image/bmp: implement a BMP decoder.

R=r
CC=golang-dev
https://golang.org/cl/4521054

src/pkg/Makefile

@@ -106,6 +106,7 @@ DIRS=\
 	http/httptest\
 	http/spdy\
 	image\
+	image/bmp\
 	image/gif\
 	image/jpeg\
 	image/png\
@@ -189,6 +190,7 @@ NOTEST+=\
 	hash\
 	http/pprof\
 	http/httptest\
+	image/bmp\
 	image/gif\
 	net/dict\
 	rand\

src/pkg/image/bmp/Makefile

+# 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.
+
+include ../../../Make.inc
+
+TARG=image/bmp
+GOFILES=\
+	reader.go\
+
+include ../../../Make.pkg

src/pkg/image/bmp/reader.go

+// 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]
+		for x := range p {
+			// BMP images are stored in BGR order rather than RGB order.
+			p[x] = image.RGBAColor{b[3*x+2], b[3*x+1], b[3*x+0], 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)
+}

src/pkg/image/decode_test.go

@@ -10,7 +10,7 @@ import (
 	"os"
 	"testing"
 
-	// TODO(nigeltao): implement bmp decoder.
+	_ "image/bmp"
 	_ "image/gif"
 	_ "image/jpeg"
 	_ "image/png"
@@ -25,7 +25,7 @@ type imageTest struct {
 }
 
 var imageTests = []imageTest{
-	//{"testdata/video-001.bmp", 0},
+	{"testdata/video-001.bmp", 0},
 	// GIF images are restricted to a 256-color palette and the conversion
 	// to GIF loses significant image quality.
 	{"testdata/video-001.gif", 64 << 8},