png, jpeg and gif: get image width and height

gif.go

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"os"
)

func main() {
	fd, err := os.Open(`your.gif`)
	if err != nil {
		panic(err)
	}
	defer fd.Close()

	var buf [6]byte
	fd.Read(buf[:6])
	if !bytes.Equal([]byte{0x47, 0x49, 0x46, 0x38, 0x39, 0x61}, buf[:6]) {
		panic("not a gif")
	}
	fd.Read(buf[:4])
	width := binary.LittleEndian.Uint16(buf[0:2])
	height := binary.LittleEndian.Uint16(buf[2:4])
	fmt.Println(width, height)
}

jpeg.go

package main

import (
	"bytes"
	"fmt"
	"os"
)

func main() {
	const F = `your.jpg`
	data, err := os.ReadFile(F)
	if err != nil {
		panic(err)
	}
	o := 0
	if !bytes.Equal([]byte{0xFF, 0xD8}, data[o:o+2]) {
		panic("not a jpeg")
	}
	o += 2
	var width, height int

	for o < len(data) {
		if data[o] != 0xFF {
			panic("bad file")
		}
		if data[o+1] == 0xC2 || data[o+1] == 0xC0 {
			height = int(data[o+5])*256 + int(data[o+6])
			width = int(data[o+7])*256 + int(data[o+8])
			break
		} else {
			o += 2
			block_len := int(data[o])*256 + int(data[o+1])
			o += int(block_len)
		}
	}
	fmt.Println(width, height)
}

png.go

package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"os"
)

func main() {
	const F = `your.png`
	fd, err := os.Open(F)
	if err != nil {
		panic(err)
	}
	defer fd.Close()

	var buf [8]byte
	fd.Read(buf[:])
	if !bytes.Equal([]byte{0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A}, buf[:]) {
		panic("not a png")
	}
	fd.Read(buf[:])
	if !bytes.Equal([]byte("IHDR"), buf[4:]) {
		panic("bad png")
	}
	fd.Read(buf[:])
	width := binary.BigEndian.Uint32(buf[0:4])
	height := binary.BigEndian.Uint32(buf[4:8])
	fmt.Println(width, height)
}