How to Properly Decode Base64 Encoded Images in Go?

Decoding Base64 Encoded Images in Go

When working with canvas, it's possible to obtain a base64 image data URL. However, when attempting to decode this image using image.DecodeConfig(), you may encounter the error "Unknown image format."

This issue arises because the data URL contains additional information beyond the base64-encoded image data. To correctly decode the image, you need to remove the prefix:

// Remove the data URL prefix
input := strings.Replace(req.PostFormValue("dataurl"), "data:image/png;base64,", "", 1)

Additionally, you need to register the PNG image format handler before calling image.DecodeConfig(). This can be achieved using:

import _ "image/png"

Alternatively, you can directly use png.DecodeConfig() if you know the exact image format.

Example:

import _ "image/png"

reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(input))
imageConfig, _, err := image.DecodeConfig(reader)
if err != nil {
    log.Fatal(err)
}

By following these steps, you can successfully decode base64-encoded images and obtain their width and height.

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