Function filterScanline

dependency/densecrf/examples/lodepng.cpp:5138–5192

#endif /*LODEPNG_COMPILE_ANCILLARY_CHUNKS*/

static void filterScanline(unsigned char* out, const unsigned char* scanline, const unsigned char* prevline,
                           size_t length, size_t bytewidth, unsigned char filterType)
{
    size_t i;
    switch(filterType)
    {
        case 0: /*None*/
            for(i = 0; i != length; ++i) out[i] = scanline[i];
            break;
        case 1: /*Sub*/
            for(i = 0; i != bytewidth; ++i) out[i] = scanline[i];
            for(i = bytewidth; i < length; ++i) out[i] = scanline[i] - scanline[i - bytewidth];
            break;
        case 2: /*Up*/
            if(prevline)
            {
                for(i = 0; i != length; ++i) out[i] = scanline[i] - prevline[i];
            }
            else
            {
                for(i = 0; i != length; ++i) out[i] = scanline[i];
            }
            break;
        case 3: /*Average*/
            if(prevline)
            {
                for(i = 0; i != bytewidth; ++i) out[i] = scanline[i] - (prevline[i] >> 1);
                for(i = bytewidth; i < length; ++i) out[i] = scanline[i] - ((scanline[i - bytewidth] + prevline[i]) >> 1);
            }
            else
            {
                for(i = 0; i != bytewidth; ++i) out[i] = scanline[i];
                for(i = bytewidth; i < length; ++i) out[i] = scanline[i] - (scanline[i - bytewidth] >> 1);
            }
            break;
        case 4: /*Paeth*/
            if(prevline)
            {
                /*paethPredictor(0, prevline[i], 0) is always prevline[i]*/
                for(i = 0; i != bytewidth; ++i) out[i] = (scanline[i] - prevline[i]);
                for(i = bytewidth; i < length; ++i)
                {
                    out[i] = (scanline[i] - paethPredictor(scanline[i - bytewidth], prevline[i], prevline[i - bytewidth]));
                }
            }
            else
            {
                for(i = 0; i != bytewidth; ++i) out[i] = scanline[i];
                /*paethPredictor(scanline[i - bytewidth], 0, 0) is always scanline[i - bytewidth]*/
                for(i = bytewidth; i < length; ++i) out[i] = (scanline[i] - scanline[i - bytewidth]);
            }
            break;
        default: return; /*unexisting filter type given*/
    }
}

/* log2 approximation. A slight bit faster than std::log. */
static float flog2(float f)