BitBlt/StretchBlt color conversion
Posted on 2002-03-07
In my program, it is necessary to convert the screen capture image from the screen's bit depth (usually 32bpp) to a 24bit format. With this code, I get about 0.5 to 1.0 seconds of lag (system freeze; the mouse cannot be moved) during StretchBlt() when converting from my desktop depth of 32bits to 24bits. As far as I can tell, this could perhaps be windows XP dependant?
Here's the code:
int ScreenWidth, ScreenHeight, Width, Height, Scale;
HDC DesktopDC, CaptureDC;
unsigned char *Data;
Scale = 50;
DesktopDC = GetDC(GetDesktopWindow());
ScreenWidth = GetDeviceCaps(DesktopDC, HORZRES);
ScreenHeight = GetDeviceCaps(DesktopDC, VERTRES);
Width = (ScreenWidth * Scale) / 100;
Height = (ScreenHeight * Scale) / 100;
memset(&BitmapInfo.bmiHeader, 0, sizeof(BitmapInfo.bmiHeader));
BitmapInfo.bmiHeader.biWidth = Width;
BitmapInfo.bmiHeader.biHeight = Height;
BitmapInfo.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
BitmapInfo.bmiHeader.biPlanes = 1;
BitmapInfo.bmiHeader.biBitCount = 24;
BitmapInfo.bmiHeader.biCompression = BI_RGB;
CaptureDC = CreateCompatibleDC(DesktopDC);
Bitmap = CreateDIBSection(DesktopDC, &BitmapInfo, DIB_RGB_COLORS, (void **)&Data, 0, 0);
StretchBlt(CaptureDC, 0, 0, Width, Height, DesktopDC, 0, 0, ScreenWidth, ScreenHeight, SRCCOPY); // 1 second freeze occurs here...
Any suggestions? It seems as if StrechBlt() is freezing up due to having to convert from 32bpp to 24bpp. I have the exact same problem when doing BitBlt() from the screen to a bitmap of similar size.