Golang : Convert []byte to image

There are times when you need to convert []byte to image for saving into file purpose. I encountered one of this situation today as I need to save a generated QR code into PNG file.

In this short tutorial, we will learn how to convert []byte into image and save to a PNG file. Basically to convert to image, you just need to process the []byte variable with bytes.NewReader() function and wrap it with image.Decode() function.

See example :

 imgByte := code.PNG()

 // convert []byte to image for saving to file
 img, _, _ := image.Decode(bytes.NewReader(imgByte))

 //save the imgByte to file
 out, err := os.Create("./QRImg.png")

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

 err = png.Encode(out, img)

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

and the entire program :

 package main

 import (
 "bytes"
 "code.google.com/p/rsc/qr"
 "crypto/rand"
 "fmt"
 "image"
 "image/png"
 "os"
 "runtime"
 )

 func randStr(strSize int, randType string) string {

 var dictionary string

 if randType == "alphanum" {
 dictionary = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
 }

 if randType == "alpha" {
 dictionary = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
 }

 if randType == "number" {
 dictionary = "0123456789"
 }

 var bytes = make([]byte, strSize)
 rand.Read(bytes)
 for k, v := range bytes {
 bytes[k] = dictionary[v%byte(len(dictionary))]
 }
 return string(bytes)
 }

 func main() {

 // maximize CPU usage for maximum performance
 runtime.GOMAXPROCS(runtime.NumCPU())

 // generate a random string - preferbly 6 or 8 characters
 randomStr := randStr(6, "alphanum")
 fmt.Println(randomStr)

 // generate the link or any data you want to
 // encode into QR codes
 // in this example, we use an example of two factor
 // authentication link. Replace the authTFAlink
 // with yours.

 authTFAlink := "otpauth://socketloop.com?key=" + randomStr + "&issuer=SocketLoop"

 // Encode authTFAlink to QR codes
 // qr.L = 20% redundant level
 // see https://godoc.org/code.google.com/p/rsc/qr#Level

 code, err := qr.Encode(authTFAlink, qr.L)

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

 imgByte := code.PNG()

 // convert []byte to image for saving to file
 img, _, _ := image.Decode(bytes.NewReader(imgByte))

 //save the imgByte to file
 out, err := os.Create("./QRImg.png")

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

 err = png.Encode(out, img)

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

 // everything ok
 fmt.Println("QR code generated and saved to QRimg.png")

 }

References :

https://www.socketloop.com/tutorials/golang-save-image-to-png-jpeg-or-gif-format

https://www.socketloop.com/tutorials/golang-convert-an-image-file-to-byte