Source code for a sample Google Cloud Function

signdance.go

# Copyright 2019 Google LLC
# SPDX-License-Identifier: Apache-2.0
package p

import (
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"image/gif"
	"log"
	"net/http"
	"strconv"

	"golang.org/x/image/font"
	"golang.org/x/image/font/basicfont"
	"golang.org/x/image/math/fixed"
)

const sourceGifURL = "https://storage.googleapis.com/deleplace-sandbox/2019/go-gcf/gopher-dance-long-3x-sign.gif"

func MakeGif(w http.ResponseWriter, r *http.Request) {
	signText := "foobar"
	if t := r.FormValue("text"); t != "" {
		fmt.Println("Custom text", t)
		signText = t
	}

	var newLightColor color.Color = color.RGBA{255, 200, 200, 255}
	if color := r.FormValue("color"); color != "" {
		fmt.Println("Custom color", color)
		var err error
		newLightColor, err = decodeHexColor(color)
		check(err)
	}
	newDimmedColor := dim(newLightColor)

	resp, err := http.Get(sourceGifURL)
	check(err)

	g, err := gif.DecodeAll(resp.Body)
	check(err)
	resp.Body.Close()
	// fmt.Println(len(g.Image), "frames")

	textColor := color.RGBA{240, 240, 240, 255}

	for i, img := range g.Image {
		modify(i, img, newLightColor, newDimmedColor, signText, textColor)
	}

	w.Header().Set("Content-Type", "image/gif")
	w.Header().Set("Cache-Control", "no-cache, no-store, no-transform")
	err = gif.EncodeAll(w, g)
	check(err)
	log.Println("Done.")
}

func modify(i int, img *image.Paletted, newLightColor, newDimmedColor color.Color, message string, textColor color.Color) {

	for j, c := range img.Palette {
		if c == originalLightColor {
			// Main light blue
			img.Palette[j] = newLightColor
		} else if c == originalDimmedColor {
			// Secondary dimmed blue
			img.Palette[j] = newDimmedColor
		}
	}
	addLabel(img, x[23-i], y[23-i], message, textColor)

}

func addLabel(img draw.Image, x, y int, label string, col color.Color) {
	words := strings.Split(label, " ")
	m := len(words)
	dy := 10 + (76 / 2) - ((m * 13) / 2)

	for _, word := range words {
		n := len(word)
		dx := 4 + (100 / 2) - ((n * 7) / 2)
		point := fixed.Point26_6{
			fixed.Int26_6((dx + x) * 64),
			fixed.Int26_6((dy + y) * 64),
		}
		face := basicfont.Face7x13

		d := &font.Drawer{
			Dst:  img,
			Src:  image.NewUniform(col),
			Face: face,
			Dot:  point,
		}
		d.DrawString(word)
		dy += 13
	}
}

func decodeHexColor(s string) (color.Color, error) {
	if len(s) != 6 {
		return nil, fmt.Errorf("Only accepting color in hex RRGGBB format")
	}
	var v [3]int64
	var err error
	for i := 0; i < 3; i++ {
		sub := s[2*i : 2+2*i]
		v[i], err = strconv.ParseInt(sub, 16, 0)
		if err != nil {
			return nil, err
		}
	}
	return color.RGBA{uint8(v[0]), uint8(v[1]), uint8(v[2]), 0xff}, nil
}

func dim(c color.Color) color.Color {
	rr, gg, bb, aa := c.RGBA()
	r, g, b, a := uint8(rr), uint8(gg), uint8(bb), uint8(aa)
	d := uint8(25)
	if r > d {
		r -= d
	} else {
		r = 0
	}
	if g > d {
		g -= d
	} else {
		g = 0
	}
	if b > d {
		b -= d
	} else {
		b = 0
	}
	return color.RGBA{r, g, b, a}
}

// Hardcoded positions of the sign
var x = []int{45, 72, 76, 70, 43, 14, 18, 27, 48, 76, 78, 73, 44, 14, 11, 25, 46, 76, 82, 74, 42, 14, 11, 27}
var y = []int{101, 116, 117, 90, 99, 100, 114, 100, 96, 114, 117, 91, 97, 99, 105, 93, 96, 110, 117, 90, 97, 97, 109, 98}

var originalLightColor = color.RGBA{0x96, 0xd6, 0xff, 0xff}
var originalDimmedColor = color.RGBA{0x8d, 0xc9, 0xf0, 0xff}

func check(err error) {
	if err != nil {
		panic(err)
	}
}