How to Decode Base64-Encoded Image Data and Retrieve Its Dimensions in Go?

Decoding Go Base64 Image Data

Problem:

You're receiving a Base64-encoded image data URL from a canvas and trying to decode it to retrieve its width and height, but you're encountering an "Unknown image format" error.

Solution:

The provided data URL format is a Data URI scheme, which includes additional information like the image type and Base64 encoding. To decode it properly, follow these steps:

import (
    "encoding/base64"
    "fmt"
    "image"
    _ "image/png" // Register the PNG format handler
    "strings"
)

func decodeBase64Image(dataURL string) (image.Config, error) {
    // Remove the prefix (e.g., "data:image/png;base64,")
    base64Data := dataURL[strings.IndexByte(dataURL, ',')+1:]

    // Decode the Base64-encoded image data
    decoded, err := base64.StdEncoding.DecodeString(base64Data)
    if err != nil {
        return image.Config{}, fmt.Errorf("could not decode Base64 data: %w", err)
    }

    // Decode the image configuration
    return image.DecodeConfig(bytes.NewReader(decoded))
}

By registering the PNG format handler (_ "image/png"), you enable the image.DecodeConfig() function to properly decode the image data. You can also use the png.DecodeConfig() function directly if you know the image format.

Avoidance of Prefix Replacement:

Instead of replacing the prefix with an empty string, slice the input string to extract the Base64-encoded data. This is a more efficient approach as it doesn't require copying the entire string in memory.

base64Data := input[strings.IndexByte(input, ',')+1:]

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