golang-image/tiff/writer.go
2012-03-07 10:32:33 +11:00

167 lines
4.5 KiB
Go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package tiff
import (
"encoding/binary"
"image"
"io"
"sort"
)
// The TIFF format allows to choose the order of the different elements freely.
// The basic structure of a TIFF file written by this package is:
//
// 1. Header (8 bytes).
// 2. Image data.
// 3. Image File Directory (IFD).
// 4. "Pointer area" for larger entries in the IFD.
// We only write little-endian TIFF files.
var enc = binary.LittleEndian
// An ifdEntry is a single entry in an Image File Directory.
// A value of type dtRational is composed of two 32-bit values,
// thus data contains two uints (numerator and denominator) for a single number.
type ifdEntry struct {
tag int
datatype int
data []uint32
}
func (e ifdEntry) putData(p []byte) {
for _, d := range e.data {
switch e.datatype {
case dtByte, dtASCII:
p[0] = byte(d)
p = p[1:]
case dtShort:
enc.PutUint16(p, uint16(d))
p = p[2:]
case dtLong, dtRational:
enc.PutUint32(p, uint32(d))
p = p[4:]
}
}
}
type byTag []ifdEntry
func (d byTag) Len() int { return len(d) }
func (d byTag) Less(i, j int) bool { return d[i].tag < d[j].tag }
func (d byTag) Swap(i, j int) { d[i], d[j] = d[j], d[i] }
func writeImgData(w io.Writer, m image.Image) error {
bounds := m.Bounds()
buf := make([]byte, 4*bounds.Dx())
for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
i := 0
for x := bounds.Min.X; x < bounds.Max.X; x++ {
r, g, b, a := m.At(x, y).RGBA()
buf[i+0] = uint8(r >> 8)
buf[i+1] = uint8(g >> 8)
buf[i+2] = uint8(b >> 8)
buf[i+3] = uint8(a >> 8)
i += 4
}
if _, err := w.Write(buf); err != nil {
return err
}
}
return nil
}
func writeIFD(w io.Writer, ifdOffset int, d []ifdEntry) error {
var buf [ifdLen]byte
// Make space for "pointer area" containing IFD entry data
// longer than 4 bytes.
parea := make([]byte, 1024)
pstart := ifdOffset + ifdLen*len(d) + 6
var o int // Current offset in parea.
// The IFD has to be written with the tags in ascending order.
sort.Sort(byTag(d))
// Write the number of entries in this IFD.
if err := binary.Write(w, enc, uint16(len(d))); err != nil {
return err
}
for _, ent := range d {
enc.PutUint16(buf[0:2], uint16(ent.tag))
enc.PutUint16(buf[2:4], uint16(ent.datatype))
count := uint32(len(ent.data))
if ent.datatype == dtRational {
count /= 2
}
enc.PutUint32(buf[4:8], count)
datalen := int(count * lengths[ent.datatype])
if datalen <= 4 {
ent.putData(buf[8:12])
} else {
if (o + datalen) > len(parea) {
newlen := len(parea) + 1024
for (o + datalen) > newlen {
newlen += 1024
}
newarea := make([]byte, newlen)
copy(newarea, parea)
parea = newarea
}
ent.putData(parea[o : o+datalen])
enc.PutUint32(buf[8:12], uint32(pstart+o))
o += datalen
}
if _, err := w.Write(buf[:]); err != nil {
return err
}
}
// The IFD ends with the offset of the next IFD in the file,
// or zero if it is the last one (page 14).
if err := binary.Write(w, enc, uint32(0)); err != nil {
return err
}
_, err := w.Write(parea[:o])
return err
}
// Encode writes the image m to w in uncompressed RGBA format.
func Encode(w io.Writer, m image.Image) error {
_, err := io.WriteString(w, leHeader)
if err != nil {
return err
}
bounds := m.Bounds()
width, height := bounds.Dx(), bounds.Dy()
// imageLen is the length of the image data in bytes.
imageLen := width * height * 4
ifdOffset := imageLen + 8 // 8 bytes for TIFF header.
err = binary.Write(w, enc, uint32(ifdOffset))
if err != nil {
return err
}
err = writeImgData(w, m)
if err != nil {
return err
}
return writeIFD(w, ifdOffset, []ifdEntry{
{tImageWidth, dtShort, []uint32{uint32(width)}},
{tImageLength, dtShort, []uint32{uint32(height)}},
{tBitsPerSample, dtShort, []uint32{8, 8, 8, 8}},
{tCompression, dtShort, []uint32{cNone}},
{tPhotometricInterpretation, dtShort, []uint32{pRGB}},
{tStripOffsets, dtLong, []uint32{8}},
{tSamplesPerPixel, dtShort, []uint32{4}},
{tRowsPerStrip, dtShort, []uint32{uint32(height)}},
{tStripByteCounts, dtLong, []uint32{uint32(imageLen)}},
// There is currently no support for storing the image
// resolution, so give a bogus value of 72x72 dpi.
{tXResolution, dtRational, []uint32{72, 1}},
{tYResolution, dtRational, []uint32{72, 1}},
{tResolutionUnit, dtShort, []uint32{resPerInch}},
{tExtraSamples, dtShort, []uint32{1}}, // RGBA.
})
}