GDI+ 滑鼠矩形命中測試：PtInRect 與動態矩形陣列

在互動式繪圖應用中，矩形命中測試 是滑鼠選取、拖曳、調整圖形的基礎。本範例示範使用 PtInRect API 搭配動態矩形陣列，實現矩形的建立、繪製與滑鼠懸停檢測。

核心技術點

• PtInRect(Rect, Point)：Windows API，判斷點是否在矩形內
• 動態陣列 array of TRect：運行時增減矩形數量
• Canvas.DrawFocusRect：繪製虛線框回饋 (滑鼠拖曳時)
• GDI+ DrawRectangle：正式繪製矩形

程式碼實作

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormPaint(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}
uses GDIPOBJ, GDIPAPI;

var
  f: Boolean;
  x1, y1, x2, y2: Integer;
  RectArr: array of TRect;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  f := True;
  x1 := X; y1 := Y;
  x2 := X; y2 := Y;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
var
  i: Integer;
begin
  if f then
  begin
    Canvas.DrawFocusRect(Rect(x1, y1, x2, y2));
    x2 := X; y2 := Y;
    Canvas.DrawFocusRect(Rect(x1, y1, x2, y2));
  end
  else
  begin
    Text := '';
    for i := Low(RectArr) to High(RectArr) do
      if PtInRect(RectArr[i], Point(X, Y)) then
        Text := Format('在第 %d 個矩形中', [i + 1]);
  end;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if not f then Exit;
  SetLength(RectArr, Length(RectArr) + 1);
  RectArr[High(RectArr)] := Rect(x1, y1, x2, y2);
  f := False;
  Repaint;
end;

procedure TForm1.FormPaint(Sender: TObject);
var
  g: TGPGraphics;
  p: TGPPen;
  i: Integer;
begin
  g := TGPGraphics.Create(Canvas.Handle);
  p := TGPPen.Create(aclRed, 1);
  g.Clear(aclWhite);
  for i := Low(RectArr) to High(RectArr) do
    g.DrawRectangle(p, MakeRect(RectArr[i]));
  g.Free;
  p.Free;
end;

end.

表單設計 (DFM)

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 179
  ClientWidth = 277
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  Position = poDesktopCenter
  OnMouseDown = FormMouseDown
  OnMouseMove = FormMouseMove
  OnMouseUp = FormMouseUp
  OnPaint = FormPaint
  PixelsPerInch = 96
  TextHeight = 13
end

執行效果

應用場景擴展

• 圖形編輯器：選取、移動、調整大小、刪除矩形
• 區域標註工具：截圖標註、OCR 區域選取
• 遊戲編輯器：碰撞區域、觸發區域定義
• 儀表板設計：拖曳佈局元件

此模式可擴展至橢圓、多邊形、路徑等任意形狀的命中測試 (使用 TGPGraphicsPath + IsVisible)。