delphi – Alphablend和TransparentBlt

我想结合两个alpha层,只应用于源层的特定部分.我试过的一种方法是将SourceConstantAlpha设置为$ff(并让函数使用源图层中的alpha通道).

这种作品 – 尽管速度很慢(我想我可以通过使用ScanLines来加快速度),但这部分是因为我无法弄清楚将alpha通道设置为什么.文档表明计算是：

st.Red  = Src.Red   + (1 - Src.Alpha) * Dst.Red

我通过猜测工作尝试了一些不同的值,但我的第一个问题是：我如何计算alpha值？

在阅读了其他几个SO问题后,我遇到了TransparentBlt函数,它可以很好地屏蔽(和快速)但不透明,有没有办法结合
这两个一起调用(可能使用第三层)？

unit MainWnd;

interface

uses
  Windows,Messages,SysUtils,Variants,Classes,Graphics,Controls,Forms,Dialogs,ExtCtrls,ControlsEx;

type
{------------------------------------------------------------------------------}
  TfrmMain = class(TForm)
    PaintBox1: TPaintBox;
    procedure PaintBox1Paint(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  frmMain: TfrmMain;

implementation

{$R *.dfm}

{..............................................................................}
procedure copyToAlpha(const in_bitmap : TBitmap; const in_transparentColor : TColor;
        const in_transparency : integer);
var
  x : integer;
  y : integer;
  p : integer;
begin
  ASSERT(in_bitmap.PixelFormat = pf32bit);

  for x := 0 to in_bitmap.Width - 1 do
  begin
    for y := 0 to in_bitmap.Height - 1 do
    begin
      p := in_bitmap.Canvas.Pixels[x,y];
      if TColor(p) <> in_transparentColor then
      begin
        in_bitmap.Canvas.Pixels[x,y] := p or (in_transparency shl 24);
      end
      else
        in_bitmap.Canvas.Pixels[x,y] := p or ($ff shl 24);
    end;
  end;  
end;

{..............................................................................}
procedure alphaBlendTest(
        const in_target : TCanvas;
        const in_width : integer;
        const in_height : integer);
const
  BARSIZE = 30;
var
  bitmap  : TBitmap;
  r       : TRect;
  blendFn : BLENDFUNCTION;
  ret     : Boolean;
begin
  blendFn.BlendOp             := AC_SRC_OVER;
  blendFn.SourceConstantAlpha := $ff;
  blendFn.BlendFlags          := 0;
  blendFn.alphaFormat         := AC_SRC_ALPHA;

  bitmap := TBitmap.Create;
  try
    bitmap.Width              := in_width;
    bitmap.Height             := in_height;
    bitmap.PixelFormat        := pf32bit;
    bitmap.HandleType         := bmDIB;
    bitmap.TransparentColor   := clFuchsia;
    bitmap.Transparent        := true;  
    bitmap.Canvas.Brush.Color := clFuchsia;
    bitmap.Canvas.FillRect(Bounds(0,in_width,in_height));
    bitmap.Canvas.Brush.Color := clGreen;

    r := Bounds(
        in_width div 2 - (in_width div 3) div 2,(in_width div 3) + 1,BARSIZE          + 1);

   bitmap.Canvas.Rectangle(r);
   // done drawing

   //copyToAlpha(bitmap,clFuchsia,1);
   ret := Windows.TransparentBlt(
        in_target.Handle,in_height,bitmap.Canvas.Handle,clFuchsia);
        //blendFn);

    ASSERT(ret);
  finally
    bitmap.Free;
  end;
end;



{..............................................................................}
procedure TfrmMain.PaintBox1Paint(Sender: TObject);
var
  r: TRect;
begin
  PaintBox1.Canvas.Brush.Color := clBlue;
  r := Bounds(0,PaintBox1.ClientWidth,PaintBox1.ClientHeight);
  PaintBox1.Canvas.FillRect(r);
  PaintBox1.Canvas.Brush.Color := clRed;
  PaintBox1.Canvas.Ellipse(0,PaintBox1.ClientHeight);

  alphaBlendTest(PaintBox1.Canvas,PaintBox1.ClientHeight);
end;

end.