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BitBlt/StretchBlt color conversion

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:

HBITMAP Bitmap;
int ScreenWidth, ScreenHeight, Width, Height, Scale;
HDC DesktopDC, CaptureDC;
BITMAPINFO BitmapInfo;
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);

SelectObject(CaptureDC, Bitmap);
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.
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Roshan Davis
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The problem is not in StreachBlt, or in BitBlt. The lag is due to the bit conversion loop.
HBITMAP Bitmap;
     int ScreenWidth, ScreenHeight, Width, Height, Scale;
     HDC DesktopDC, CaptureDC;
     BITMAPINFO BitmapInfo;
     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);
     HBITMAP hCurrentBMP = (HBITMAP)GetCurrentObject(DesktopDC, OBJ_BITMAP);
     Bitmap = CreateDIBSection(DesktopDC, &BitmapInfo, DIB_RGB_COLORS, (void **)&Data, 0, 0);
     DWORD     dw1 = GetTickCount();
     ::GetDIBits(DesktopDC, hCurrentBMP, 0, BitmapInfo.bmiHeader.biHeight, Data, &BitmapInfo, DIB_RGB_COLORS);
     TRACE("Diff = %d \n", GetTickCount() - dw1);
     SelectObject(CaptureDC, Bitmap);

     I think this will get some more performance...
     These things can be apply to avoid the freezing,
     1. U can do ur own converion method and use SetDIBits .. So in that loop u can Peek and Dispatch Message
     2. Do these things in a seperate thread

GOOD LUCK
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ASKER

Will GetDIBits do the same as BitBlt?
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chensu
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Have you tried other computers? This could be a hardware design problem. During a Blt operation, the bus is very busy.
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ASKER

Yes, I have tried on 4 Windows XP computers and 2 Windows 98 computers; all the windows XPs have this problem.
ASKER CERTIFIED SOLUTION
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Roshan Davis
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