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POPerforming full screen grab in windows
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copied!<p>I am working on an idea that involves getting a full capture of the screen including windows and apps, analyzing it, and then drawing items back onto the screen, as an overlay. </p> <p>I want to learn image processing techniques and I could get lots of data to work with if I can directly access the Windows screen. I could use this to build automation tools the likes of which have never been seen before. More on that later.</p> <p>I have full screen capture working for the most part. </p> <pre><code>HWND hwind = GetDesktopWindow(); HDC hdc = GetDC(hwind); int resx = GetSystemMetrics(SM_CXSCREEN); int resy = GetSystemMetrics(SM_CYSCREEN); int BitsPerPixel = GetDeviceCaps(hdc,BITSPIXEL); HDC hdc2 = CreateCompatibleDC(hdc); BITMAPINFO info; info.bmiHeader.biSize = sizeof(BITMAPINFOHEADER); info.bmiHeader.biWidth = resx; info.bmiHeader.biHeight = resy; info.bmiHeader.biPlanes = 1; info.bmiHeader.biBitCount = BitsPerPixel; info.bmiHeader.biCompression = BI_RGB; void *data; hbitmap = CreateDIBSection(hdc2,&info,DIB_RGB_COLORS,(void**)&data,0,0); SelectObject(hdc2,hbitmap); </code></pre> <p>Once this is done, I can call this repeatedly: </p> <pre><code>BitBlt(hdc2,0,0,resx,resy,hdc,0,0,SRCCOPY); </code></pre> <p>The cleanup code (I have no idea if this is correct): </p> <pre><code>DeleteObject(hbitmap); ReleaseDC(hwind,hdc); if (hdc2) { DeleteDC(hdc2); } </code></pre> <p>Every time BitBlt is called it grabs the screen and saves it in memory I can access thru <code>data</code>. </p> <p>Performance is somewhat satisfactory. BitBlt executes in 50 milliseconds (sometimes as low as 33ms) at 1920x1200x32.</p> <p>What surprises me is that when I switch display mode to 16 bit, 1920x1200x16, either through my graphics settings beforehand, or by using <code>ChangeDisplaySettings</code>, I get a massively improved screen grab time between 1ms and 2ms, which cannot be explained by the factor of two reduction in bit-depth. Using <code>CreateDIBSection</code> (as above) offers a significant speed up when in 16-bit mode, compared to if I set up with <code>CreateCompatibleBitmap</code> (6-7ms/f). </p> <p>Does anybody know why dropping to 16bit causes such a speed increase? Is there any hope for me to grab 32bit at such speeds? if not for the color depth, but for not forcing a change of screen buffer modes and the awful flickering. </p>

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