Brutal extraction of the thumbnail PNG of a Sketchup file

extractSkpThumb.go

package main

import (
	"bytes"
	"encoding/hex"
	"fmt"
	"io/ioutil"
	"os"
	"strconv"
	"strings"
)

// PNG 8-byte header
const hexPNGSignature = "89504E470D0A1A0A"

// Four bytes "IEND"
const hexIEND = "49454E44"

func main() {
	if len(os.Args) != 2 {
		fmt.Fprintln(os.Stderr, "Usage:")
		fmt.Fprintln(os.Stderr, "  ", os.Args[0], "file.skp")
		return
	}
	skpFilename := os.Args[1]

	data, err := ioutil.ReadFile(skpFilename)
	checkerr(err)

	PNGsignature, err := hex.DecodeString(hexPNGSignature)
	checkerr(err)

	IEND, err := hex.DecodeString(hexIEND)
	checkerr(err)

	var offset = 0
	n := 0
	for {
		i := bytes.Index(data[offset+1:], PNGsignature)
		if i == -1 {
			if offset == 0 {
				fmt.Fprintln(os.Stderr, "Signature", hexPNGSignature, "not found in", skpFilename)
			}
			break
		}
		offset += 1 + i

		// This is an approximation: jump to next IEND occurrence.
		// But it would be more correct to scan PNG chunks until a proper IEND chunk is encountered.
		j := bytes.Index(data[offset:], IEND)
		if j == -1 {
			panic("IEND not found :(")
		}

		pngdata := data[offset : offset+j]
		pngfilename := strings.Replace(skpFilename, ".skp", "", -1) + "." + strconv.Itoa(n) + ".png"
		ioutil.WriteFile(pngfilename, pngdata, 0777)

		n++

		if n == 2 {
			// We are currently interested only in first 2 pictures
			break
		}
	}

	fmt.Println("Extracted", n, "png files (out of a total of", bytes.Count(data, PNGsignature), "png).")
}

func checkerr(err error) {
	if err != nil {
		panic(err)
	}
}