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writer.go
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writer.go
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// ◄◄◄ gobmp/writer.go ►►►
// Copyright © 2012 Jason Summers
// Use of this code is governed by an MIT-style license that can
// be found in the readme.md file.
//
// BMP file encoder
//
package gobmp
import "image"
import "io"
// EncoderOptions stores options that can be passed to EncodeWithOptions().
// Create an EncoderOptions object with new().
type EncoderOptions struct {
densitySet bool
xDens, yDens int
supportTrns bool
}
// SetDensity sets the density to write to the output image's metadata, in
// pixels per meter.
func (opts *EncoderOptions) SetDensity(xDens, yDens int) {
opts.densitySet = true
opts.xDens = xDens
opts.yDens = yDens
}
// SupportTransparency indicates whether to retain transparency information
// when writing the BMP file. Transparency requires the use of a
// not-so-portable version of BMP.
func (opts *EncoderOptions) SupportTransparency(t bool) {
opts.supportTrns = t
}
type encoder struct {
opts *EncoderOptions
w io.Writer
m image.Image
m_AsPaletted *image.Paletted
srcBounds image.Rectangle
width int
height int
dstStride int
dstBitsSize int
dstBitCount int
dstBitsOffset int
dstFileSize int
writeAlpha bool
writePaletted bool
srcIsGray bool
nColors int // Number of colors in palette; 0 if no palette
headerSize int // 40 (for BMPv3) or 124 (for BMPv5)
}
func setWORD(b []byte, n uint16) {
b[0] = byte(n)
b[1] = byte(n >> 8)
}
func setDWORD(b []byte, n uint32) {
b[0] = byte(n)
b[1] = byte(n >> 8)
b[2] = byte(n >> 16)
b[3] = byte(n >> 24)
}
// Write the BITMAPFILEHEADER structure to a slice[14].
func (e *encoder) generateFileHeader(h []byte) {
h[0] = 0x42 // 'B'
h[1] = 0x4d // 'M'
setDWORD(h[2:6], uint32(e.dstFileSize))
setDWORD(h[10:14], uint32(e.dstBitsOffset))
}
// Write the BITMAPINFOHEADER structure to a slice[40] or [124].
func (e *encoder) generateInfoHeader(h []byte) {
setDWORD(h[0:4], uint32(e.headerSize))
setDWORD(h[4:8], uint32(e.width))
setDWORD(h[8:12], uint32(e.height))
setWORD(h[12:14], 1) // biPlanes
setWORD(h[14:16], uint16(e.dstBitCount))
if e.writeAlpha {
setWORD(h[16:20], 3) // "Compression" = BI_BITFIELDS
}
setDWORD(h[20:24], uint32(e.dstBitsSize))
if e.opts.densitySet {
setDWORD(h[24:28], uint32(e.opts.xDens))
setDWORD(h[28:32], uint32(e.opts.yDens))
} else {
setDWORD(h[24:28], 2835)
setDWORD(h[28:32], 2835)
}
setDWORD(h[32:36], uint32(e.nColors))
if len(h) == 124 {
// Set V5 header fields
setDWORD(h[40:44], 0x00ff0000) // RedMask
setDWORD(h[44:48], 0x0000ff00) // GreenMask
setDWORD(h[48:52], 0x000000ff) // BlueMask
setDWORD(h[52:56], 0xff000000) // AlphaMask
setDWORD(h[56:60], 0x73524742) // CSType = sRGB
setDWORD(h[108:112], 4) // Intent = IMAGES (perceptual)
}
}
func (e *encoder) writeHeaders() error {
h := make([]byte, 14+e.headerSize)
e.generateFileHeader(h[:14])
e.generateInfoHeader(h[14:])
_, err := e.w.Write(h[:])
return err
}
func (e *encoder) writePalette() error {
if !e.writePaletted {
return nil
}
pal := make([]uint8, 4*e.nColors)
for i := 0; i < e.nColors; i++ {
var r, g, b uint32
if e.srcIsGray {
// Manufacture a grayscale palette.
r = uint32(i) << 8
g, b = r, r
} else {
r, g, b, _ = e.m_AsPaletted.Palette[i].RGBA()
}
pal[4*i+0] = uint8(b >> 8)
pal[4*i+1] = uint8(g >> 8)
pal[4*i+2] = uint8(r >> 8)
}
_, err := e.w.Write(pal)
return err
}
// Read a row from the (paletted) source image, and store it in rowBuf in 1-bit
// BMP format.
func generateRow_1(e *encoder, j int, rowBuf []byte) {
for i := range rowBuf {
rowBuf[i] = 0
}
for i := 0; i < e.width; i++ {
if e.m_AsPaletted.Pix[j*e.m_AsPaletted.Stride+i] != 0 {
rowBuf[i/8] |= uint8(1 << uint(7-i%8))
}
}
}
// Read a row from the (paletted) source image, and store it in rowBuf in 4-bit
// BMP format.
func generateRow_4(e *encoder, j int, rowBuf []byte) {
for i := range rowBuf {
rowBuf[i] = 0
}
for i := 0; i < e.width; i++ {
v := e.m_AsPaletted.Pix[j*e.m_AsPaletted.Stride+i]
if i%2 == 0 {
v <<= 4
}
rowBuf[i/2] |= v
}
}
// Read a row from the (paletted) source image, and store it in rowBuf in 8-bit
// BMP format.
func generateRow_8(e *encoder, j int, rowBuf []byte) {
copy(rowBuf[0:e.width], e.m_AsPaletted.Pix[j*e.m_AsPaletted.Stride:])
}
// Read a row from the (grayscale) source image, and store it in rowBuf in
// 8-bit BMP format.
func generateRow_GrayPal(e *encoder, j int, rowBuf []byte) {
for i := 0; i < e.width; i++ {
srcclr := e.m.At(e.srcBounds.Min.X+i, e.srcBounds.Min.Y+j)
r, _, _, _ := srcclr.RGBA()
rowBuf[i] = uint8(r >> 8)
}
}
// Read a row from the source image, and store it in rowBuf in 24-bit BMP format.
func generateRow_24(e *encoder, j int, rowBuf []byte) {
var s [3]uint32
for i := 0; i < e.width; i++ {
srcclr := e.m.At(e.srcBounds.Min.X+i, e.srcBounds.Min.Y+j)
s[2], s[1], s[0], _ = srcclr.RGBA()
for k := 0; k < 3; k++ {
rowBuf[i*3+k] = uint8(s[k] >> 8)
}
}
}
// Read a row from the source image, and store it in rowBuf in 32-bit BMP format.
func generateRow_32(e *encoder, j int, rowBuf []byte) {
var s [4]uint32
for i := 0; i < e.width; i++ {
srcclr := e.m.At(e.srcBounds.Min.X+i, e.srcBounds.Min.Y+j)
s[2], s[1], s[0], s[3] = srcclr.RGBA()
for k := 0; k < 4; k++ {
if s[3] == 0 {
rowBuf[i*4+k] = 0
} else if k == 3 || s[3] == 0xffff {
rowBuf[i*4+k] = uint8(s[k] >> 8)
} else {
// Convert to unassociated alpha
rowBuf[i*4+k] = uint8(0.5 + 255.0*(float64(s[k])/float64(s[3])))
}
}
}
}
func (e *encoder) writeBits() error {
var err error
var genRowFunc func(e *encoder, j int, rowBuf []byte)
if e.writePaletted {
if e.srcIsGray {
genRowFunc = generateRow_GrayPal
} else {
switch e.dstBitCount {
case 1:
genRowFunc = generateRow_1
case 4:
genRowFunc = generateRow_4
default:
genRowFunc = generateRow_8
}
}
} else {
if e.dstBitCount == 32 {
genRowFunc = generateRow_32
} else {
genRowFunc = generateRow_24
}
}
rowBuf := make([]byte, e.dstStride)
for j := 0; j < e.height; j++ {
genRowFunc(e, e.height-j-1, rowBuf)
_, err = e.w.Write(rowBuf)
if err != nil {
return err
}
}
return nil
}
// If the image can be written as a paletted image, sets e.writePaletted
// to true, and sets related fields.
func (e *encoder) checkPaletted() {
if e.writeAlpha {
return
}
switch e.m.(type) {
case *image.Paletted:
e.m_AsPaletted = e.m.(*image.Paletted)
e.nColors = len(e.m_AsPaletted.Palette)
if e.nColors < 1 || e.nColors > 256 {
e.m_AsPaletted = nil
e.nColors = 0
return
}
e.writePaletted = true
case *image.Gray, *image.Gray16:
e.srcIsGray = true
e.writePaletted = true
e.nColors = 256
}
}
func (e *encoder) srcIsOpaque() bool {
switch e.m.(type) {
// If the image's type doesn't even support transparency, it must be opaque.
case *image.YCbCr, *image.Gray, *image.Gray16:
return true
}
for j := e.srcBounds.Min.Y; j < e.srcBounds.Max.Y; j++ {
for i := e.srcBounds.Min.X; i < e.srcBounds.Max.X; i++ {
_, _, _, a := e.m.At(i, j).RGBA()
if a < 0xffff {
return false
}
}
}
return true
}
// Plot out the structure of the file that we're going to write.
func (e *encoder) strategize() error {
e.srcBounds = e.m.Bounds()
e.width = e.srcBounds.Dx()
e.height = e.srcBounds.Dy()
if e.opts.supportTrns && !e.srcIsOpaque() {
e.writeAlpha = true
e.headerSize = 124
} else {
e.headerSize = 40
}
e.checkPaletted()
if e.writePaletted {
if e.nColors <= 2 {
e.dstBitCount = 1
} else if e.nColors <= 16 {
e.dstBitCount = 4
} else {
e.dstBitCount = 8
}
} else {
if e.writeAlpha {
e.dstBitCount = 32
} else {
e.dstBitCount = 24
}
}
e.dstStride = ((e.width*e.dstBitCount + 31) / 32) * 4
e.dstBitsOffset = 14 + e.headerSize + 4*e.nColors
e.dstBitsSize = e.height * e.dstStride
e.dstFileSize = e.dstBitsOffset + e.dstBitsSize
return nil
}
// EncodeWithOptions writes the Image m to w in BMP format, using the options
// recorded in opts.
// opts may be nil, in which case it behaves the same as Encode.
func EncodeWithOptions(w io.Writer, m image.Image, opts *EncoderOptions) error {
var err error
e := new(encoder)
e.w = w
e.m = m
if opts != nil {
e.opts = opts
} else {
e.opts = new(EncoderOptions)
}
err = e.strategize()
if err != nil {
return err
}
err = e.writeHeaders()
if err != nil {
return err
}
err = e.writePalette()
if err != nil {
return err
}
err = e.writeBits()
if err != nil {
return err
}
return nil
}
// Encode writes the Image m to w in BMP format.
func Encode(w io.Writer, m image.Image) error {
return EncodeWithOptions(w, m, nil)
}