extracting color samples from a picture of a big pumpkin

pumpkin_color.go

// https://www.devdungeon.com/content/working-images-go#reading_image_from_file
// https://socketloop.com/tutorials/golang-get-rgba-values-of-each-image-pixel

package main

import (
	"fmt"
	"image"
	"image/jpeg"
	"os"
	"sort"
	"strconv"
	"strings"
)

var colorMap = map[string]bool{}

func init() {
	// damn important or else At(), Bounds() functions will
	// caused memory pointer error!!
	image.RegisterFormat("jpeg", "jpeg", jpeg.Decode, jpeg.DecodeConfig)
}

func main() {
	imgfile, err := os.Open("path/to/pics/pumpkin.jpg")

	if err != nil {
		fmt.Println("img.jpg file not found!")
		os.Exit(1)
	}

	defer imgfile.Close()

	out, _ := os.Create("image_analysis.html")
	// l := log.New(out, "", 0777)
	fmt.Fprintln(out, "<style>div{height:50px;width:200px}</style>")

	// get image height and width with image/jpeg
	// change accordinly if file is png or gif

	imgCfg, _, err := image.DecodeConfig(imgfile)

	if err != nil {
		fmt.Println(err)
		os.Exit(1)
	}

	sampleSize := 0.5

	width := imgCfg.Width
	height := imgCfg.Height

	fmt.Println("Width : ", width)
	fmt.Println("Height : ", height)

	// we need to reset the io.Reader again for image.Decode() function below to work
	// otherwise we will  - panic: runtime error: invalid memory address or nil pointer dereference
	// there is no build in rewind for io.Reader, use Seek(0,0)
	imgfile.Seek(0, 0)
	// get the image
	img, _, err := image.Decode(imgfile)

	fmt.Println(img.At(10, 10).RGBA())
	// for y := 0; y < height  ; y++ {
	for y := 0; y < int(float64(height)*sampleSize); y++ {
		for x := 0; x < int(float64(width)*sampleSize); x++ {
			r, g, b, a := img.At(x, y).RGBA()
			R := r >> 8
			G := g >> 8
			B := b >> 8
			A := a >> 8
			colorMap[fmt.Sprintf("%v:%v:%v:%v", R, G, B, A)] = true
		}
	}

	// To store the keys in slice in sorted order
	var keys []string
	for k := range colorMap {
		keys = append(keys, k)
	}
	sort.Strings(keys)

	for _, c := range keys {
		split := strings.Split(c, ":")
		R, G, B, A := split[0], split[1], split[2], split[3]
		i, _ := strconv.Atoi(R)
		if i >= 200 {
			fmt.Fprintf(out, "<div style=\"background-color: rgba(%v, %v, %v, %v)\">rgba(%v, %v, %v, %v)</div>", R, G, B, A, R, G, B, A)
		}
	}
}