utils.go

package imaging

import (
	"image"
	"math"
	"runtime"
	"sync"
)

// parallel processes the data in separate goroutines.
func parallel(start, stop int, fn func(<-chan int)) {
	count := stop - start
	if count < 1 {
		return
	}

	procs := runtime.GOMAXPROCS(0)
	if procs > count {
		procs = count
	}

	c := make(chan int, count)
	for i := start; i < stop; i++ {
		c <- i
	}
	close(c)

	var wg sync.WaitGroup
	for i := 0; i < procs; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			fn(c)
		}()
	}
	wg.Wait()
}

// absint returns the absolute value of i.
func absint(i int) int {
	if i < 0 {
		return -i
	}
	return i
}

// clamp rounds and clamps float64 value to fit into uint8.
func clamp(x float64) uint8 {
	v := int64(x + 0.5)
	if v > 255 {
		return 255
	}
	if v > 0 {
		return uint8(v)
	}
	return 0
}

func reverse(pix []uint8) {
	if len(pix) <= 4 {
		return
	}
	i := 0
	j := len(pix) - 4
	for i < j {
		pix[i+0], pix[j+0] = pix[j+0], pix[i+0]
		pix[i+1], pix[j+1] = pix[j+1], pix[i+1]
		pix[i+2], pix[j+2] = pix[j+2], pix[i+2]
		pix[i+3], pix[j+3] = pix[j+3], pix[i+3]
		i += 4
		j -= 4
	}
}

func toNRGBA(img image.Image) *image.NRGBA {
	if img, ok := img.(*image.NRGBA); ok {
		return &image.NRGBA{
			Pix:    img.Pix,
			Stride: img.Stride,
			Rect:   img.Rect.Sub(img.Rect.Min),
		}
	}
	return Clone(img)
}

// rgbToHSL converts a color from RGB to HSL.
func rgbToHSL(r, g, b uint8) (float64, float64, float64) {
	rr := float64(r) / 255
	gg := float64(g) / 255
	bb := float64(b) / 255

	max := math.Max(rr, math.Max(gg, bb))
	min := math.Min(rr, math.Min(gg, bb))

	l := (max + min) / 2

	if max == min {
		return 0, 0, l
	}

	var h, s float64
	d := max - min
	if l > 0.5 {
		s = d / (2 - max - min)
	} else {
		s = d / (max + min)
	}

	switch max {
	case rr:
		h = (gg - bb) / d
		if g < b {
			h += 6
		}
	case gg:
		h = (bb-rr)/d + 2
	case bb:
		h = (rr-gg)/d + 4
	}
	h /= 6

	return h, s, l
}

// hslToRGB converts a color from HSL to RGB.
func hslToRGB(h, s, l float64) (uint8, uint8, uint8) {
	var r, g, b float64
	if s == 0 {
		v := clamp(l * 255)
		return v, v, v
	}

	var q float64
	if l < 0.5 {
		q = l * (1 + s)
	} else {
		q = l + s - l*s
	}
	p := 2*l - q

	r = hueToRGB(p, q, h+1/3.0)
	g = hueToRGB(p, q, h)
	b = hueToRGB(p, q, h-1/3.0)

	return clamp(r * 255), clamp(g * 255), clamp(b * 255)
}

func hueToRGB(p, q, t float64) float64 {
	if t < 0 {
		t++
	}
	if t > 1 {
		t--
	}
	if t < 1/6.0 {
		return p + (q-p)*6*t
	}
	if t < 1/2.0 {
		return q
	}
	if t < 2/3.0 {
		return p + (q-p)*(2/3.0-t)*6
	}
	return p
}