faq37.htm

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   <TITLE>Screen capture the desktop.</TITLE>

   <META NAME="Author" CONTENT="Harold Howe">

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<H3>

Screen capture the desktop.

</H3>

<P>

This FAQ is an extension to the FAQ on how to <A HREF="faq36.htm">create a TCanvas

object for the entire screen</A>. This technique first creates a <TT>TCanvas</TT> object for

the screen. An <TT>Image</TT> control then calls the <TT>TCanvas::CopyRect</TT> function to

copy the screen image into the <TT>TImage</TT> object.

</P>

<pre>

  <font color="navy">// First, get an HDC for the entire screen with the API GetDC function.</font>

  <font color="navy">// Then create a Canvas based on the DC.</font>

  HDC dc <b>=</b> GetDC<b>(</b><font color="blue">0</font><b>)</b><b>;</b>

  Graphics<b>:</b><b>:</b>TCanvas <b>*</b>ScreenCanvas <b>=</b> <b>new</b> Graphics<b>:</b><b>:</b>TCanvas<b>;</b>

  ScreenCanvas<b>-></b>Handle <b>=</b> dc<b>;</b>



  <font color="navy">// Resize the TImage control to match the size of the screen.</font>

  <font color="navy">// Then copy from the screen Canvas to the TImage Canvas.</font>

  Image1<b>-></b>Picture<b>-></b>Bitmap<b>-></b>Width <b>=</b> Screen<b>-></b>Width<b>;</b>

  Image1<b>-></b>Picture<b>-></b>Bitmap<b>-></b>Height<b>=</b> Screen<b>-></b>Height<b>;</b>

  TRect rect <b>=</b> Rect<b>(</b><font color="blue">0</font><b>,</b><font color="blue">0</font><b>,</b>Screen<b>-></b>Width<b>,</b> Screen<b>-></b>Height<b>)</b><b>;</b>

  Image1<b>-></b>Picture<b>-></b>Bitmap<b>-></b>Canvas<b>-></b>CopyRect<b>(</b>rect<b>,</b> ScreenCanvas<b>,</b> rect<b>)</b><b>;</b>



  <font color="navy">// cleanup</font>

  <b>delete</b> ScreenCanvas<b>;</b>

  ReleaseDC<b>(</b>NULL<b>,</b>dc<b>)</b><b>;</b>

</pre>

<P>

You can capture a specific window by obtaining a <TT>DC</TT> for that window instead

of getting a <TT>DC</TT> for the entire screen. This code captures the C++Builder main

window if it's visible.

</P>

<pre>

  <font color="navy">// The BCB IDE window has the window class "TAppBuilder"</font>

  HWND BCBHandle <b>=</b> FindWindow<b>(</b><font color="blue">"TAppBuilder"</font><b>,</b>NULL<b>)</b><b>;</b>

  <b>if</b><b>(</b>BCBHandle<b>)</b>

  <b>{</b>

    <font color="navy">// Create a TCanvas for the BCB IDE window. GetWindowDC returns a DC for the</font>

    <font color="navy">// whole window, including the menu, title bar and border.</font>

    HDC dc <b>=</b> GetWindowDC<b>(</b>BCBHandle<b>)</b><b>;</b>

    Graphics<b>:</b><b>:</b>TCanvas <b>*</b>ScreenCanvas <b>=</b> <b>new</b> Graphics<b>:</b><b>:</b>TCanvas<b>;</b>

    ScreenCanvas<b>-></b>Handle <b>=</b> dc<b>;</b>



    <font color="navy">// need to create a TRect that is the same width and height as the ClipRect</font>

    <font color="navy">// of the Canvas, but whose left and top members are zero.</font>

    TRect rect <b>=</b> ScreenCanvas<b>-></b>ClipRect<b>;</b>

    rect<b>.</b>Right  <b>=</b> rect<b>.</b>Right  <b>-</b> rect<b>.</b>Left<b>;</b>

    rect<b>.</b>Bottom <b>=</b> rect<b>.</b>Bottom <b>-</b> rect<b>.</b>Top<b>;</b>

    rect<b>.</b>Top <b>=</b> rect<b>.</b>Left <b>=</b> <font color="blue">0</font><b>;</b>



    <font color="navy">// resize the bitmap of the Image, and then copy the contents of the</font>

    <font color="navy">// BCB ide canvas to the bitmap's canvas. Then clean up the canvas and DC</font>

    Image1<b>-></b>Picture<b>-></b>Bitmap<b>-></b>Width <b>=</b> rect<b>.</b>Right<b>;</b>

    Image1<b>-></b>Picture<b>-></b>Bitmap<b>-></b>Height<b>=</b> rect<b>.</b>Bottom<b>;</b>

    Image1<b>-></b>Picture<b>-></b>Bitmap<b>-></b>Canvas<b>-></b>CopyRect<b>(</b>rect<b>,</b>

                                              ScreenCanvas<b>,</b>

                                              ScreenCanvas<b>-></b>ClipRect<b>)</b><b>;</b>

    <b>delete</b> ScreenCanvas<b>;</b>

    ReleaseDC<b>(</b>BCBHandle<b>,</b>dc<b>)</b><b>;</b>

  <b>}</b>

</pre>

<P>

<B>Note:</B> The <TT>ClipRect</TT> property of <TT>TCanvas</TT> is recalculated every time

the property is read. This is good, because the clipping rect of a window changes

every time the window is moved or resized. <TT>ClipRect</TT> re-reads the info so the

<TT>Canvas</TT> keeps up with changes to its underlying <TT>HWND</TT> and <TT>HDC</TT> (this

did not play a role in the example code)

</P>

<P>

<B>Note:</B> You must remember to call <TT>ReleaseDC</TT> for the <TT>HDC</TT> handle.

</P>

<P>

<B>Note:</B> Both <TT>GetDC</TT> and <TT>GetWindowDC</TT> take an <TT>HWND</TT> as their

argument. <TT>GetDC</TT> returns a <TT>DC</TT> for the client area of the specified window.

<TT>GetWindowDC</TT> returns a <TT>DC</TT> that can draw on both the non-client and client

areas of the window. In other words, you can paint the title bar and menu of a window if

you obtain the <TT>DC</TT> with <TT>GetWindowDC</TT>.

</P>





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