data structure of Sobel and Scharr derivatives about jsfeat HOT 2 CLOSED

sobel_derivatives(src, dst);
var w = src.cols, h = src.rows;
var dstep = w<<1, dptr=0, sptr=0;
for(y=0; y < h; y++, dptr += dstep, sptr += w) {
    for(x=0; x < w; x++) {
        var dx = dst.data[x*2+dptr];
        var dy = dst.data[x*2+dptr+1];
        var gx = Math.sqrt(dx*dx + dy*dy);
        src.data[x+sptr] = gx&0xff;
    }
}