emoji and text renderer

emoji.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"image"
	"image/jpeg"
	_ "image/png"
	"io"
	"os"
	"strings"

	"github.com/golang/freetype/truetype"
	"golang.org/x/image/draw"
	"golang.org/x/image/font"
	"golang.org/x/image/font/gofont/gobold"
	"golang.org/x/image/math/fixed"
)

const (
	exitSuccess = 0
	exitFailure = 1
)

const (
	imagePath     = "./noto-emoji/png/128"
	aliasFile     = "./noto-emoji/emoji_aliases.txt"
	fontSize      = 64  // point
	imageWidth    = 640 // pixel
	imageHeight   = 120 // pixel
	textTopMargin = 80  // fixed.I
	lineHeight    = 70  // fixed.I
)

// emoji alias
var alias = map[string]string{}

func init() {
	fp, err := os.Open(aliasFile)
	if err != nil {
		panic(err)
	}

	s := bufio.NewScanner(fp)
	for s.Scan() {
		line := s.Text()

		/*
		   alias file format:
		       # 'fe0f' is not in these sequences
		       1f3c3;1f3c3_200d_2642 # RUNNER -> man running
		*/

		if strings.HasPrefix(line, "#") {
			continue
		}

		s := strings.Split(line, ";")
		if len(s) != 2 {
			continue
		}

		i := strings.Index(s[1], " ")
		if i < 0 {
			continue
		}

		from := s[0]
		to := s[1][:i]

		alias[from] = to
	}

	if err := s.Err(); err != nil {
		fmt.Fprintln(os.Stderr, "reading alias file:", err)
	}

}

func exist(path string) bool {
	_, err := os.Stat(path)
	return err == nil
}

// GetPath if unicode codepoint is included in list and file exists return file path
func getPath(r rune) (string, error) {
	name := fmt.Sprintf("%.4x", r)
	var path string

	if v, ok := alias[name]; ok {
		path = fmt.Sprintf("%s/emoji_u%s.png", imagePath, v)
	} else {
		path = fmt.Sprintf("%s/emoji_u%s.png", imagePath, name)
	}

	if !exist(path) {
		return "", fmt.Errorf("%s does NOT exist", path)
	}

	return path, nil
}

func loadEmoji(r rune, size int) (image.Image, bool) {
	var img image.Image

	path, err := getPath(r)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		return img, false
	}

	fp, err := os.Open(path)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		return img, false
	}
	defer fp.Close()

	img, _, err = image.Decode(fp)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		return img, false
	}

	rect := image.Rect(0, 0, size, size)
	dst := image.NewRGBA(rect)
	draw.ApproxBiLinear.Scale(dst, rect, img, img.Bounds(), draw.Over, nil)

	return dst, true
}

func renderLine(img draw.Image, dr *font.Drawer, s string) {
	size := dr.Face.Metrics().Ascent.Floor() + dr.Face.Metrics().Descent.Floor()

	for _, r := range s {
		emoji, ok := loadEmoji(r, size)

		if ok {
			// Drawer.Dot is glyph baseline of next glyph
			// get left/top coordinates for draw.Draw().
			p := image.Pt(dr.Dot.X.Floor(), dr.Dot.Y.Floor()-dr.Face.Metrics().Ascent.Floor())
			rect := image.Rect(0, 0, size, size).Add(p)

			// draw emoji and ascend baseline
			draw.Draw(img, rect, emoji, image.ZP, draw.Over)
			dr.Dot.X += fixed.I(size)
		} else {
			// fallback: use normal glyph
			dr.DrawString(string(r))
		}
	}
}

func renderText(img draw.Image, face font.Face, text string) error {
	dr := &font.Drawer{
		Dst:  img,
		Src:  image.White,
		Face: face,
		Dot:  fixed.Point26_6{},
	}

	for i, s := range strings.Split(text, "\n") {
		dr.Dot.X = (fixed.I(imageWidth) - dr.MeasureString(s)) / 2
		dr.Dot.Y = fixed.I(textTopMargin + i*lineHeight)

		renderLine(img, dr, s)
	}

	return nil
}

func outputJPEG(img image.Image) error {
	buf := new(bytes.Buffer)
	err := jpeg.Encode(buf, img, nil)
	if err != nil {
		return err
	}

	_, err = io.Copy(os.Stdout, buf)
	if err != nil {
		return err
	}

	return nil
}

func main() {
	img := image.NewRGBA(image.Rect(0, 0, imageWidth, imageHeight))

	ft, err := truetype.Parse(gobold.TTF)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(exitFailure)
	}

	opt := truetype.Options{
		Size:              fontSize,
		DPI:               0,
		Hinting:           0,
		GlyphCacheEntries: 0,
		SubPixelsX:        0,
		SubPixelsY:        0,
	}

	renderText(img, truetype.NewFace(ft, &opt), "Hello, world! 🙋")

	err = outputJPEG(img)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(exitFailure)
	}
}