Golang : Convert PNG transparent background image to JPG or JPEG image

For this tutorial, we will learn how to convert PNG image file to JPEG image file. Because JPEG format does not support transparent background, you will have to expect some quality loss when converting from PNG to JPEG image.

A PNG file with transparent background

Converting PNG to JPEG in straight forward manner will cause the transparent background to become black in the final result.

A converted JPEG file with black background

We need to take extra steps to create a new image with white background and paste the PNG image over the new image before saving as JPEG.

Here you go!

 package main

 import (
 "fmt"
 "image"
 "image/color"
 "image/draw"
 "image/jpeg"
 "image/png"
 "os"
 )

 func main() {
 pngImgFile, err := os.Open("./PNG-file.png")

 if err != nil {
 fmt.Println("PNG-file.png file not found!")
 os.Exit(1)
 }

 defer pngImgFile.Close()

 // create image from PNG file
 imgSrc, err := png.Decode(pngImgFile)

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

 // create a new Image with the same dimension of PNG image
 newImg := image.NewRGBA(imgSrc.Bounds())

 // we will use white background to replace PNG's transparent background
 // you can change it to whichever color you want with
 // a new color.RGBA{} and use image.NewUniform(color.RGBA{<fill in color>}) function

 draw.Draw(newImg, newImg.Bounds(), &image.Uniform{color.White}, image.Point{}, draw.Src)

 // paste PNG image OVER to newImage
 draw.Draw(newImg, newImg.Bounds(), imgSrc, imgSrc.Bounds().Min, draw.Over)

 // create new out JPEG file
 jpgImgFile, err := os.Create("./JPEG-file.jpg")

 if err != nil {
 fmt.Println("Cannot create JPEG-file.jpg !")
 fmt.Println(err)
 os.Exit(1)
 }

 defer jpgImgFile.Close()

 var opt jpeg.Options
 opt.Quality = 80

 // convert newImage to JPEG encoded byte and save to jpgImgFile
 // with quality = 80
 err = jpeg.Encode(jpgImgFile, newImg, &opt)

 //err = jpeg.Encode(jpgImgFile, newImg, nil) -- use nil if ignore quality options

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

 fmt.Println("Converted PNG file to JPEG file")
 }

Final result. A proper JPEG file with white background.

JPEG file with white background converted from PNG

References :

https://golang.org/pkg/image/draw/#Draw

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