func (d *decoder) parsePLTE(r io.Reader, crc hash.Hash32, length uint32) os.Error {
np := int(length / 3) // The number of palette entries.
if length%3 != 0 || np <= 0 || np > 256 {
return FormatError("bad PLTE length")
}
n, err := io.ReadFull(r, d.tmp[0:3*np])
if err != nil {
return err
}
crc.Write(d.tmp[0:n])
switch d.colorType {
case ctPaletted:
palette := make([]image.Color, np)
for i := 0; i < np; i++ {
palette[i] = image.RGBAColor{d.tmp[3*i+0], d.tmp[3*i+1], d.tmp[3*i+2], 0xff}
}
d.image.(*image.Paletted).Palette = image.PalettedColorModel(palette)
case ctTrueColor, ctTrueColorAlpha:
// As per the PNG spec, a PLTE chunk is optional (and for practical purposes,
// ignorable) for the ctTrueColor and ctTrueColorAlpha color types (section 4.1.2).
return nil
default:
return FormatError("PLTE, color type mismatch")
}
return nil
}
func render(x, y, z int) []byte {
// tileX and tileY is the absolute position of this tile at the current zoom level.
tileX, tileY := x*tileSize, y*tileSize
scale := 1 / float64(int(1<<uint(z))*tileSize)
img := image.NewPaletted(tileSize, tileSize, image.PalettedColorModel(color[:]))
for i := 0; i < tileSize; i++ {
for j := 0; j < tileSize; j++ {
c := complex(float64(tileX+i)*scale, float64(tileY+j)*scale)
img.SetColorIndex(i, j, mandelbrotValue(c))
}
}
buf := new(bytes.Buffer)
png.Encode(buf, img)
return buf.Bytes()
}
func newDecoder(r io.Reader) (*decoder, os.Error) {
d := &decoder{
r: newReaderAt(r),
features: make(map[int][]uint),
}
p := make([]byte, 8)
if _, err := d.r.ReadAt(p, 0); err != nil {
return nil, err
}
switch string(p[0:4]) {
case leHeader:
d.byteOrder = binary.LittleEndian
case beHeader:
d.byteOrder = binary.BigEndian
default:
return nil, FormatError("malformed header")
}
ifdOffset := int64(d.byteOrder.Uint32(p[4:8]))
// The first two bytes contain the number of entries (12 bytes each).
if _, err := d.r.ReadAt(p[0:2], ifdOffset); err != nil {
return nil, err
}
numItems := int(d.byteOrder.Uint16(p[0:2]))
// All IFD entries are read in one chunk.
p = make([]byte, ifdLen*numItems)
if _, err := d.r.ReadAt(p, ifdOffset+2); err != nil {
return nil, err
}
for i := 0; i < len(p); i += ifdLen {
if err := d.parseIFD(p[i : i+ifdLen]); err != nil {
return nil, err
}
}
d.config.Width = int(d.firstVal(tImageWidth))
d.config.Height = int(d.firstVal(tImageLength))
// Determine the image mode.
switch d.firstVal(tPhotometricInterpretation) {
case pRGB:
d.config.ColorModel = image.RGBAColorModel
// RGB images normally have 3 samples per pixel.
// If there are more, ExtraSamples (p. 31-32 of the spec)
// gives their meaning (usually an alpha channel).
switch len(d.features[tBitsPerSample]) {
case 3:
d.mode = mRGB
case 4:
switch d.firstVal(tExtraSamples) {
case 1:
d.mode = mRGBA
case 2:
d.mode = mNRGBA
d.config.ColorModel = image.NRGBAColorModel
default:
// The extra sample is discarded.
d.mode = mRGB
}
default:
return nil, FormatError("wrong number of samples for RGB")
}
case pPaletted:
d.mode = mPaletted
d.config.ColorModel = image.PalettedColorModel(d.palette)
case pWhiteIsZero:
d.mode = mGrayInvert
d.config.ColorModel = image.GrayColorModel
case pBlackIsZero:
d.mode = mGray
d.config.ColorModel = image.GrayColorModel
default:
return nil, UnsupportedError("color model")
}
if _, ok := d.features[tBitsPerSample]; !ok {
return nil, FormatError("BitsPerSample tag missing")
}
for _, b := range d.features[tBitsPerSample] {
if b != 8 {
return nil, UnsupportedError("not an 8-bit image")
}
}
return d, nil
}