byjp.me
A tool for archiving & converting scans of postcards, and information about them.
1package resolution
2
3import (
4 "fmt"
5 "math/big"
6)
7
8var readers = []struct {
9 magicBytes []byte
10 fn func([]byte) (*big.Rat, *big.Rat, error)
11}{
12 {[]byte(pngHeader), decodePNG},
13 {[]byte("\xff\xd8"), decodeJPEG},
14 {[]byte("RIFF????WEBPVP8"), decodeWebP},
15 {[]byte{0x4D, 0x4D, 0x00, 0x2A}, decodeTIFF},
16 {[]byte{0x49, 0x49, 0x2A, 0x00}, decodeTIFF},
17}
18
19// Decode returns the width and height resolution (number of pixels per centimetre) an image declares it is stored with
20func Decode(data []byte) (*big.Rat, *big.Rat, error) {
21 for _, r := range readers {
22 if isMagic(data[0:len(r.magicBytes)], r.magicBytes) {
23 return r.fn(data)
24 }
25 }
26 return nil, nil, fmt.Errorf("unknown image format")
27}
28
29func isMagic(data, magic []byte) bool {
30 if len(magic) != len(data) {
31 return false
32 }
33
34 for i, b := range data {
35 if magic[i] != b && magic[i] != '?' {
36 return false
37 }
38 }
39
40 return true
41}