Premute Image Alpha

void ImagePreMultAlpha(Image *pImage)
{
    // The per pixel alpha blending API for layered windows deals with
    // pre-multiplied alpha values in the RGB channels. For further details see
    // the MSDN documentation for the BLENDFUNCTION structure. It basically
    // means we have to multiply each red, green, and blue channel in our image
    // with the alpha value divided by 255.
    //
    // Notes:
    // 1. ImagePreMultAlpha() needs to be called before every call to
    //    UpdateLayeredWindow() (in the RedrawLayeredWindow() function).
    //
    // 2. Must divide by 255.0 instead of 255 to prevent alpha values in range
    //    [1, 254] from causing the pixel to become black. This will cause a
    //    conversion from 'float' to 'BYTE' possible loss of data warning which
    //    can be safely ignored.

    if (!pImage)
        return;

    BYTE *pPixel = NULL;

    if (pImage->width * 4 == pImage->pitch)
    {
        // This is a special case. When the image width is already a multiple
        // of 4 the image does not require any padding bytes at the end of each
        // scan line. Consequently we do not need to address each scan line
        // separately. This is much faster than the below case where the image
        // width is not a multiple of 4.

        int totalBytes = pImage->width * pImage->height * 4;

        for (int i = 0; i < totalBytes; i += 4)
        {
            pPixel = &pImage->pPixels[i];
            pPixel[0] = (BYTE)(pPixel[0] * (float)pPixel[3] / 255.0f);
            pPixel[1] = (BYTE)(pPixel[1] * (float)pPixel[3] / 255.0f);
            pPixel[2] = (BYTE)(pPixel[2] * (float)pPixel[3] / 255.0f);
        }
    }
    else
    {
        // Width of the image is not a multiple of 4. So padding bytes have
        // been included in the DIB's pixel data. Need to address each scan
        // line separately. This is much slower than the above case where the
        // width of the image is already a multiple of 4.

        for (int y = 0; y < pImage->height; ++y)
        {
            for (int x = 0; x < pImage->width; ++x)
            {
                pPixel = &pImage->pPixels[(y * pImage->pitch) + (x * 4)];
                pPixel[0] = (BYTE)(pPixel[0] * (float)pPixel[3] / 255.0f);
                pPixel[1] = (BYTE)(pPixel[1] * (float)pPixel[3] / 255.0f);
                pPixel[2] = (BYTE)(pPixel[2] * (float)pPixel[3] / 255.0f);
            }
        }
    }
}