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davidcoleman

StretchDIBits funny colors

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I am using StretchDIBits to paint a graph in a dialog but instead black and windowColor I get green and windowColor!!! Function looks like this CDC *pDC = m_container->GetDC(); pDC->SetStretchBltMode(HALFTONE); // tried other also BITMAPINFOHEADER bmiHeader; bmiHeader.biWidth=320; bmiHeader.biHeight=240; bmiHeader.biPlanes=1; bmiHeader.biBitCount=8; bmiHeader.biCompression=BI_RGB; bmiHeader.biSizeImage=320*240; byte MyBWImage[320*240]; for (i=1;i<320*100;i++) MyBWImage[i]=200; //graph drawing // depending on above number I get different colors: "200" green, "3" pale yellow... what are these colors and how do I set some normal ones? int aaa=StretchDIBits (pDC->GetSafeHdc(), 0,0,320,240, 0,0,320,240, MyBWImage, reinterpret_cast<LPBITMAPINFO>( &bmiHeader), DIB_RGB_COLORS, SRCCOPY); m_container->ReleaseDC();

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An 8 bit BMP has a palette associated with it, palette index 200 will be green, index 3 is pale yellow, etc. The BITMAPINFO struct has a palette in it, and you're supposed to pass one to StretchDIBits. However, because of the cast, you're reading into random memory - StretchDIBits expects an array of 256 RGBQUADs after the BITMAPINFOHEADER, so it tries to read that and happens to read green and pale yellow colours.

Use something like the following instead:

BYTE* pbyBuffer = new BYTE[sizeof(BITMAPINFO) + sizeof(RGBQUAD)*256];
BITMAPINFO* pBitmap = (BITMAPINFO*)pbyBuffer;

// Setup header
pBitmap->bmiHeader.biWidth=320;
pBitmap->bmiHeader.biHeight=240;
pBitmap->bmiHeader.biPlanes=1;
pBitmap->bmiHeader.biBitCount=8;
pBitmap->bmiHeader.biCompression=BI_RGB;
pBitmap->bmiHeader.biSizeImage=320*240;

// Zero out palette
memset(pBitmap->bmiColors, 0, sizeof(RGBQUAD)*256);

// Set entry 200 to red
pBitmap->bmiColors[200].rgbRed = 255;

BYTE MyBWImage[320*240];
for (i=1;i<320*100;i++)
MyBWImage[i]=200;

int aaa=StretchDIBits (pDC->GetSafeHdc(),
0,0,320,240,
0,0,320,240,
MyBWImage,
pBitmap,
DIB_RGB_COLORS,
SRCCOPY);

delete[] pbyBuffer;

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