go.image/tiff: fix (TIFF-specific) LZW decompression.
The blue-purple-pink.lzwcompressed.tiff image was created by
"bmp2tiff -c lzw".
LGTM=djd
R=djd
CC=bsiegert, golang-codereviews
https://golang.org/cl/105750045
diff --git a/testdata/blue-purple-pink.lzwcompressed.tiff b/testdata/blue-purple-pink.lzwcompressed.tiff
new file mode 100644
index 0000000..5978f7a
--- /dev/null
+++ b/testdata/blue-purple-pink.lzwcompressed.tiff
Binary files differ
diff --git a/testdata/blue-purple-pink.png b/testdata/blue-purple-pink.png
new file mode 100644
index 0000000..d4fbf6b
--- /dev/null
+++ b/testdata/blue-purple-pink.png
Binary files differ
diff --git a/tiff/lzw/reader.go b/tiff/lzw/reader.go
index ef59699..48579a7 100644
--- a/tiff/lzw/reader.go
+++ b/tiff/lzw/reader.go
@@ -6,13 +6,32 @@
// described in T. A. Welch, ``A Technique for High-Performance Data
// Compression'', Computer, 17(6) (June 1984), pp 8-19.
//
-// In particular, it implements LZW as used by the GIF, TIFF and PDF file
-// formats, which means variable-width codes up to 12 bits and the first
-// two non-literal codes are a clear code and an EOF code.
+// In particular, it implements LZW as used by the TIFF file format, including
+// an "off by one" algorithmic difference when compared to standard LZW.
package lzw
-// TODO(nigeltao): check that TIFF and PDF use LZW in the same way as GIF,
-// modulo LSB/MSB packing order.
+/*
+This file was branched from src/pkg/compress/lzw/reader.go in the
+standard library. Differences from the original are marked with "NOTE".
+
+The tif_lzw.c file in the libtiff C library has this comment:
+
+----
+The 5.0 spec describes a different algorithm than Aldus
+implements. Specifically, Aldus does code length transitions
+one code earlier than should be done (for real LZW).
+Earlier versions of this library implemented the correct
+LZW algorithm, but emitted codes in a bit order opposite
+to the TIFF spec. Thus, to maintain compatibility w/ Aldus
+we interpret MSB-LSB ordered codes to be images written w/
+old versions of this library, but otherwise adhere to the
+Aldus "off by one" algorithm.
+----
+
+The Go code doesn't read (invalid) TIFF files written by old versions of
+libtiff, but the LZW algorithm in this package still differs from the one in
+Go's standard package library to accomodate this "off by one" in valid TIFFs.
+*/
import (
"bufio"
@@ -53,7 +72,7 @@
// The next two codes mean clear and EOF.
// Other valid codes are in the range [lo, hi] where lo := clear + 2,
// with the upper bound incrementing on each code seen.
- // overflow is the code at which hi overflows the code width.
+ // overflow is the code at which hi overflows the code width. NOTE: TIFF's LZW is "off by one".
// last is the most recently seen code, or decoderInvalidCode.
clear, eof, hi, overflow, last uint16
@@ -189,7 +208,7 @@
return
}
d.last, d.hi = code, d.hi+1
- if d.hi >= d.overflow {
+ if d.hi+1 >= d.overflow { // NOTE: the "+1" is where TIFF's LZW differs from the standard algorithm.
if d.width == maxWidth {
d.last = decoderInvalidCode
} else {
diff --git a/tiff/reader.go b/tiff/reader.go
index ea4f1f2..f4ccd23 100644
--- a/tiff/reader.go
+++ b/tiff/reader.go
@@ -8,7 +8,6 @@
package tiff
import (
- "compress/lzw"
"compress/zlib"
"encoding/binary"
"fmt"
@@ -16,6 +15,8 @@
"image/color"
"io"
"io/ioutil"
+
+ "code.google.com/p/go.image/tiff/lzw"
)
// A FormatError reports that the input is not a valid TIFF image.
diff --git a/tiff/reader_test.go b/tiff/reader_test.go
index d62e277..cceb7aa 100644
--- a/tiff/reader_test.go
+++ b/tiff/reader_test.go
@@ -125,6 +125,21 @@
compare(t, img0, img4)
}
+// TestDecodeLZW tests that decoding a PNG image and a LZW-compressed TIFF image
+// result in the same pixel data.
+func TestDecodeLZW(t *testing.T) {
+ img0, err := load("blue-purple-pink.png")
+ if err != nil {
+ t.Fatal(err)
+ }
+ img1, err := load("blue-purple-pink.lzwcompressed.tiff")
+ if err != nil {
+ t.Fatal(err)
+ }
+
+ compare(t, img0, img1)
+}
+
// TestDecompress tests that decoding some TIFF images that use different
// compression formats result in the same pixel data.
func TestDecompress(t *testing.T) {
