再學 GDI+[17]: FillRectangle、ColorRefToARGB、TGPSolidBrush 和顏色透明度

本例效果圖:

　　代碼文件:unit Unit1;
interface
uses
　Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
　Dialogs, ComCtrls, StdCtrls, ExtCtrls;
type
　TForm1 = class(TForm)
　　ColorListBox1: TColorListBox;
　　TrackBar1: TTrackBar;
　　procedure FormCreate(Sender: TObject);
　　procedure FormPaint(Sender: TObject);
　　procedure ColorListBox1Click(Sender: TObject);
　　procedure TrackBar1Change(Sender: TObject);
　end;
var
　Form1: TForm1;
implementation
{$R *.dfm}
uses GDIPOBJ,GDIPAPI;
procedure TForm1.FormCreate(Sender: TObject);
begin
　Self.Position := poDesktopCenter;
　ColorListBox1.Align := alRight;
　TrackBar1.ShowSelRange := False;
　TrackBar1.PageSize := 20;
　TrackBar1.Min := 0;
　TrackBar1.Max := 255;
　TrackBar1.Position := TrackBar1.Max;
end;
procedure TForm1.FormPaint(Sender: TObject);
var
　g: TGPGraphics;
　r: TGPRect;
　sb: TGPSolidBrush;
　clr: TGPColor;
　Alpha: Byte;
begin
　g := TGPGraphics.Create(Canvas.Handle);
　g.Clear(MakeColor(255,255,255));
　r := MakeRect(20, 20, 200, 150);
　clr := ColorRefToARGB(ColorListBox1.Selected);
　Alpha := TrackBar1.Position;
　clr := MakeColor(Alpha, GetRed(clr), GetGreen(clr), GetBlue(clr));
　sb := TGPSolidBrush.Create(clr);
　g.FillRectangle(sb, r);
　sb.Free;
　g.Free;
end;
procedure TForm1.ColorListBox1Click(Sender: TObject);
begin
　Repaint;
end;
procedure TForm1.TrackBar1Change(Sender: TObject);
begin
　Repaint;
　Text := Format('透明度: %d', [TrackBar1.Position]);
end;
end.窗體文件:object Form1: TForm1
　Left = 0
　Top = 0
　Caption = 'Form1'
　ClIEntHeight = 206
　ClIEntWidth = 339
　Color = clBtnFace
　Font.Charset = DEFAULT_CHARSET
　Font.Color = clWindowText
　Font.Height = -11
　Font.Name = 'Tahoma'
　Font.Style = []
　OldCreateOrder = False
　OnCreate = FormCreate
　OnPaint = FormPaint
　PixelsPerInch = 96
　TextHeight = 13
　object ColorListBox1: TColorListBox
　　Left = 248
　　Top = 0
　　Width = 91
　　Height = 206
　　Align = alRight
　　ItemHeight = 16
　　TabOrder = 0
　　OnClick = ColorListBox1Click
　end
　object TrackBar1: TTrackBar
　　Left = 0
　　Top = 184
　　Width = 249
　　Height = 45
　　TabOrder = 1
　　OnChange = TrackBar1Change
　end
end