Hi All,

I tried to figure out the meaning of that function, I couldn't, even tried on paper, but didn't get it. Would someone explain that to me ? If that's possible with Ascii text or some images would be so much appreciated.

static void inline resizeRow(uint32_t *dst, uint32_t *src, uint32_t pixelsPerRow)

{
        uint8_t * pSrc8 = (uint8_t *)src;
        uint8_t * pDest8 = (uint8_t *)dst;
        int stride = pixelsPerRow * sizeof(uint32_t);
        int x;
        int r, g, b, a;


        for (x=0; x<pixelsPerRow; x++)
        {
                r = pSrc8[0] + pSrc8[4] + pSrc8[stride+0] + pSrc8[stride+4];
                g = pSrc8[1] + pSrc8[5] + pSrc8[stride+1] + pSrc8[stride+5];
                b = pSrc8[2] + pSrc8[6] + pSrc8[stride+2] + pSrc8[stride+6];
                a = pSrc8[3] + pSrc8[7] + pSrc8[stride+3] + pSrc8[stride+7];
                pDest8[0] = (uint8_t)((r + 2)/4); // average with rounding
                pDest8[1] = (uint8_t)((g + 2)/4);
                pDest8[2] = (uint8_t)((b + 2)/4);
                pDest8[3] = (uint8_t)((a + 2)/4);
                pSrc8 += 8; // skip forward 2 source pixels
                pDest8 += 4; // skip forward 1 destination pixel
        }
}
 

Thanks for your answer, I have made an imaginary bitmap, so I got 4*4 pixels assuming 1 Pixel is 3 bytes RGB, and I added them, but how come from two rows I get 4*4 pixels?

Would someone please show me some pics ?

Regarding also those lines:

                pDest8[0] = (uint8_t)((r + 2)/4); // average with rounding
                pDest8[1] = (uint8_t)((g + 2)/4);
                pDest8[2] = (uint8_t)((b + 2)/4);
                pDest8[3] = (uint8_t)((a + 2)/4);
                pSrc8 += 8; // skip forward 2 source pixels
                pDest8 += 4; // skip forward 1 destination pixel
 

He skipped 4 bytes, but at the second iteration he writes again at 0,1,2,3 at the dest, should be 4,5,6,7 ?

@Paradimg Shift,

Thanks for your explanation, you know what I have worked a lot in C,C++, and I never know that info!! I'm gonna review pointers now..

If its ok for you, would you please just draw a simple bitmap to get the algorithm ?

I would like also to modify it so that it works for any scale, not just by two.

I have used opencv function cv::resize and it is slow on ARM... that's why I wanna write my own, and convert it to ARM Neon...

What is meant by stride + 0 , stride + 1 ?