Quick code to resize the DIB image and maintain a good quality img


There is many algorithms to do image resizing - lancorz, bicubic, bilinear, e.g. But most of them are pretty complex and therefore consume too much CPU.

What I need is fast relatively simple C++ code to resize images with acceptable quality.

Here is an example of what I'm currently doing:

for (int y = 0; y < height; y ++)
    int         srcY1Coord = int((double)(y * srcHeight) / height);
    int         srcY2Coord = min(srcHeight - 1, max(srcY1Coord, int((double)((y + 1) * srcHeight) / height) - 1));

    for (int x = 0; x < width; x ++)
        int     srcX1Coord = int((double)(x * srcWidth) / width);
        int     srcX2Coord = min(srcWidth - 1, max(srcX1Coord, int((double)((x + 1) * srcWidth) / width) - 1));
        int     srcPixelsCount = (srcX2Coord - srcX1Coord + 1) * (srcY2Coord - srcY1Coord + 1);
        RGB32       color32;
        UINT32      r(0), g(0), b(0), a(0);

        for (int xSrc = srcX1Coord; xSrc <= srcX2Coord; xSrc ++)
            for (int ySrc = srcY1Coord; ySrc <= srcY2Coord; ySrc ++)
                RGB32   curSrcColor32 = pSrcDIB->GetDIBPixel(xSrc, ySrc);
                r += curSrcColor32.r; g += curSrcColor32.g; b += curSrcColor32.b; a += curSrcColor32.alpha;

            color32.r = BYTE(r / srcPixelsCount); color32.g = BYTE(g / srcPixelsCount); color32.b = BYTE(b / srcPixelsCount); color32.alpha = BYTE(a / srcPixelsCount);

            SetDIBPixel(x, y, color32);

The code above is fast enough, but the quality is not ok on scaling pictures up.

Therefore, possibly someone already has fast and good C++ code sample for scaling DIBs?

Note: I was using StretchDIBits before - it was super-slow when was needed to downsize 10000x10000 picture down to 100x100 size, my code is much, much faster, I just want to have a bit higher quality

P.S. I'm using my own SetPixel/GetPixel functions, to work directly with data array and fast, that's not device context!

Why are you doing it on the CPU? Using GDI, there's a good chance of some hardware acceleration. Use StretchBlt and SetStretchBltMode.

In pseudocode:

 create source dc and destination dc using CreateCompatibleDC
 create source and destination bitmaps
 SelectObject source bitmap into source DC and dest bitmap into dest DC
 release DCs