1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
|
/*
* Copyright 2002-2017 Drew Noakes
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
* More information about this project is available at:
*
* https://drewnoakes.com/code/exif/
* https://github.com/drewnoakes/metadata-extractor
*/
package com.drew.imaging.png;
import com.drew.lang.SequentialReader;
import com.drew.lang.annotations.NotNull;
import com.drew.lang.annotations.Nullable;
import java.io.IOException;
import java.util.*;
/**
* @author Drew Noakes https://drewnoakes.com
*/
public class PngChunkReader
{
private static final byte[] PNG_SIGNATURE_BYTES = {(byte)0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A};
public Iterable<PngChunk> extract(@NotNull final SequentialReader reader, @Nullable final Set<PngChunkType> desiredChunkTypes) throws PngProcessingException, IOException
{
//
// PNG DATA STREAM
//
// Starts with a PNG SIGNATURE, followed by a sequence of CHUNKS.
//
// PNG SIGNATURE
//
// Always composed of these bytes: 89 50 4E 47 0D 0A 1A 0A
//
// CHUNK
//
// 4 - length of the data field (unsigned, but always within 31 bytes), may be zero
// 4 - chunk type, restricted to [65,90] and [97,122] (A-Za-z)
// * - data field
// 4 - CRC calculated from chunk type and chunk data, but not length
//
// CHUNK TYPES
//
// Critical Chunk Types:
//
// IHDR - image header, always the first chunk in the data stream
// PLTE - palette table, associated with indexed PNG images
// IDAT - image data chunk, of which there may be many
// IEND - image trailer, always the last chunk in the data stream
//
// Ancillary Chunk Types:
//
// Transparency information: tRNS
// Colour space information: cHRM, gAMA, iCCP, sBIT, sRGB
// Textual information: iTXt, tEXt, zTXt
// Miscellaneous information: bKGD, hIST, pHYs, sPLT
// Time information: tIME
//
reader.setMotorolaByteOrder(true); // network byte order
if (!Arrays.equals(PNG_SIGNATURE_BYTES, reader.getBytes(PNG_SIGNATURE_BYTES.length))) {
throw new PngProcessingException("PNG signature mismatch");
}
boolean seenImageHeader = false;
boolean seenImageTrailer = false;
List<PngChunk> chunks = new ArrayList<PngChunk>();
Set<PngChunkType> seenChunkTypes = new HashSet<PngChunkType>();
while (!seenImageTrailer) {
// Process the next chunk.
int chunkDataLength = reader.getInt32();
if (chunkDataLength < 0)
throw new PngProcessingException("PNG chunk length exceeds maximum");
PngChunkType chunkType = new PngChunkType(reader.getBytes(4));
boolean willStoreChunk = desiredChunkTypes == null || desiredChunkTypes.contains(chunkType);
byte[] chunkData = reader.getBytes(chunkDataLength);
// Skip the CRC bytes at the end of the chunk
// TODO consider verifying the CRC value to determine if we're processing bad data
reader.skip(4);
if (willStoreChunk && seenChunkTypes.contains(chunkType) && !chunkType.areMultipleAllowed()) {
throw new PngProcessingException(String.format("Observed multiple instances of PNG chunk '%s', for which multiples are not allowed", chunkType));
}
if (chunkType.equals(PngChunkType.IHDR)) {
seenImageHeader = true;
} else if (!seenImageHeader) {
throw new PngProcessingException(String.format("First chunk should be '%s', but '%s' was observed", PngChunkType.IHDR, chunkType));
}
if (chunkType.equals(PngChunkType.IEND)) {
seenImageTrailer = true;
}
if (willStoreChunk) {
chunks.add(new PngChunk(chunkType, chunkData));
}
seenChunkTypes.add(chunkType);
}
return chunks;
}
}
|
