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