246 lines
8.3 KiB
Go
246 lines
8.3 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 and encoder.
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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 // import "golang.org/x/image/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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// If topDown is false, the image rows will be read bottom-up.
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func decodePaletted(r io.Reader, c image.Config, topDown bool) (image.Image, error) {
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paletted := image.NewPaletted(image.Rect(0, 0, c.Width, c.Height), c.ColorModel.(color.Palette))
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if c.Width == 0 || c.Height == 0 {
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return paletted, nil
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}
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var tmp [4]byte
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y0, y1, yDelta := c.Height-1, -1, -1
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if topDown {
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y0, y1, yDelta = 0, c.Height, +1
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}
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for y := y0; y != y1; y += yDelta {
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p := paletted.Pix[y*paletted.Stride : y*paletted.Stride+c.Width]
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if _, err := io.ReadFull(r, p); 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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// decodeRGB reads a 24 bit-per-pixel BMP image from r.
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// If topDown is false, the image rows will be read bottom-up.
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func decodeRGB(r io.Reader, c image.Config, topDown bool) (image.Image, error) {
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rgba := image.NewRGBA(image.Rect(0, 0, c.Width, c.Height))
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if c.Width == 0 || c.Height == 0 {
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return rgba, nil
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}
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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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y0, y1, yDelta := c.Height-1, -1, -1
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if topDown {
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y0, y1, yDelta = 0, c.Height, +1
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}
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for y := y0; y != y1; y += yDelta {
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if _, err := io.ReadFull(r, b); 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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// decodeNRGBA reads a 32 bit-per-pixel BMP image from r.
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// If topDown is false, the image rows will be read bottom-up.
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func decodeNRGBA(r io.Reader, c image.Config, topDown, allowAlpha bool) (image.Image, error) {
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rgba := image.NewNRGBA(image.Rect(0, 0, c.Width, c.Height))
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if c.Width == 0 || c.Height == 0 {
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return rgba, nil
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}
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y0, y1, yDelta := c.Height-1, -1, -1
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if topDown {
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y0, y1, yDelta = 0, c.Height, +1
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}
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for y := y0; y != y1; y += yDelta {
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p := rgba.Pix[y*rgba.Stride : y*rgba.Stride+c.Width*4]
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if _, err := io.ReadFull(r, p); err != nil {
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return nil, err
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}
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for i := 0; i < len(p); i += 4 {
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// BMP images are stored in BGRA order rather than RGBA order.
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p[i+0], p[i+2] = p[i+2], p[i+0]
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if !allowAlpha {
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p[i+3] = 0xFF
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}
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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, 24 or 32 bits per pixel.
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func Decode(r io.Reader) (image.Image, error) {
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c, bpp, topDown, allowAlpha, 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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switch bpp {
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case 8:
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return decodePaletted(r, c, topDown)
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case 24:
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return decodeRGB(r, c, topDown)
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case 32:
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return decodeNRGBA(r, c, topDown, allowAlpha)
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}
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panic("unreachable")
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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, 24 or 32 bits per pixel.
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func DecodeConfig(r io.Reader) (image.Config, error) {
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config, _, _, _, err := decodeConfig(r)
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return config, err
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}
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func decodeConfig(r io.Reader) (config image.Config, bitsPerPixel int, topDown bool, allowAlpha bool, err error) {
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// We only support those BMP images with one of the following DIB headers:
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// - BITMAPINFOHEADER (40 bytes)
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// - BITMAPV4HEADER (108 bytes)
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// - BITMAPV5HEADER (124 bytes)
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const (
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fileHeaderLen = 14
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infoHeaderLen = 40
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v4InfoHeaderLen = 108
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v5InfoHeaderLen = 124
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)
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var b [1024]byte
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if _, err := io.ReadFull(r, b[:fileHeaderLen+4]); err != nil {
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if err == io.EOF {
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err = io.ErrUnexpectedEOF
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}
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return image.Config{}, 0, false, false, err
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}
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if string(b[:2]) != "BM" {
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return image.Config{}, 0, false, false, errors.New("bmp: invalid format")
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}
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offset := readUint32(b[10:14])
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infoLen := readUint32(b[14:18])
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if infoLen != infoHeaderLen && infoLen != v4InfoHeaderLen && infoLen != v5InfoHeaderLen {
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return image.Config{}, 0, false, false, ErrUnsupported
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}
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if _, err := io.ReadFull(r, b[fileHeaderLen+4:fileHeaderLen+infoLen]); err != nil {
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if err == io.EOF {
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err = io.ErrUnexpectedEOF
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}
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return image.Config{}, 0, false, false, err
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}
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width := int(int32(readUint32(b[18:22])))
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height := int(int32(readUint32(b[22:26])))
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if height < 0 {
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height, topDown = -height, true
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}
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if width < 0 || height < 0 {
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return image.Config{}, 0, false, false, ErrUnsupported
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}
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// We only support 1 plane and 8, 24 or 32 bits per pixel and no
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// 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 compression is set to BI_BITFIELDS, but the bitmask is set to the default bitmask
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// that would be used if compression was set to 0, we can continue as if compression was 0
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if compression == 3 && infoLen > infoHeaderLen &&
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readUint32(b[54:58]) == 0xff0000 && readUint32(b[58:62]) == 0xff00 &&
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readUint32(b[62:66]) == 0xff && readUint32(b[66:70]) == 0xff000000 {
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compression = 0
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}
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if planes != 1 || compression != 0 {
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return image.Config{}, 0, false, false, ErrUnsupported
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}
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switch bpp {
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case 8:
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if offset != fileHeaderLen+infoLen+256*4 {
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return image.Config{}, 0, false, false, ErrUnsupported
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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 image.Config{}, 0, false, false, err
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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{ColorModel: pcm, Width: width, Height: height}, 8, topDown, false, nil
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case 24:
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if offset != fileHeaderLen+infoLen {
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return image.Config{}, 0, false, false, ErrUnsupported
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}
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return image.Config{ColorModel: color.RGBAModel, Width: width, Height: height}, 24, topDown, false, nil
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case 32:
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if offset != fileHeaderLen+infoLen {
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return image.Config{}, 0, false, false, ErrUnsupported
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}
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// 32 bits per pixel is possibly RGBX (X is padding) or RGBA (A is
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// alpha transparency). However, for BMP images, "Alpha is a
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// poorly-documented and inconsistently-used feature" says
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// https://source.chromium.org/chromium/chromium/src/+/bc0a792d7ebc587190d1a62ccddba10abeea274b:third_party/blink/renderer/platform/image-decoders/bmp/bmp_image_reader.cc;l=621
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//
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// That goes on to say "BITMAPV3HEADER+ have an alpha bitmask in the
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// info header... so we respect it at all times... [For earlier
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// (smaller) headers we] ignore alpha in Windows V3 BMPs except inside
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// ICO files".
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//
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// "Ignore" means to always set alpha to 0xFF (fully opaque):
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// https://source.chromium.org/chromium/chromium/src/+/bc0a792d7ebc587190d1a62ccddba10abeea274b:third_party/blink/renderer/platform/image-decoders/bmp/bmp_image_reader.h;l=272
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//
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// Confusingly, "Windows V3" does not correspond to BITMAPV3HEADER, but
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// instead corresponds to the earlier (smaller) BITMAPINFOHEADER:
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// https://source.chromium.org/chromium/chromium/src/+/bc0a792d7ebc587190d1a62ccddba10abeea274b:third_party/blink/renderer/platform/image-decoders/bmp/bmp_image_reader.cc;l=258
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//
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// This Go package does not support ICO files and the (infoLen >
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// infoHeaderLen) condition distinguishes BITMAPINFOHEADER (40 bytes)
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// vs later (larger) headers.
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allowAlpha = infoLen > infoHeaderLen
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return image.Config{ColorModel: color.RGBAModel, Width: width, Height: height}, 32, topDown, allowAlpha, nil
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}
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return image.Config{}, 0, false, false, ErrUnsupported
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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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