code: func filterPaeth(cdat, pdat []byte, bytesPerPixel int) {
  	var a, b, c, pa, pb, pc int
  	for i := 0; i < bytesPerPixel; i++ {
  		a, c = 0, 0
  		for j := i; j < len(cdat); j += bytesPerPixel {
  			b = int(pdat[j])
  			pa = b - c
  			pb = a - c
  			pc = abs(pa + pb)
  			pa = abs(pa)
  			pb = abs(pb)
  			if pa <= pb && pa <= pc {
  				// No-op.
  			} else if pb <= pc {
  				a = b
  			} else {
  				a = c
  			}
  			a += int(cdat[j])
  			a &= 0xff
  			cdat[j] = uint8(a)
  			c = b
  		}
  	}
  }
docs: // filterPaeth applies the Paeth filter to the cdat slice.
  // cdat is the current row's data, pdat is the previous row's data.