Golang : Get RGBA values of each image pixel

Problem :

You need to find out each pixels R,G,B,A values on an image. Maybe to do ASCII art, edge detection algorithm for cognitive purpose or some advanced image processing. How to do that?

Solution :

Get the width and height of an image file with image.DecodeConfig() function and then iterate each pixel base on width/height on the image covering entire image. At each pixel, use image.At() function's RGBA() method to retrieve the R,G,B,A values.

For example :

 package main

 import (
 "fmt"
 "image"
 "image/jpeg"
 "os"
 )

 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("./img.jpg")

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

 defer imgfile.Close()

 // 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)
 }

 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 x := 0; x < width; x++ {
 r, g, b, a := img.At(x, y).RGBA()
 fmt.Printf("[X : %d Y : %v] R : %v, G : %v, B : %v, A : %v  \n", x, y, r, g, b, a)
 }
 }

 }