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.

var
  dc:HDC;

2.在适当时候调用GetDC，將得到的句柄赋值给变量dc,并將得到的句柄赋值给Canvas的句柄（fullCanvas是变量）

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

3.然后就是对fullCanvas进行操作，这里也要用到CopyRect函数

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. 

一句话就是，将Canvas中的Source大小的区域拷贝到目标canvas中的Dest區域中，如果这二个面积不相等，会有缩放效果，如果不想有此效果，这二个区域的长宽相等

 1       if SavePictureDialog1.Execute then
 2       begin
 3         bm:=TBitmap.Create;
 4         try
 5           bm.Width:=abs(x2-x1);
 6           bm.Height:=abs(y2-y1);
 7           bm.Canvas.CopyRect(Rect(0,0,bm.Width,bm.Height),fullCanvas,Rect(x1,y1,x2,y2));
 8           bm.SaveToFile(SavePictureDialog1.FileName+'.bmp');
 9         finally
10           bm.Free;
11         end;

其中bm.width:=ABS(x2-x1),bm.heigth:=ABS(y2-y1),如果要全屏截图，则将二个RECT都设置为Rect(0,0,Screen.Width,Screen.heigth)

程序运行如下

要实现有背景的黑层行是这样操作的

(1).设置窗体 AlphaBlend为True,AlphaBlendVaue为150,BorderStyle:=bsNone;

(2).在窗体创建时写如下语句

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