixelFormat:=pf24bit;
    bmpImg1.Canvas.Draw(0,0,Image1);
    bmpImg2.Canvas.Draw(0,0,Image2);
    h := Min(bmpImg1.Height, bmpImg2.height);
    w := Min(bmpImg1.Width, bmpImg2.Width);
    for y :=0 to h-1 do begin
      p1 := bmpImg1.ScanLine[y];
      p2 := bmpImg2.ScanLine[y];
      for x:= 0 to w-1 do begin
        p1^[0] := ((p1^[0]*(AlphaValue+1) + (p2^[0]*(256-AlphaValue))) shr 8);
        p1^[1] := ((p1^[1]*(AlphaValue+1) + (p2^[1]*(256-AlphaValue))) shr 8);
        p1^[2] := ((p1^[2]*(AlphaValue+1) + (p2^[2]*(256-AlphaValue))) shr 8);
        inc(p1);
        inc(p2);
      end;
    end;
    ResultBitmap.Assign(bmpImg1);
  finally
    bmpImg1.Free;
    bmpImg2.Free;
  end;
end;					

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procedure TfrmMain.btnblendClick(Sender: TObject);
var bmpresult:TBitmap;
begin
  ImageResult.Picture.Assign(nil);
  bmpresult:=TBitmap.Create;
  AlphaImgs(Image1.Picture.Graphic,Image2.Picture.Graphic,bmpresult,Byte(sbAlpha.Position));
  ImageResult.Picture.Assign(bmpresult);
  bmpresult.Free;
end;					

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if image1.Height < image2.Height then
  minHeight := image1.Height
else
  minHeight := image2.Height;

for y := 0 to minHeight-1 do
...					

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  p1^[0] := ((p1^[0]*(AlphaValue+1) + (p2^[0]*(256-AlphaValue))) shr 8);
  p1^[1] := ((p1^[1]*(AlphaValue+1) + (p2^[1]*(256-AlphaValue))) shr 8);
  p1^[2] := ((p1^[2]*(AlphaValue+1) + (p2^[2]*(256-AlphaValue))) shr 8);