Delphi屏幕截圖的實現

    首先要獲得設備環境的句柄，可以通過GetDC函數來獲得，對於這個函數，MSDN上是這樣說明的

The GetDC function retrieves a handle to a device context (DC) for the client area of a specified window or for the entire screen. You can use the returned handle in subsequent GDI functions to draw in the DC. The device context is an opaque data structure, whose values are used internally by GDI.
The GetDCEx function is an extension to GetDC, which gives an application more control over how and whether clipping occurs in the client area.
Syntax

HDC GetDC(
__in HWND hWnd
);
Parameters
hWnd [in]
A handle to the window whose DC is to be retrieved. If this value is NULL, GetDC retrieves the DC for the entire screen.
Return value
If the function succeeds, the return value is a handle to the DC for the specified window's client area.
If the function fails, the return value is NULL.

1.申明一全變量存儲GetDC方法得到的句柄，HDC類型

var
  dc:HDC;

2.在適當時候調用GetDC，將得到的句柄賦值給變量dc,並將得到的句柄賦值給Canvas的句柄（fullCanvas是變量）

  dc:=GetDC(0);
  fullCanvas:=TCanvas.Create;
  fullCanvas.Handle:=dc;

3.然后就是對fullCanvas進行操作，這里也要用到CopyRect函數

procedure CopyRect(const Dest: TRect; Canvas: TCanvas; const Source: TRect);

Copies part of an image from another canvas into the canvas.Use CopyRect to transfer part of the image on another canvas to the image of the TCanvas object.

Dest specifies the rectangle on the canvas where the source image will be copied. The Canvas parameter specifies the canvas with the source image.

Source specifies a rectangle bounding the portion of the source canvas that will be copied.The portion of the source canvas is copied using the mode specified by CopyMode. 

一句話就是，將Canvas中的Source大小的區域拷貝到目標canvas中的Dest區域中，如果這二個面積不相等，會有縮放效果，如果不想有此效果，這二個區域的長寬相等

      if SavePictureDialog1.Execute then
      begin
        bm:=TBitmap.Create;
        try
          bm.Width:=abs(x2-x1);
          bm.Height:=abs(y2-y1);
          bm.Canvas.CopyRect(Rect(0,0,bm.Width,bm.Height),fullCanvas,Rect(x1,y1,x2,y2));
          bm.SaveToFile(SavePictureDialog1.FileName+'.bmp');
        finally
          bm.Free;
        end;

其中bm.width:=ABS(x2-x1),bm.heigth:=ABS(y2-y1),如果要全屏截圖，則將二個RECT都設置為Rect(0,0,Screen.Width,Screen.heigth)

程序運行如下

要實現有背景的黑層行是這樣操作的

(1).設置窗體 AlphaBlend為True,AlphaBlendVaue為150,BorderStyle:=bsNone;

(2).在窗體創建時寫如下語句

      SetWindowPos(0,HWND_TOP,0,0,Screen.Width,Screen.Height,SWP_DRAWFRAME);
      ClientHeight:=Screen.Height;
      ClientWidth:=Screen.Width;