Standard library provides the foundation for working with images.

Package image provides:

• an image.Image interface describing a bitmap image
• an implementation for most common ways of representing an image in memory, for example Image.RGBA
• a way to register image format decoders (e.g. PNG, JPEG etc. with image.RegisterFormat.

Standard library provides:

• GIF decoder and encoder
• PNG decoder and encoder
• JPEG decoder and encoder

The are libraries for:

• BMP decoder and encoder
• TIFF decoder and encoder
• WEBP decoder
• vp8 decoder
• vp8l decoder.

The image.Image interface is minimal:

type Image interface {
        // ColorModel returns the Image's color model.
        ColorModel() color.Model
        // Bounds returns the domain for which At can return non-zero color.
        // The bounds do not necessarily contain the point (0, 0).
        Bounds() Rectangle
        // At returns the color of the pixel at (x, y).
        // At(Bounds().Min.X, Bounds().Min.Y) returns the upper-left pixel of the grid.
        // At(Bounds().Max.X-1, Bounds().Max.Y-1) returns the lower-right one.
        At(x, y int) color.Color
}

Here's an example of decoding a PNG image from an URL and printing the size of the image using Bounds() method:

package main

import (
	"fmt"
	"image/png"
	"log"
	"net/http"
)

// :show start
func showImageSize(uri string) {
	resp, err := http.Get(uri)
	if err != nil {
		log.Fatalf("http.Get('%s') failed with %s\\n", uri, err)
	}
	defer resp.Body.Close()
	if resp.StatusCode >= 400 {
		log.Fatalf("http.Get() failed with '%s'\\n", resp.Status)
	}
	img, err := png.Decode(resp.Body)
	if err != nil {
		log.Fatalf("png.Decode() failed with '%s'\\n", err)
	}
	size := img.Bounds().Size()
	fmt.Printf("Image '%s'\\n", uri)
	fmt.Printf("  size: %dx%d\\n", size.X, size.Y)
	fmt.Printf("  format in memory: '%T'\\n", img)
}

// :show end

func main() {
	showImageSize("<https://www.programming-books.io/covers/Go.png>")
}