Convert base64 string to JPG

Here's something which may help, demonstrates quite a few other things too (you need to have two files: flower.jpg and mouse.png to make this work, available in the repository https://github.com/rravishankar/golangtraining/tree/master/test/jpegencode):

package main

import (
    "bufio"
    "encoding/base64"
    "fmt"
    "image"
    "image/jpeg"
    "image/png"
    "strings"

    "log"
    "os"
)

func main() {

    createJpg()
    createPng()
    base64toPng()
    base64toJpg(getJPEGbase64("flower.jpg"))
}

//Take an existing jpg srcFileName and decode/encode it
func createJpg() {

    srcFileName := "flower.jpg"
    dstFileName := "newFlower.jpg"
    // Decode the JPEG data. If reading from file, create a reader with
    reader, err := os.Open(srcFileName)
    if err != nil {
        log.Fatal(err)
    }
    defer reader.Close()
    //Decode from reader to image format
    m, formatString, err := image.Decode(reader)
    if err != nil {
        log.Fatal(err)
        return
    }
    fmt.Println("Got format String", formatString)
    fmt.Println(m.Bounds())

    //Encode from image format to writer
    f, err := os.OpenFile(dstFileName, os.O_WRONLY|os.O_CREATE, 0777)
    if err != nil {
        log.Fatal(err)
        return
    }

    err = jpeg.Encode(f, m, &jpeg.Options{Quality: 75})
    if err != nil {
        log.Fatal(err)
        return
    }
    fmt.Println("Jpg file", dstFileName, "created")

}

//Take an existing png srcFileName and decode/encode it
func createPng() {
    srcFileName := "mouse.png"
    dstFileName := "newMouse.png"
    reader, err := os.Open(srcFileName)
    if err != nil {
        log.Fatal(err)
    }
    defer reader.Close()
    //Decode from reader to image format
    m, formatString, err := image.Decode(reader)
    if err != nil {
        log.Fatal(err)
        return
    }
    fmt.Println("Got format String", formatString)
    fmt.Println(m.Bounds())

    //Encode from image format to writer
    f, err := os.OpenFile(dstFileName, os.O_WRONLY|os.O_CREATE, 0777)
    if err != nil {
        log.Fatal(err)
        return
    }

    err = png.Encode(f, m)
    if err != nil {
        log.Fatal(err)
        return
    }
    fmt.Println("Png file", dstFileName, "created")

}

//Converts pre-existing base64 data (found in example of https://golang.org/pkg/image/#Decode) to test.png
func base64toPng() {
    const data = `
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`

    reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
    m, formatString, err := image.Decode(reader)
    if err != nil {
        log.Fatal(err)
    }
    bounds := m.Bounds()
    fmt.Println(bounds, formatString)

    //Encode from image format to writer
    pngFilename := "test.png"
    f, err := os.OpenFile(pngFilename, os.O_WRONLY|os.O_CREATE, 0777)
    if err != nil {
        log.Fatal(err)
        return
    }

    err = png.Encode(f, m)
    if err != nil {
        log.Fatal(err)
        return
    }
    fmt.Println("Png file", pngFilename, "created")

}

//Given a base64 string of a JPEG, encodes it into an JPEG image test.jpg
func base64toJpg(data string) {

    reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
    m, formatString, err := image.Decode(reader)
    if err != nil {
        log.Fatal(err)
    }
    bounds := m.Bounds()
    fmt.Println("base64toJpg", bounds, formatString)

    //Encode from image format to writer
    pngFilename := "test.jpg"
    f, err := os.OpenFile(pngFilename, os.O_WRONLY|os.O_CREATE, 0777)
    if err != nil {
        log.Fatal(err)
        return
    }

    err = jpeg.Encode(f, m, &jpeg.Options{Quality: 75})
    if err != nil {
        log.Fatal(err)
        return
    }
    fmt.Println("Jpg file", pngFilename, "created")

}

//Gets base64 string of an existing JPEG file
func getJPEGbase64(fileName string) string {

    imgFile, err := os.Open(fileName)

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

    defer imgFile.Close()

    // create a new buffer base on file size
    fInfo, _ := imgFile.Stat()
    var size = fInfo.Size()
    buf := make([]byte, size)

    // read file content into buffer
    fReader := bufio.NewReader(imgFile)
    fReader.Read(buf)

    imgBase64Str := base64.StdEncoding.EncodeToString(buf)
    //fmt.Println("Base64 string is:", imgBase64Str)
    return imgBase64Str

}

You may also want to read the example in https://golang.org/pkg/image/#Decode Hope this helps.

You're passing your io.Reader to png.Decode(), which begins consuming the reader, only to discover that the input is not a valid PNG, so returns an error.

Then your partly-consumed reader is passed to jpeg.Decode(), which reads the data not yet read, which is not a valid JPEG, and returns the error you observe.

You need to create a new reader for each decoder:

pngI, errPng := png.Decode(bytes.NewReader(unbased))

// ...

jpgI, errJpg := jpeg.Decode(bytes.NewReader(unbased))

Or better yet, consider the MIME type, and only call the proper decoder:

switch strings.TrimSuffix(image[5:coI], ";base64") {
case "image/png":
    pngI, err = png.Decode(res)
    // ...
case "image/jpeg":
    jpgI, err = jpeg.Decode(res)
    // ...
}