Make a mosaic image (bitmap format)

Question

I want to make a mosaic photo with different window-size (that has been determined by user). This is just like a first draft of the code but I have problems to get the pixels and calculating averages. Then put the avarage value in each pixel and continue to the end. Even I get error to converting them of diffrent types: (Also the other part manufacturers a gray-scale image) p.s: sorry that I am in the very first steps of learning image processing.

''' void CImageProcessingDoc::OnProcessMosaic()
{
if (m_pImage) {
    DlgMosaicOption dlg;

    if (dlg.DoModal() == IDOK) {

        DWORD dwWindowSize = dlg.m_dwWindowSize;

        DWORD width = m_pImage->GetWidth();
        DWORD height = m_pImage->GetHeight();

        RGBQUAD color;
        RGBQUAD newcolor;
        float X_step = width / dwWindowSize;
        float Y_step = height / dwWindowSize;
        int avg, pixel;
        for (DWORD y = 0; y < dwWindowSize; y++) {
            for (DWORD x = 0; x < dwWindowSize; x++) {
                color = m_pImage->GetPixelColor(x, y);
                (RGBQUAD) pixel =  m_pImage->GetPixelColor(x, y);
                avR += (int)(color.red(pixel);
                avG += (int)(color.green(pixel);
                avB += (int)(color.blue(pixel);
                newcolor.rgbBlue  = (BYTE)RGB2GRAY(color.rgbRed, color.rgbGreen, color.rgbBlue);
                newcolor.rgbGreen = (BYTE)RGB2GRAY(color.rgbRed, color.rgbGreen, color.rgbBlue);
                newcolor.rgbRed   = (BYTE)RGB2GRAY(color.rgbRed, color.rgbGreen, color.rgbBlue);

                m_pImage->SetPixelColor(x, y, newcolor);
            }
        }
    }
}
 } '''

Could anyone please help me to understand the problem?

Answer 1

I think you are mixing up spatial, spectral and temporal average here.

Spatial average

This is the operation of computing average of pixels over an area.

You have to compute eR = 1/N * (P0.R + P1.R + P2.R + P3.R + ...), eG = 1/N * (P0.G + P1.G + ...), eB = 1/N * (P0.B + P1.B + ...)

You'll get a pixel with as many color as there was in the input picture, but with limited spatial frequency, a picture computed like this will appear blurred, with no details

Spectral average

This is the operation of computing average of the components (spectrum) of each pixels.

You have to compute e = 1/3 * (P0.R + P0.G + P0.B)

You'll get a monochromic picture with the exact same spatial frequency as the initial picture.

Temporal average

While you haven't talked about it, this is for reference. The idea is to compute the average of each pixel, and each component for N pictures in a temporal sequence

This gives a kind of motion blurred picture.

Answer

If I understand your question correctly, you want spectral average to convert a RGB to the average grey value taken that grey = (R+G+B)/3.

Thus, you pixel loop should look like this:

for (DWORD y = 0; y < dwWindowSize; y++) {
            for (DWORD x = 0; x < dwWindowSize; x++) {
                color = m_pImage->GetPixelColor(x, y);
                float avg = (color.rgbRed + color.rgbGreen + color.rgbBlue) / 3.f;
                m_pImage->SetPixelColor(x, y, RGBQUAD(avg, avg, avg, 1.0f));
            }
        }

Please notice that converting non linear RGB (usually called sRGB) to luminance using the average is a poor formula for RGB to grayscale conversion. You should read about RGB to Lab* conversion (you are interested in L part only) or at least RGB to YUV (you are interested to Y part only).

If your question is about resizing the input picture, then you are not using the appropriate algorithm, what you want is called resampling.