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// Copyright 2011 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 bmp implements a BMP image decoder.
//
// The BMP specification is at http://www.digicamsoft.com/bmp/bmp.html.
package bmp
import (
"image"
"io"
"os"
)
// ErrUnsupported means that the input BMP image uses a valid but unsupported
// feature.
var ErrUnsupported = os.NewError("bmp: unsupported BMP image")
func readUint16(b []byte) uint16 {
return uint16(b[0]) | uint16(b[1])<<8
}
func readUint32(b []byte) uint32 {
return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
}
// decodePaletted reads an 8 bit-per-pixel BMP image from r.
func decodePaletted(r io.Reader, c image.Config) (image.Image, os.Error) {
var tmp [4]byte
paletted := image.NewPaletted(c.Width, c.Height, c.ColorModel.(image.PalettedColorModel))
// BMP images are stored bottom-up rather than top-down.
for y := c.Height - 1; y >= 0; y-- {
p := paletted.Pix[y*paletted.Stride : y*paletted.Stride+c.Width]
_, err := io.ReadFull(r, p)
if err != nil {
return nil, err
}
// Each row is 4-byte aligned.
if c.Width%4 != 0 {
_, err := io.ReadFull(r, tmp[:4-c.Width%4])
if err != nil {
return nil, err
}
}
}
return paletted, nil
}
// decodeRGBA reads a 24 bit-per-pixel BMP image from r.
func decodeRGBA(r io.Reader, c image.Config) (image.Image, os.Error) {
rgba := image.NewRGBA(c.Width, c.Height)
// There are 3 bytes per pixel, and each row is 4-byte aligned.
b := make([]byte, (3*c.Width+3)&^3)
// BMP images are stored bottom-up rather than top-down.
for y := c.Height - 1; y >= 0; y-- {
_, err := io.ReadFull(r, b)
if err != nil {
return nil, err
}
p := rgba.Pix[y*rgba.Stride : y*rgba.Stride+c.Width*4]
for i, j := 0, 0; i < len(p); i, j = i+4, j+3 {
// BMP images are stored in BGR order rather than RGB order.
p[i+0] = b[j+2]
p[i+1] = b[j+1]
p[i+2] = b[j+0]
p[i+3] = 0xFF
}
}
return rgba, nil
}
// Decode reads a BMP image from r and returns it as an image.Image.
// Limitation: The file must be 8 or 24 bits per pixel.
func Decode(r io.Reader) (image.Image, os.Error) {
c, err := DecodeConfig(r)
if err != nil {
return nil, err
}
if c.ColorModel == image.RGBAColorModel {
return decodeRGBA(r, c)
}
return decodePaletted(r, c)
}
// DecodeConfig returns the color model and dimensions of a BMP image without
// decoding the entire image.
// Limitation: The file must be 8 or 24 bits per pixel.
func DecodeConfig(r io.Reader) (config image.Config, err os.Error) {
// We only support those BMP images that are a BITMAPFILEHEADER
// immediately followed by a BITMAPINFOHEADER.
const (
fileHeaderLen = 14
infoHeaderLen = 40
)
var b [1024]byte
if _, err = io.ReadFull(r, b[:fileHeaderLen+infoHeaderLen]); err != nil {
return
}
if string(b[:2]) != "BM" {
err = os.NewError("bmp: invalid format")
return
}
offset := readUint32(b[10:14])
if readUint32(b[14:18]) != infoHeaderLen {
err = ErrUnsupported
return
}
width := int(readUint32(b[18:22]))
height := int(readUint32(b[22:26]))
if width < 0 || height < 0 {
err = ErrUnsupported
return
}
// We only support 1 plane, 8 or 24 bits per pixel and no compression.
planes, bpp, compression := readUint16(b[26:28]), readUint16(b[28:30]), readUint32(b[30:34])
if planes != 1 || compression != 0 {
err = ErrUnsupported
return
}
switch bpp {
case 8:
if offset != fileHeaderLen+infoHeaderLen+256*4 {
err = ErrUnsupported
return
}
_, err = io.ReadFull(r, b[:256*4])
if err != nil {
return
}
pcm := make(image.PalettedColorModel, 256)
for i := range pcm {
// BMP images are stored in BGR order rather than RGB order.
// Every 4th byte is padding.
pcm[i] = image.RGBAColor{b[4*i+2], b[4*i+1], b[4*i+0], 0xFF}
}
return image.Config{pcm, width, height}, nil
case 24:
if offset != fileHeaderLen+infoHeaderLen {
err = ErrUnsupported
return
}
return image.Config{image.RGBAColorModel, width, height}, nil
}
err = ErrUnsupported
return
}
func init() {
image.RegisterFormat("bmp", "BM????\x00\x00\x00\x00", Decode, DecodeConfig)
}
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