go.image/bmp: support decoding 32-bit BMPs, as well as top-down BMPs.
Tested manually, from "dwebp -bmp" output. dwebp comes from
https://developers.google.com/speed/webp/download
LGTM=bsiegert
R=bsiegert
CC=golang-codereviews
https://golang.org/cl/148020043
diff --git a/bmp/reader.go b/bmp/reader.go
index a124018..4c96d44 100644
--- a/bmp/reader.go
+++ b/bmp/reader.go
@@ -27,14 +27,17 @@
}
// decodePaletted reads an 8 bit-per-pixel BMP image from r.
-func decodePaletted(r io.Reader, c image.Config) (image.Image, error) {
+// If topDown is false, the image rows will be read bottom-up.
+func decodePaletted(r io.Reader, c image.Config, topDown bool) (image.Image, error) {
var tmp [4]byte
paletted := image.NewPaletted(image.Rect(0, 0, c.Width, c.Height), c.ColorModel.(color.Palette))
- // BMP images are stored bottom-up rather than top-down.
- for y := c.Height - 1; y >= 0; y-- {
+ y0, y1, yDelta := c.Height-1, -1, -1
+ if topDown {
+ y0, y1, yDelta = 0, c.Height, +1
+ }
+ for y := y0; y != y1; y += yDelta {
p := paletted.Pix[y*paletted.Stride : y*paletted.Stride+c.Width]
- _, err := io.ReadFull(r, p)
- if err != nil {
+ if _, err := io.ReadFull(r, p); err != nil {
return nil, err
}
// Each row is 4-byte aligned.
@@ -48,15 +51,18 @@
return paletted, nil
}
-// decodeRGBA reads a 24 bit-per-pixel BMP image from r.
-func decodeRGBA(r io.Reader, c image.Config) (image.Image, error) {
+// decodeRGB reads a 24 bit-per-pixel BMP image from r.
+// If topDown is false, the image rows will be read bottom-up.
+func decodeRGB(r io.Reader, c image.Config, topDown bool) (image.Image, error) {
rgba := image.NewRGBA(image.Rect(0, 0, 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 {
+ y0, y1, yDelta := c.Height-1, -1, -1
+ if topDown {
+ y0, y1, yDelta = 0, c.Height, +1
+ }
+ for y := y0; y != y1; y += yDelta {
+ if _, err := io.ReadFull(r, b); err != nil {
return nil, err
}
p := rgba.Pix[y*rgba.Stride : y*rgba.Stride+c.Width*4]
@@ -71,23 +77,54 @@
return rgba, nil
}
+// decodeNRGBA reads a 32 bit-per-pixel BMP image from r.
+// If topDown is false, the image rows will be read bottom-up.
+func decodeNRGBA(r io.Reader, c image.Config, topDown bool) (image.Image, error) {
+ rgba := image.NewNRGBA(image.Rect(0, 0, c.Width, c.Height))
+ y0, y1, yDelta := c.Height-1, -1, -1
+ if topDown {
+ y0, y1, yDelta = 0, c.Height, +1
+ }
+ for y := y0; y != y1; y += yDelta {
+ p := rgba.Pix[y*rgba.Stride : y*rgba.Stride+c.Width*4]
+ if _, err := io.ReadFull(r, p); err != nil {
+ return nil, err
+ }
+ for i := 0; i < len(p); i += 4 {
+ // BMP images are stored in BGRA order rather than RGBA order.
+ p[i+0], p[i+2] = p[i+2], p[i+0]
+ }
+ }
+ 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.
+// Limitation: The file must be 8, 24 or 32 bits per pixel.
func Decode(r io.Reader) (image.Image, error) {
- c, err := DecodeConfig(r)
+ c, bpp, topDown, err := decodeConfig(r)
if err != nil {
return nil, err
}
- if c.ColorModel == color.RGBAModel {
- return decodeRGBA(r, c)
+ switch bpp {
+ case 8:
+ return decodePaletted(r, c, topDown)
+ case 24:
+ return decodeRGB(r, c, topDown)
+ case 32:
+ return decodeNRGBA(r, c, topDown)
}
- return decodePaletted(r, c)
+ panic("unreachable")
}
// 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 error) {
+// Limitation: The file must be 8, 24 or 32 bits per pixel.
+func DecodeConfig(r io.Reader) (image.Config, error) {
+ config, _, _, err := decodeConfig(r)
+ return config, err
+}
+
+func decodeConfig(r io.Reader) (config image.Config, bitsPerPixel int, topDown bool, err error) {
// We only support those BMP images that are a BITMAPFILEHEADER
// immediately followed by a BITMAPINFOHEADER.
const (
@@ -95,39 +132,37 @@
infoHeaderLen = 40
)
var b [1024]byte
- if _, err = io.ReadFull(r, b[:fileHeaderLen+infoHeaderLen]); err != nil {
- return
+ if _, err := io.ReadFull(r, b[:fileHeaderLen+infoHeaderLen]); err != nil {
+ return image.Config{}, 0, false, err
}
if string(b[:2]) != "BM" {
- err = errors.New("bmp: invalid format")
- return
+ return image.Config{}, 0, false, errors.New("bmp: invalid format")
}
offset := readUint32(b[10:14])
if readUint32(b[14:18]) != infoHeaderLen {
- err = ErrUnsupported
- return
+ return image.Config{}, 0, false, ErrUnsupported
}
- width := int(readUint32(b[18:22]))
- height := int(readUint32(b[22:26]))
+ width := int(int32(readUint32(b[18:22])))
+ height := int(int32(readUint32(b[22:26])))
+ if height < 0 {
+ height, topDown = -height, true
+ }
if width < 0 || height < 0 {
- err = ErrUnsupported
- return
+ return image.Config{}, 0, false, ErrUnsupported
}
// 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
+ return image.Config{}, 0, false, ErrUnsupported
}
switch bpp {
case 8:
if offset != fileHeaderLen+infoHeaderLen+256*4 {
- err = ErrUnsupported
- return
+ return image.Config{}, 0, false, ErrUnsupported
}
_, err = io.ReadFull(r, b[:256*4])
if err != nil {
- return
+ return image.Config{}, 0, false, err
}
pcm := make(color.Palette, 256)
for i := range pcm {
@@ -135,16 +170,19 @@
// Every 4th byte is padding.
pcm[i] = color.RGBA{b[4*i+2], b[4*i+1], b[4*i+0], 0xFF}
}
- return image.Config{ColorModel: pcm, Width: width, Height: height}, nil
+ return image.Config{ColorModel: pcm, Width: width, Height: height}, 8, topDown, nil
case 24:
if offset != fileHeaderLen+infoHeaderLen {
- err = ErrUnsupported
- return
+ return image.Config{}, 0, false, ErrUnsupported
}
- return image.Config{ColorModel: color.RGBAModel, Width: width, Height: height}, nil
+ return image.Config{ColorModel: color.RGBAModel, Width: width, Height: height}, 24, topDown, nil
+ case 32:
+ if offset != fileHeaderLen+infoHeaderLen {
+ return image.Config{}, 0, false, ErrUnsupported
+ }
+ return image.Config{ColorModel: color.RGBAModel, Width: width, Height: height}, 32, topDown, nil
}
- err = ErrUnsupported
- return
+ return image.Config{}, 0, false, ErrUnsupported
}
func init() {
