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+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tiff
+
+// A tiff image file contains one or more images. The metadata
+// of each image is contained in an Image File Directory (IFD),
+// which contains entries of 12 bytes each and is described
+// on page 14-16 of the specification. An IFD entry consists of
+//
+// - a tag, which describes the signification of the entry,
+// - the data type and length of the entry,
+// - the data itself or a pointer to it if it is more than 4 bytes.
+//
+// The presence of a length means that each IFD is effectively an array.
+
+const (
+ leHeader = "II\x2A\x00" // Header for little-endian files.
+ beHeader = "MM\x00\x2A" // Header for big-endian files.
+
+ ifdLen = 12 // Length of an IFD entry in bytes.
+)
+
+// Data types (p. 14-16 of the spec).
+const (
+ dtByte = 1
+ dtASCII = 2
+ dtShort = 3
+ dtLong = 4
+ dtRational = 5
+)
+
+// The length of one instance of each data type in bytes.
+var lengths = [...]uint32{0, 1, 1, 2, 4, 8}
+
+// Tags (see p. 28-41 of the spec).
+const (
+ tImageWidth = 256
+ tImageLength = 257
+ tBitsPerSample = 258
+ tCompression = 259
+ tPhotometricInterpretation = 262
+
+ tStripOffsets = 273
+ tSamplesPerPixel = 277
+ tRowsPerStrip = 278
+ tStripByteCounts = 279
+
+ tTileWidth = 322
+ tTileLength = 323
+ tTileOffsets = 324
+ tTileByteCounts = 325
+
+ tXResolution = 282
+ tYResolution = 283
+ tResolutionUnit = 296
+
+ tPredictor = 317
+ tColorMap = 320
+ tExtraSamples = 338
+ tSampleFormat = 339
+)
+
+// Compression types (defined in various places in the spec and supplements).
+const (
+ cNone = 1
+ cCCITT = 2
+ cG3 = 3 // Group 3 Fax.
+ cG4 = 4 // Group 4 Fax.
+ cLZW = 5
+ cJPEGOld = 6 // Superseded by cJPEG.
+ cJPEG = 7
+ cDeflate = 8 // zlib compression.
+ cPackBits = 32773
+ cDeflateOld = 32946 // Superseded by cDeflate.
+)
+
+// Photometric interpretation values (see p. 37 of the spec).
+const (
+ pWhiteIsZero = 0
+ pBlackIsZero = 1
+ pRGB = 2
+ pPaletted = 3
+ pTransMask = 4 // transparency mask
+ pCMYK = 5
+ pYCbCr = 6
+ pCIELab = 8
+)
+
+// Values for the tPredictor tag (page 64-65 of the spec).
+const (
+ prNone = 1
+ prHorizontal = 2
+)
+
+// Values for the tResolutionUnit tag (page 18).
+const (
+ resNone = 1
+ resPerInch = 2 // Dots per inch.
+ resPerCM = 3 // Dots per centimeter.
+)
+
+// imageMode represents the mode of the image.
+type imageMode int
+
+const (
+ mBilevel imageMode = iota
+ mPaletted
+ mGray
+ mGrayInvert
+ mRGB
+ mRGBA
+ mNRGBA
+)
+
+// CompressionType describes the type of compression used in Options.
+type CompressionType int
+
+const (
+ Uncompressed CompressionType = iota
+ Deflate
+)
+
+// specValue returns the compression type constant from the TIFF spec that
+// is equivalent to c.
+func (c CompressionType) specValue() uint32 {
+ switch c {
+ case Deflate:
+ return cDeflate
+ }
+ return cNone
+}