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150 lines
3.3 KiB
Go
150 lines
3.3 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 tiff
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// A tiff image file contains one or more images. The metadata
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// of each image is contained in an Image File Directory (IFD),
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// which contains entries of 12 bytes each and is described
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// on page 14-16 of the specification. An IFD entry consists of
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//
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// - a tag, which describes the signification of the entry,
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// - the data type and length of the entry,
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// - the data itself or a pointer to it if it is more than 4 bytes.
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//
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// The presence of a length means that each IFD is effectively an array.
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const (
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leHeader = "II\x2A\x00" // Header for little-endian files.
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beHeader = "MM\x00\x2A" // Header for big-endian files.
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ifdLen = 12 // Length of an IFD entry in bytes.
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)
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// Data types (p. 14-16 of the spec).
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const (
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dtByte = 1
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dtASCII = 2
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dtShort = 3
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dtLong = 4
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dtRational = 5
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)
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// The length of one instance of each data type in bytes.
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var lengths = [...]uint32{0, 1, 1, 2, 4, 8}
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// Tags (see p. 28-41 of the spec).
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const (
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tImageWidth = 256
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tImageLength = 257
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tBitsPerSample = 258
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tCompression = 259
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tPhotometricInterpretation = 262
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tFillOrder = 266
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tStripOffsets = 273
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tSamplesPerPixel = 277
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tRowsPerStrip = 278
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tStripByteCounts = 279
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tT4Options = 292 // CCITT Group 3 options, a set of 32 flag bits.
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tT6Options = 293 // CCITT Group 4 options, a set of 32 flag bits.
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tTileWidth = 322
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tTileLength = 323
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tTileOffsets = 324
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tTileByteCounts = 325
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tXResolution = 282
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tYResolution = 283
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tResolutionUnit = 296
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tPredictor = 317
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tColorMap = 320
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tExtraSamples = 338
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tSampleFormat = 339
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)
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// Compression types (defined in various places in the spec and supplements).
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const (
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cNone = 1
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cCCITT = 2
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cG3 = 3 // Group 3 Fax.
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cG4 = 4 // Group 4 Fax.
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cLZW = 5
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cJPEGOld = 6 // Superseded by cJPEG.
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cJPEG = 7
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cDeflate = 8 // zlib compression.
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cPackBits = 32773
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cDeflateOld = 32946 // Superseded by cDeflate.
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)
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// Photometric interpretation values (see p. 37 of the spec).
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const (
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pWhiteIsZero = 0
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pBlackIsZero = 1
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pRGB = 2
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pPaletted = 3
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pTransMask = 4 // transparency mask
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pCMYK = 5
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pYCbCr = 6
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pCIELab = 8
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)
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// Values for the tPredictor tag (page 64-65 of the spec).
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const (
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prNone = 1
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prHorizontal = 2
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)
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// Values for the tResolutionUnit tag (page 18).
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const (
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resNone = 1
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resPerInch = 2 // Dots per inch.
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resPerCM = 3 // Dots per centimeter.
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)
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// imageMode represents the mode of the image.
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type imageMode int
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const (
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mBilevel imageMode = iota
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mPaletted
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mGray
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mGrayInvert
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mRGB
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mRGBA
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mNRGBA
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mCMYK
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)
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// CompressionType describes the type of compression used in Options.
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type CompressionType int
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// Constants for supported compression types.
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const (
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Uncompressed CompressionType = iota
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Deflate
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LZW
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CCITTGroup3
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CCITTGroup4
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)
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// specValue returns the compression type constant from the TIFF spec that
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// is equivalent to c.
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func (c CompressionType) specValue() uint32 {
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switch c {
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case LZW:
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return cLZW
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case Deflate:
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return cDeflate
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case CCITTGroup3:
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return cG3
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case CCITTGroup4:
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return cG4
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}
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return cNone
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}
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