如何在TMetaFileCanvas上透明地绘制PNG

Question

我需要在MetaFileCanvas上透明地绘制PNG.当我绘制到位图(我在屏幕上显示时使用)时这很好用,但是对于打印我需要一个MetaFile.我怎样才能做到这一点？

这是一些演示此问题的代码.将附加的PNG放在某处并更新代码中的路径.

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

可能做到这一点的唯一方法是使用AlphaBlend () 窗口函数。当您将 PNG 转换为 32 位 RGBA 文件时。你需要做什么：

1. 将 PNG 加载到 32 位 BMP 中

2. 将 RGB 值与位图中的 A 值预乘（是 AlphaBlend 调用的先决条件）

             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. 调用 Alphablend，使用源 HDC 作为预乘位图的 Canvas.Handle 和目标句柄作为图元文件画布句柄。

   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 );