draw.go

// Package letteravatar generates letter-avatars.
package letteravatar

import (
	"image"
	"image/color"
	"image/draw"
	"math/rand"
	"sync"
	"time"

	"github.com/golang/freetype"
	"github.com/golang/freetype/truetype"
	"golang.org/x/image/math/fixed"
)

// Options are letter-avatar parameters.
type Options struct {
	Font        *truetype.Font
	Palette     []color.Color
	LetterColor color.Color

	// PaletteKey is used to pick the background color from the Palette.
	// Using the same PaletteKey leads to the same background color being picked.
	// If PaletteKey is empty (default) the background color is picked randomly.
	PaletteKey string
}

var defaultLetterColor = color.RGBA{0xf0, 0xf0, 0xf0, 0xf0}

// Draw generates a new letter-avatar image of the given size using the given letter
// with the given options. Default parameters are used if a nil *Options is passed.
func Draw(size int, letter rune, options *Options) (image.Image, error) {
	font := defaultFont
	if options != nil && options.Font != nil {
		font = options.Font
	}

	palette := defaultPalette
	if options != nil && options.Palette != nil {
		palette = options.Palette
	}

	var letterColor color.Color = defaultLetterColor
	if options != nil && options.LetterColor != nil {
		letterColor = options.LetterColor
	}

	var bgColor color.Color = color.RGBA{0x00, 0x00, 0x00, 0xff}
	if len(palette) > 0 {
		if options != nil && len(options.PaletteKey) > 0 {
			bgColor = palette[keyindex(len(palette), options.PaletteKey)]
		} else {
			bgColor = palette[randint(len(palette))]
		}
	}

	return drawAvatar(bgColor, letterColor, font, size, letter)
}

func drawAvatar(bgColor, fgColor color.Color, font *truetype.Font, size int, letter rune) (image.Image, error) {
	dst := newRGBA(size, size, bgColor)

	fontSize := float64(size) * 0.6
	src, err := drawString(bgColor, fgColor, font, fontSize, string(letter))
	if err != nil {
		return nil, err
	}

	r := src.Bounds().Add(dst.Bounds().Size().Div(2)).Sub(src.Bounds().Size().Div(2))
	draw.Draw(dst, r, src, src.Bounds().Min, draw.Src)

	return dst, nil
}

func drawString(bgColor, fgColor color.Color, font *truetype.Font, fontSize float64, str string) (image.Image, error) {
	c := freetype.NewContext()
	c.SetDPI(72)

	bb := font.Bounds(c.PointToFixed(fontSize))
	w := bb.Max.X.Ceil() - bb.Min.X.Floor()
	h := bb.Max.Y.Ceil() - bb.Min.Y.Floor()

	dst := newRGBA(w, h, bgColor)
	src := image.NewUniform(fgColor)

	c.SetDst(dst)
	c.SetSrc(src)
	c.SetClip(dst.Bounds())
	c.SetFontSize(fontSize)
	c.SetFont(font)

	p, err := c.DrawString(str, fixed.Point26_6{X: 0, Y: bb.Max.Y})
	if err != nil {
		return nil, err
	}

	return dst.SubImage(image.Rect(0, 0, p.X.Ceil(), h)), nil
}

func newRGBA(w, h int, c color.Color) *image.RGBA {
	img := image.NewRGBA(image.Rect(0, 0, w, h))
	rgba := color.RGBAModel.Convert(c).(color.RGBA)
	for x := 0; x < w; x++ {
		for y := 0; y < h; y++ {
			img.SetRGBA(x, y, rgba)
		}
	}
	return img
}

func keyindex(n int, key string) int {
	var index int64
	for _, r := range key {
		index = (index + int64(r)) % int64(n)
	}
	return int(index)
}

var (
	rng   = rand.New(rand.NewSource(time.Now().UnixNano()))
	rngMu = new(sync.Mutex)
)

func randint(n int) int {
	rngMu.Lock()
	defer rngMu.Unlock()
	return rng.Intn(n)
}