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tiff/consts.go (3.3K)
1 // Copyright 2011 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4
5 package tiff
6
7 // A tiff image file contains one or more images. The metadata
8 // of each image is contained in an Image File Directory (IFD),
9 // which contains entries of 12 bytes each and is described
10 // on page 14-16 of the specification. An IFD entry consists of
11 //
12 // - a tag, which describes the signification of the entry,
13 // - the data type and length of the entry,
14 // - the data itself or a pointer to it if it is more than 4 bytes.
15 //
16 // The presence of a length means that each IFD is effectively an array.
17
18 const (
19 leHeader = "II\x2A\x00" // Header for little-endian files.
20 beHeader = "MM\x00\x2A" // Header for big-endian files.
21
22 ifdLen = 12 // Length of an IFD entry in bytes.
23 )
24
25 // Data types (p. 14-16 of the spec).
26 const (
27 dtByte = 1
28 dtASCII = 2
29 dtShort = 3
30 dtLong = 4
31 dtRational = 5
32 )
33
34 // The length of one instance of each data type in bytes.
35 var lengths = [...]uint32{0, 1, 1, 2, 4, 8}
36
37 // Tags (see p. 28-41 of the spec).
38 const (
39 tImageWidth = 256
40 tImageLength = 257
41 tBitsPerSample = 258
42 tCompression = 259
43 tPhotometricInterpretation = 262
44
45 tFillOrder = 266
46
47 tStripOffsets = 273
48 tSamplesPerPixel = 277
49 tRowsPerStrip = 278
50 tStripByteCounts = 279
51
52 tT4Options = 292 // CCITT Group 3 options, a set of 32 flag bits.
53 tT6Options = 293 // CCITT Group 4 options, a set of 32 flag bits.
54
55 tTileWidth = 322
56 tTileLength = 323
57 tTileOffsets = 324
58 tTileByteCounts = 325
59
60 tXResolution = 282
61 tYResolution = 283
62 tResolutionUnit = 296
63
64 tPredictor = 317
65 tColorMap = 320
66 tExtraSamples = 338
67 tSampleFormat = 339
68 )
69
70 // Compression types (defined in various places in the spec and supplements).
71 const (
72 cNone = 1
73 cCCITT = 2
74 cG3 = 3 // Group 3 Fax.
75 cG4 = 4 // Group 4 Fax.
76 cLZW = 5
77 cJPEGOld = 6 // Superseded by cJPEG.
78 cJPEG = 7
79 cDeflate = 8 // zlib compression.
80 cPackBits = 32773
81 cDeflateOld = 32946 // Superseded by cDeflate.
82 )
83
84 // Photometric interpretation values (see p. 37 of the spec).
85 const (
86 pWhiteIsZero = 0
87 pBlackIsZero = 1
88 pRGB = 2
89 pPaletted = 3
90 pTransMask = 4 // transparency mask
91 pCMYK = 5
92 pYCbCr = 6
93 pCIELab = 8
94 )
95
96 // Values for the tPredictor tag (page 64-65 of the spec).
97 const (
98 prNone = 1
99 prHorizontal = 2
100 )
101
102 // Values for the tResolutionUnit tag (page 18).
103 const (
104 resNone = 1
105 resPerInch = 2 // Dots per inch.
106 resPerCM = 3 // Dots per centimeter.
107 )
108
109 // imageMode represents the mode of the image.
110 type imageMode int
111
112 const (
113 mBilevel imageMode = iota
114 mPaletted
115 mGray
116 mGrayInvert
117 mRGB
118 mRGBA
119 mNRGBA
120 mCMYK
121 )
122
123 // CompressionType describes the type of compression used in Options.
124 type CompressionType int
125
126 // Constants for supported compression types.
127 const (
128 Uncompressed CompressionType = iota
129 Deflate
130 LZW
131 CCITTGroup3
132 CCITTGroup4
133 )
134
135 // specValue returns the compression type constant from the TIFF spec that
136 // is equivalent to c.
137 func (c CompressionType) specValue() uint32 {
138 switch c {
139 case LZW:
140 return cLZW
141 case Deflate:
142 return cDeflate
143 case CCITTGroup3:
144 return cG3
145 case CCITTGroup4:
146 return cG4
147 }
148 return cNone
149 }