怎样托动鼠标在桌面窗口上画一个矩形？

Question

1.以下得到桌面窗口的代码有没有错误？
var
dc:hdc;
canvas:tcanvas;
begin
dc:=getdc(getdesktopwindow);
canvas:=tcanvas.create;
canvas.handle:=dc;...releasedc(dc);
end;2.如果得到了canvas,那么用framerectange等方法确实可以画一个矩形，
但是我想在鼠标按下后托动的过程中，动态的出现一个虚线匡矩形，起点就是鼠标按下的点，终点就是现在鼠标的位置。
但是鼠标是在不断移动的，矩形的尺寸也应该是随鼠标移动变化的，但是单纯的画矩形就会留下一连串矩形，很难看，请问已经画完的矩形怎样消除？请注意，我要在image上画图，并不是单色的背景。谢谢！

Answer 1

canvas.Pen.Mode := pmNotXor;

Answer 2

好像不行啊，我的image里面已经装载了一幅图片，但是一画矩形，就变成白背景了，原来的位图没了阿？！unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, EnJpgGr, ExtCtrls,jpeg;type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Panel1: TPanel;
    Image1: TImage;
    OpenDialog1: TOpenDialog;
    procedure FormCreate(Sender: TObject);
    procedure Image1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Image1MouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure Button1Click(Sender: TObject);
  private
    fselecting:boolean;
    fbeginpt,fendpt:tpoint;
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation{$R *.dfm}procedure TForm1.FormCreate(Sender: TObject);
begin
  
  fselecting:=false;  
end;procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  fselecting:=true;
  fbeginpt:=image1.ScreenToClient(mouse.CursorPos);
  image1.Canvas.Pen.Mode:=pmNotXor;
  //image1.Canvas.Pen.Color:=clblue;
end;procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  fselecting:=false;
  fendpt:=image1.ScreenToClient(mouse.CursorPos);
end;procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  fendpt:=image1.ScreenToClient(mouse.CursorPos);
  if fselecting then
  begin    image1.Canvas.FillRect(image1.Canvas.ClipRect);
    //image1.Canvas.DrawFocusRect(rect(fbeginpt,fendpt));
    image1.Canvas.Rectangle(rect(fbeginpt,fendpt));
  end;
end;procedure TForm1.Button1Click(Sender: TObject);
var
jpg:tjpegimage;
begin
  if opendialog1.Execute then
  begin
    jpg:=tjpegimage.Create;
    jpg.LoadFromFile(opendialog1.FileName);
    image1.Picture.Bitmap.Assign(jpg);
    jpg.Free;
  end;
end;end.

Answer 3

unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, EnJpgGr, ExtCtrls,jpeg;type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Panel1: TPanel;
    Image1: TImage;
    OpenDialog1: TOpenDialog;
    procedure FormCreate(Sender: TObject);
    procedure Image1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Image1MouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure Button1Click(Sender: TObject);
  private
    fselecting:boolean;
    fbeginpt,fendpt:tpoint;
    FRect:TRect;    //全局rect;
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation{$R *.dfm}procedure TForm1.FormCreate(Sender: TObject);
begin
  
  fselecting:=false;  
end;procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  fselecting:=true;
  fbeginpt:=image1.ScreenToClient(mouse.CursorPos);
end;procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  fselecting:=false;
  fendpt:=image1.ScreenToClient(mouse.CursorPos);
end;procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  fendpt:=image1.ScreenToClient(mouse.CursorPos);
  if fselecting then
  begin
    image1.Canvas.Pen.Mode:=pmNotXor;
    Image1.Canvas.Brush.Style := bsClear;
    image1.Canvas.Pen.Color:=clblue;    //image1.Canvas.FillRect(image1.Canvas.ClipRect);//不要
    //image1.Canvas.DrawFocusRect(rect(fbeginpt,fendpt));
    image1.Canvas.Rectangle(Frect);
    FRect:=rect(fbeginpt,fendpt);
    image1.Canvas.Rectangle(Frect);  
  end;
end;procedure TForm1.Button1Click(Sender: TObject);
var
jpg:tjpegimage;
begin
  if opendialog1.Execute then
  begin
    jpg:=tjpegimage.Create;
    jpg.LoadFromFile(opendialog1.FileName);
    image1.Picture.Bitmap.Assign(jpg);
    jpg.Free;
  end;
end;end.

Answer 4

var
  OldRect,NewRect:TRect;
  MouseDown:Boolean;
//在MouseDown事件中：
OldRect.Left:=x;
OldRect.Top:=y;
OldRect.Right:=x;
OldRect.Bottom:=y;
MouseDown:=true;
//在MouseMove事件中：
if MouseDown=true then
begin
  image1.Canvas.DrawFocusRect(OldRect);
  NewRect.Right:=x;
  NewRect.Bottom:=y;
  Image1.Canvas.DrawFocusRect(NewRect);
end;//这样写只能从左上角画到右下角，如果需要全方位的话，自己考虑处理新矩形区域的关系吧！//在mouseUp事件中：
MouseDown:=false;

Answer 5

十分感谢各位的帮助！
还有个小问题，就是画矩形的时候经常出现图片闪动，但是把image.strech:=true就没有了闪动，请问这是怎么回事？我想考屏，用全局鼠标钩子再桌面上选个区域，请问怎样避免闪动？谢谢！

Answer 6

{以下是dll的代码，请问为什么不能实现在桌面canvas上画出矩形？还有个大问题，似乎只有delphi程序的窗口中鼠标按下是513消息，鼠标松开是514,其他窗口都不是，我这个全局鼠标钩子肯定是挂上了，但是在其他窗口，比如资源管理器里面按下鼠标就不会beep,就是说在那里的鼠标按下消息不是513，请问怎样解决？}
unit captureunit;interface
uses
windows,sysutils,messages,global,debug_san,
classes,graphics;
function beginselect(awnd:integer):integer;stdcall;
function endselect():integer;stdcall;implementation
var
selecting:boolean=false;
startpoint,endpoint:tpoint;
hhook:cardinal;
wnd:cardinal;
dc:hdc;
canvas:tcanvas;
frect:trect;
//////////////////////
function hookproc(ncode:smallint;wparam,lparam:integer):integer;stdcall;
begin
  result:=1;
  
  case wparam of
  sm_mouseup:
  begin
    beep;
    selecting:=false;
    endpoint:=PMOUSEHOOKSTRUCT(lparam)^.pt;
    postmessage(wnd,sm_selected,integer(@startpoint),integer(@endpoint));
    
  end;
  sm_mousedown:
  begin
    beep;
    selecting:=true;
    //getcursorpos(endpoint);
    startpoint:=PMOUSEHOOKSTRUCT(lparam)^.pt;  end;
  WM_MOUSEMOVE:
  begin
     //getcursorpos(endpoint);
//     result:=0;     if selecting then
     begin
       endpoint:=PMOUSEHOOKSTRUCT(lparam)^.pt;
       //canvas.Lock;
       Canvas.Rectangle(frect);
       fRect:=rect(startpoint,endpoint);
       Canvas.Rectangle(frect);
       //canvas.TextOut(0,0,'sss');
       //canvas.Unlock;
       beep;
     end;  end;
  else
     result:=0;
  end;
  if ncode<0 then
    result:=callnexthookex(hhook,ncode,wparam,lparam);
end;
//////////////////function beginselect(awnd:integer):integer;stdcall;
begin
  wnd:=awnd;
  //msg2(wnd);
  dc:=getdc(getdesktopwindow);
  canvas.Handle:=dc;
  hhook:=setwindowshookex(WH_MOUSE,@hookproc,hinstance,0);
  result:=hhook;
  canvas.Pen.Mode:=pmNotXor;
  Canvas.Pen.Color:=clblue;
  Canvas.Brush.Style := bsClear;end;function endselect():integer;stdcall;
begin
  releasedc(getdesktopwindow,dc);
  unhookwindowshookex(hhook);
end;
initialization
  canvas:=tcanvas.Create;
finalization
  canvas.Free;  
end.
///////////////////////////////////////////unit global;interfaceuses
windows,sysutils,messages;const
sm_mouseup=514;
sm_mousedown=513;sm_selected=wm_user+1000;c_capturedll='capture.dll';
c_beginselect='beginselect';
c_endselect='endselect';implementationend.

Answer 7

library capture;uses
  SysUtils,
  captureunit in 'captureunit.pas';{$R *.res}exports
  beginselect,
  endselect;
end.

Answer 8

解决闪动问题的办法是：在窗体的oncreate事件中，将Image1.parent.DoubleBuffered:=true;设置图像双缓冲就可以了！