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package jpeg
import (
"fmt"
"io"
)
var defaultTables = [][]int{
{ // Luminance
16, 11, 12, 14, 12, 10, 16, 14,
13, 14, 18, 17, 16, 19, 24, 40,
26, 24, 22, 22, 24, 49, 35, 37,
29, 40, 58, 51, 61, 60, 57, 51,
56, 55, 64, 72, 92, 78, 64, 68,
87, 69, 55, 56, 80, 109, 81, 87,
95, 98, 103, 104, 103, 62, 77, 113,
121, 112, 100, 120, 92, 101, 103, 99,
},
{ // Chrominance
17, 18, 18, 24, 21, 24, 47, 26,
26, 47, 99, 66, 56, 66, 99, 99,
99, 99, 99, 99, 99, 99, 99, 99,
99, 99, 99, 99, 99, 99, 99, 99,
99, 99, 99, 99, 99, 99, 99, 99,
99, 99, 99, 99, 99, 99, 99, 99,
99, 99, 99, 99, 99, 99, 99, 99,
99, 99, 99, 99, 99, 99, 99, 99,
},
}
func read2Bytes(r io.Reader) ([]byte, error) {
b := make([]byte, 2)
if _, err := r.Read(b); err != nil {
return nil, err
}
return b, nil
}
// ReadJPEGQuality returns the JPEG quality estimate.
//
// Cleaned up version of github.com/liut/jpegquality
func ReadJPEGQuality(r io.ReadSeeker) (int, error) {
sign, err := read2Bytes(r)
if err != nil {
return 0, err
}
if sign[0] != 0xff && sign[1] != 0xd8 {
return 0, fmt.Errorf("invalid jpeg header")
}
for {
mark, err := read2Bytes(r)
if err != nil {
return 0, err
}
if mark[0] != 0xff || mark[1] == 0xff || mark[1] == 0x00 {
// Would also be valid to just keep re-reading to find the marker
return 0, fmt.Errorf("invalid marker")
}
marker := int(mark[0])<<8 + int(mark[1])
if marker == 0 {
return 0, fmt.Errorf("invalid jpeg header")
}
sign, err := read2Bytes(r)
if err != nil {
return 0, err
}
length := int(sign[0])<<8 + int(sign[1]) - 2
if length < 0 {
return 0, fmt.Errorf("short segment read")
}
if (marker & 0xff) != 0xdb { // not a quantization table
if _, err := r.Seek(int64(length), 1); err != nil {
return 0, err
}
continue
}
if length%65 != 0 {
return 0, fmt.Errorf("wrong size for quantization table")
}
tabuf := make([]byte, length)
n, err := r.Read(tabuf)
if err != nil {
return 0, err
}
tabuf = tabuf[0:n]
allones := 1
var reftable []int
var cumsf, cumsf2 float64
for a := 0; a < n; {
tableindex := int(tabuf[a] & 0x0f)
a++
if tableindex < 2 {
reftable = defaultTables[tableindex]
}
// Read in the table, compute statistics relative to reference table
if a+64 > n {
return 0, fmt.Errorf("DQT segment too short")
}
for coefindex := 0; coefindex < 64 && a < n; coefindex++ {
var val int
if tableindex>>4 != 0 {
temp := int(tabuf[a])
a++
temp *= 256
val = int(tabuf[a]) + temp
a++
} else {
val = int(tabuf[a])
a++
}
// scaling factor in percent
x := 100.0 * float64(val) / float64(reftable[coefindex])
cumsf += x
cumsf2 += x * x
// separate check for all-ones table (Q 100)
if val != 1 {
allones = 0
}
}
if 0 != len(reftable) { // terse output includes quality
var qual float64
cumsf /= 64.0 // mean scale factor
cumsf2 /= 64.0
if allones == 1 { // special case for all-ones table
qual = 100.0
} else if cumsf <= 100.0 {
qual = (200.0 - cumsf) / 2.0
} else {
qual = 5000.0 / cumsf
}
if tableindex == 0 {
return (int)(qual + 0.5), nil
}
}
}
}
}
|
