GDI+ 實心筆刷與透明度控制：TGPSolidBrush、ColorRefToARGB

TGPSolidBrush 是最基礎的填充筆刷，支援完整的 ARGB 32 位色彩 (含 Alpha 透明通道)。結合 ColorRefToARGB 與 MakeColor 函數，可靈活實現 Delphi TColor 與 GDI+ 顏色格式的雙向轉換與透明度控制。

核心函數

• ColorRefToARGB(ColorRef): TGPColor：Delphi TColor (BGR) → GDI+ ARGB
• MakeColor(A, R, G, B): TGPColor：組合 ARGB 元件為 32 位色值
• GetRed/Green/Blue/Alpha(Color): Byte：提取色彩元件

程式碼實作

右側色彩列表選取基礎色，底部軌跡條 (0-255) 即時調整 Alpha 透明度：

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.

表單設計 (DFM)

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

執行效果

拖動軌跡條可見矩形從完全不透明 (Alpha=255) 漸變至完全透明 (Alpha=0)。掌握 ARGB 色彩模型是 GDI+ 進階繪圖 (如疊加混合、漸變透明、玻璃效果) 的基礎。