WinAPI: AlphaBlend - 图像透明度

AlphaBlend(

  DC: HDC;                  {目标设备环境句柄}

  p2, p3, p4, p5: Integer;  {目标位置与宽高}

  DC6: HDC;                 {源设备环境句柄}

  p7, p8, p9, p10: Integer; {源位置与宽高}

  p11: TBlendFunction       {合成方式的数据结构, 见下表}

): BOOL;



//TBlendFunction 结构是 _BLENDFUNCTION 的重命名:

_BLENDFUNCTION = packed record

  BlendOp: BYTE;             {本例使用了: AC_SRC_OVER = 0}

  BlendFlags: BYTE;          {必须为 0}

  SourceConstantAlpha: BYTE; {透明度: 0..255}

  AlphaFormat: BYTE;         {通常为 0，如果源位图为32位真彩色，可为 AC_SRC_ALPHA }

end;


 
   

本例效果图:

---

代码文件:

---

unit Unit1;



interface



uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls, ExtCtrls, ComCtrls;



type

  TForm1 = class(TForm)

    TrackBar1: TTrackBar;

    procedure FormCreate(Sender: TObject);

    procedure FormPaint(Sender: TObject);

    procedure TrackBar1Change(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

  end;



var

  Form1: TForm1;



implementation



{$R *.dfm}



var

  Bit,BitBack: TBitmap;

  Transparency: Byte = 255; {透明度}



procedure TForm1.FormCreate(Sender: TObject);

begin

  {设定窗体背景}

  BitBack := TBitmap.Create;

  BitBack.LoadFromFile('c:\temp\Back.bmp');

  Brush.Bitmap := BitBack;



  {调入图片}

  Bit := TBitmap.Create;

  Bit.LoadFromFile('c:\temp\test.bmp');



  {给 TrackBar1 初始化}

  TrackBar1.Max := 255;

  TrackBar1.Min := 0;

  TrackBar1.ShowSelRange := False;

  TrackBar1.Position := TrackBar1.Max;

end;



procedure TForm1.FormPaint(Sender: TObject);

var

  Blend: TBlendFunction; {定义 AlphaBlend 要使用的 TBlendFunction 结构}

begin

  {给 TBlendFunction 结构赋值}

  Blend.BlendOp := AC_SRC_OVER;

  Blend.BlendFlags := 0;

  Blend.AlphaFormat := 0;

  Blend.SourceConstantAlpha := Transparency;



  Windows.AlphaBlend(Canvas.Handle, {因 VCL 有同名属性, 所以指定了是来自 Windows 单元}

                     10,

                     10,

                     Bit.Width + 10,

                     Bit.Height + 10,

                     Bit.Canvas.Handle,

                     0,

                     0,

                     Bit.Width,

                     Bit.Height,

                     Blend

                     );

end;



procedure TForm1.TrackBar1Change(Sender: TObject);

begin

  Transparency := TrackBar1.Position;

  Text := IntToStr(Transparency);

  Self.Repaint; {命令重绘}

end;



procedure TForm1.FormDestroy(Sender: TObject);

begin

  Bit.Free;

  BitBack.Free;

end;



end.


 
   

窗体设计:

---

object Form1: TForm1

  Left = 352

  Top = 238

  Caption = 'Form1'

  ClientHeight = 209

  ClientWidth = 211

  Color = clBtnFace

  Font.Charset = DEFAULT_CHARSET

  Font.Color = clWindowText

  Font.Height = -11

  Font.Name = 'Tahoma'

  Font.Style = []

  OldCreateOrder = False

  Position = poDesigned

  OnCreate = FormCreate

  OnDestroy = FormDestroy

  OnPaint = FormPaint

  PixelsPerInch = 96

  TextHeight = 13

  object TrackBar1: TTrackBar

    Left = 0

    Top = 186

    Width = 201

    Height = 24

    TabOrder = 0

    OnChange = TrackBar1Change

  end

end