Why Am I Getting an 'Unknown Image Format' Error When Decoding Base64 Images in Go?

Go Base64 Image Decode: Troubleshooting "Unknown Image Format" Error

In Go, you can decode a base64 image to obtain its width and height using the image package. However, you may encounter the "Unknown image format" error during this process.

Decoding Considerations:

To correctly decode an image, the specific image format handler must be registered. For example, to decode a PNG image, the PNG format handler should be imported using:

import _ "image/png"

Alternatively, you can use the format-specific function png.DecodeConfig().

Data URI Scheme

The base64 image data you receive is typically part of a Data URI scheme, which consists of:

1. Data scheme type: data:
2. Media type: image/png
3. Base64-encoded image data

To isolate the base64-encoded data, slice the string from the comma after the media type:

input := "data:image/png;base64,iVkhdfjdAjdfirtn="
b64data := input[strings.IndexByte(input, ',')+1:]

Sample Code:

Using the provided information, here's a revised code:

package main

import (
    "fmt"
    "image/png"
    "log"
    "os"
    "strings"

    "github.com/nfnt/resize"
)

func main() {
    dataURL := os.Getenv("IMAGE_DATA_URL")

    if dataURL == "" {
        log.Fatal("Missing IMAGE_DATA_URL environment variable")
    }

    // Remove Data URI scheme prefix
    b64data := dataURL[strings.IndexByte(dataURL, ',')+1:]

    // Decode base64 string
    reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(b64data))

    // Register PNG format handler and decode the image config
    _ = png.RegisterFormat("png", "png", png.PNGConfig{})
    config, _, err := image.DecodeConfig(reader)
    if err != nil {
        log.Fatal(err)
    }

    // Resize the image
    newSize := resize.Thumbnail(0, 0, config.Width, config.Height)
    scaledImage := resize.Resize(uint(newSize.Width), uint(newSize.Height), reader, resize.Lanczos3)

    // Save the resized image
    outFile, err := os.Create("resized-image.png")
    if err != nil {
        log.Fatal(err)
    }

    err = png.Encode(outFile, scaledImage)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Printf("Image saved to: %s", "resized-image.png")
}

Note that this sample code also resizes the image using the resize package, but this is not necessary for just decoding the image config.

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