code: func paeth(a, b, c uint8) uint8 {
  	// This is an optimized version of the sample code in the PNG spec.
  	// For example, the sample code starts with:
  	//	p := int(a) + int(b) - int(c)
  	//	pa := abs(p - int(a))
  	// but the optimized form uses fewer arithmetic operations:
  	//	pa := int(b) - int(c)
  	//	pa = abs(pa)
  	pc := int(c)
  	pa := int(b) - pc
  	pb := int(a) - pc
  	pc = abs(pa + pb)
  	pa = abs(pa)
  	pb = abs(pb)
  	if pa <= pb && pa <= pc {
  		return a
  	} else if pb <= pc {
  		return b
  	}
  	return c
  }
docs: // paeth implements the Paeth filter function, as per the PNG specification.