Question

I have got a trouble using the StretchDIBits function.
I want to draw a bitmap made from a buffer. However, the colors I define in the buffer are different from the result on screen.
I have read the documentation and I played with the biCompression (BI_RGB and BI_BITFIELDS) and biClrUsed (0 / 3) parameters of the BITMAPINFOHEADER. I can see some differences depending on their values, but the result is still different from what I am expecting.

Here is the code I am using (it can be inserted in the OnDraw method of a template SDI project to demonstrate the problem).

void CTestStretchDIBitsView::OnDraw(CDC* /*pDC*/)
{
    ...

    CClientDC dc(this);
    CRect rect;
    GetClientRect(&rect);

    DWORD* pBuffer = new DWORD[500 * 500];
    memset(pBuffer, RGB(255, 255, 0), 500 * 500 * sizeof(DWORD));

    LPBITMAPINFO pBmpInfo = (LPBITMAPINFO) new BYTE[sizeof(BITMAPINFOHEADER) + 256 * sizeof(RGBQUAD)];

    pBmpInfo->bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    pBmpInfo->bmiHeader.biWidth = 500;
    pBmpInfo->bmiHeader.biHeight = 500;
    pBmpInfo->bmiHeader.biPlanes = 1;
    pBmpInfo->bmiHeader.biBitCount = 32;
    pBmpInfo->bmiHeader.biCompression = BI_BITFIELDS;
    pBmpInfo->bmiHeader.biSizeImage = 500 * 500;
    pBmpInfo->bmiHeader.biXPelsPerMeter = 0;
    pBmpInfo->bmiHeader.biYPelsPerMeter = 0;
    pBmpInfo->bmiHeader.biClrUsed = 0;
    pBmpInfo->bmiHeader.biClrImportant = 0;

    SetStretchBltMode(dc.m_hDC, STRETCH_DELETESCANS);
    StretchDIBits(dc.m_hDC, 
                    0, 
                    rect.Height(), 
                    rect.Width(), 
                    -rect.Height(), 
                    0, 
                    0, 
                    500, 
                    500, 
                    pBuffer, 
                    pBmpInfo, 
                    DIB_RGB_COLORS, 
                    SRCCOPY);

    delete[] pBmpInfo;
    delete[] pBuffer;
}

Answer 1

The problem didn’t come from the StretchDIBits function but from the initialization of the buffer used as the bitmap here.
memset(…) function was misused.
With an initialization such as :

int Color = RGB(255, 0, 0);

for (int i = 0 ; i < 500 * 500 ; i++)
    pBuffer[i] = Color;

I get a perfectly blue image as expected.