github.com/varialus/godfly@v0.0.0-20130904042352-1934f9f095ab/src/pkg/image/png/paeth.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package png

// paeth implements the Paeth filter function, as per the PNG specification.
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 = pa + pb
	if pa < 0 {
		pa = -pa
	}
	if pb < 0 {
		pb = -pb
	}
	if pc < 0 {
		pc = -pc
	}
	if pa <= pb && pa <= pc {
		return a
	} else if pb <= pc {
		return b
	}
	return c
}

// filterPaeth applies the Paeth filter to the cdat slice.
// cdat is the current row's data, pdat is the previous row's data.
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 = pa + pb
			if pa < 0 {
				pa = -pa
			}
			if pb < 0 {
				pb = -pb
			}
			if pc < 0 {
				pc = -pc
			}
			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
		}
	}
}