最近在读取客户发过来的tiff文件是，底层竟然报错了，错误： bandOffsets.length is wrong! 没办法，因为错误消息出现在tiff的read中，因此就对
底层序中tiff读取的代码进行了研究。
之前有一篇文章，我简单的介绍了Geotools读取Tiff的代码， Java 通过geotools读取tiff ，其实通过深入研究发现，原来幕后的大佬竟然是imageio-ext中的TiffImageReader，
imageio做为Java开发的人员肯定都知道，而ImageIO-ext是imageio的扩展类，我们可以到github上看到它的源码，这是一个非常强大的库，对于Java处理各种栅格数据的读写非常有帮助！
借助这篇文章，我们需要先了解Tiff文件的具体结构，可以参考这篇文章， TIFF文件结构详解 https://blog.csdn.net/oYinHeZhiGuang/article/details/121710467 讲的很好！
下面我们来看下imageio-ext中的tiff读取代码，主要类TiffImageReader，我们来看下Java程序是如何读取tiff文件的。
构造方法：
public TIFFImageReader(ImageReaderSpi originatingProvider) {
super(originatingProvider);
}
这个类需要通过一个ImageReaderSpi来实例化，其实这种SPI的设计模式，Java的很多开源项目都在用到，这里我们通过TIFFImageReaderSpi这个类即可。
其次设置文件的路径，以及其它一些参数，通过该类的如下方法：
public void setInput(Object input,
boolean seekForwardOnly,
boolean ignoreMetadata)
这个方法，里面有input就是需要读取的文件， seekForwardOnly设置为true表示： 只能从这个输入源按升序读取图像和元数据。 ignoreMetadata设置为true表示读取忽略元数据
接下来就是对tiff元数据的读取，具体参见getImageMetadata(int imageIndex)这个方法：
public IIOMetadata getImageMetadata(int imageIndex) throws IIOException {
seekToImage(imageIndex, true);
TIFFImageMetadata im =
new TIFFImageMetadata(imageMetadata.getRootIFD().getTagSetList());
Node root =
imageMetadata.getAsTree(TIFFImageMetadata.nativeMetadataFormatName);
im.setFromTree(TIFFImageMetadata.nativeMetadataFormatName, root);
if (noData != null) {
im.setNoData(new double[] {noData, noData});
}
if (scales != null && offsets != null) {
im.setScales(scales);
im.setOffsets(offsets);
}
return im;
}
其中的seekToImage(imageIndex, true)为最主要的逻辑处理，这个方法中，第一个参数，imageIndex为tiff多页中的第几个，第二参数设置标示该tiff页是否已经被解析过
private void seekToImage(int imageIndex, boolean optimized) throws IIOException {
checkIndex(imageIndex);
// TODO we should do this initialization just once!!!
int index = locateImage(imageIndex);
if (index != imageIndex) {
throw new IndexOutOfBoundsException("imageIndex out of bounds!");
}
final Integer i= Integer.valueOf(index);
//optimized branch
if(!optimized){
readMetadata();
initializeFromMetadata();
return;
}
// in case we have cache the info for this page
if(pagesInfo.containsKey(i)){
// initialize from cachedinfo only if needed
// TODO Improve
if(imageMetadata == null || !initialized) {// this means the curindex has changed
final PageInfo info = pagesInfo.get(i);
final TIFFImageMetadata metadata = info.imageMetadata.get();
if (metadata != null) {
initializeFromCachedInfo(info, metadata);
return;
}
pagesInfo.put(i,null);
}
}
readMetadata();
initializeFromMetadata();
}
这个方法当中，第一次加载tiff，通过readMetadata()和initializeFromMetadata()将tiff的元信息缓存起来，方便后面再次读取。
读取过程
主要是要结合Tiff的格式进行理解，大体主要是解析tiff头，然后获取到IFD(tiff的图像目录信息)，然后再依次去解析每个目录的具体内容，代码就不再这里罗列了。
这里主要说下，解析目录信息是获取tiff的元信息的过程，通常是解析每个tag的信息，解析代码TIFFIFD类的initialize(ImageInputStream stream, boolean ignoreUnknownFields, final boolean isBTIFF)方法中
public void initialize(ImageInputStream stream,
boolean ignoreUnknownFields, final boolean isBTIFF) throws IOException {
removeTIFFFields();
List tagSetList = getTagSetList();
final long numEntries;
if(isBTIFF)
numEntries= stream.readLong();
else
numEntries= stream.readUnsignedShort();
for (int i = 0; i < numEntries; i++) {
// Read tag number, value type, and value count.
int tag = stream.readUnsignedShort();
int type = stream.readUnsignedShort();
int count;
if(isBTIFF)
{
long count_=stream.readLong();
count = (int)count_;
if(count!=count_)
throw new IllegalArgumentException("unable to use long number of values");
}
else
count = (int)stream.readUnsignedInt();
// Get the associated TIFFTag.
TIFFTag tiffTag = getTag(tag, tagSetList);
// Ignore unknown fields.
if(ignoreUnknownFields && tiffTag == null) {
// Skip the value/offset so as to leave the stream
// position at the start of the next IFD entry.
if(isBTIFF)
stream.skipBytes(8);
else
stream.skipBytes(4);
// XXX Warning message ...
// Continue with the next IFD entry.
continue;
}
long nextTagOffset;
if(isBTIFF){
nextTagOffset = stream.getStreamPosition() + 8;
int sizeOfType = TIFFTag.getSizeOfType(type);
if (count*sizeOfType > 8) {
long value = stream.readLong();
stream.seek(value);
}
}
else{
nextTagOffset = stream.getStreamPosition() + 4;
int sizeOfType = TIFFTag.getSizeOfType(type);
if (count*sizeOfType > 4) {
long value = stream.readUnsignedInt();
stream.seek(value);
}
}
if (tag == BaselineTIFFTagSet.TAG_STRIP_BYTE_COUNTS ||
tag == BaselineTIFFTagSet.TAG_TILE_BYTE_COUNTS ||
tag == BaselineTIFFTagSet.TAG_JPEG_INTERCHANGE_FORMAT_LENGTH) {
this.stripOrTileByteCountsPosition =
stream.getStreamPosition();
if (LAZY_LOADING) {
type = type == TIFFTag.TIFF_LONG ? TIFFTag.TIFF_LAZY_LONG : TIFFTag.TIFF_LAZY_LONG8;
}
} else if (tag == BaselineTIFFTagSet.TAG_STRIP_OFFSETS ||
tag == BaselineTIFFTagSet.TAG_TILE_OFFSETS ||
tag == BaselineTIFFTagSet.TAG_JPEG_INTERCHANGE_FORMAT) {
this.stripOrTileOffsetsPosition =
stream.getStreamPosition();
if (LAZY_LOADING) {
type = type == TIFFTag.TIFF_LONG ? TIFFTag.TIFF_LAZY_LONG : TIFFTag.TIFF_LAZY_LONG8;
}
}
Object obj = null;
try {
switch (type) {
case TIFFTag.TIFF_BYTE:
case TIFFTag.TIFF_SBYTE:
case TIFFTag.TIFF_UNDEFINED:
case TIFFTag.TIFF_ASCII:
byte[] bvalues = new byte[count];
stream.readFully(bvalues, 0, count);
if (type == TIFFTag.TIFF_ASCII) {
// Can be multiple strings
final List<String> v = new ArrayList<String>();
boolean inString = false;
int prevIndex = 0;
for (int index = 0; index <= count; index++) {
if (index < count && bvalues[index] != 0) {
if (!inString) {
// start of string
prevIndex = index;
inString = true;
}
} else { // null or special case at end of string
if (inString) {
// end of string
final String s = new String(bvalues, prevIndex,index - prevIndex);
v.add(s);
inString = false;
}
}
}
count = v.size();
String[] strings;
if(count != 0) {
strings = new String[count];
for (int c = 0 ; c < count; c++) {
strings[c] = v.get(c);
}
} else {
// This case has been observed when the value of
// 'count' recorded in the field is non-zero but
// the value portion contains all nulls.
count = 1;
strings = new String[] {""};
}
obj = strings;
} else {
obj = bvalues;
}
break;
case TIFFTag.TIFF_SHORT:
char[] cvalues = new char[count];
for (int j = 0; j < count; j++) {
cvalues[j] = (char)(stream.readUnsignedShort());
}
obj = cvalues;
break;
case TIFFTag.TIFF_LONG:
case TIFFTag.TIFF_IFD_POINTER:
long[] lvalues = new long[count];
for (int j = 0; j < count; j++) {
lvalues[j] = stream.readUnsignedInt();
}
obj = lvalues;
break;
case TIFFTag.TIFF_RATIONAL:
long[][] llvalues = new long[count][2];
for (int j = 0; j < count; j++) {
llvalues[j][0] = stream.readUnsignedInt();
llvalues[j][1] = stream.readUnsignedInt();
}
obj = llvalues;
break;
case TIFFTag.TIFF_SSHORT:
short[] svalues = new short[count];
for (int j = 0; j < count; j++) {
svalues[j] = stream.readShort();
}
obj = svalues;
break;
case TIFFTag.TIFF_SLONG:
int[] ivalues = new int[count];
for (int j = 0; j < count; j++) {
ivalues[j] = stream.readInt();
}
obj = ivalues;
break;
case TIFFTag.TIFF_SRATIONAL:
int[][] iivalues = new int[count][2];
for (int j = 0; j < count; j++) {
iivalues[j][0] = stream.readInt();
iivalues[j][1] = stream.readInt();
}
obj = iivalues;
break;
case TIFFTag.TIFF_FLOAT:
float[] fvalues = new float[count];
for (int j = 0; j < count; j++) {
fvalues[j] = stream.readFloat();
}
obj = fvalues;
break;
case TIFFTag.TIFF_DOUBLE:
double[] dvalues = new double[count];
for (int j = 0; j < count; j++) {
dvalues[j] = stream.readDouble();
}
obj = dvalues;
break;
case TIFFTag.TIFF_LONG8:
case TIFFTag.TIFF_SLONG8:
case TIFFTag.TIFF_IFD8:
long[] lBvalues = new long[count];
for (int j = 0; j < count; j++) {
lBvalues[j] = stream.readLong();
}
obj = lBvalues;
break;
case TIFFTag.TIFF_LAZY_LONG8:
case TIFFTag.TIFF_LAZY_LONG:
obj = new TIFFLazyData(stream, type, count);
break;
default:
// XXX Warning
break;
}
} catch(EOFException eofe) {
// The TIFF 6.0 fields have tag numbers less than or equal
// to 532 (ReferenceBlackWhite) or equal to 33432 (Copyright).
// If there is an error reading a baseline tag, then re-throw
// the exception and fail; otherwise continue with the next
// field.
if(BaselineTIFFTagSet.getInstance().getTag(tag) == null) {
throw eofe;
}
}
if (tiffTag == null) {
// XXX Warning: unknown tag
} else if (!tiffTag.isDataTypeOK(type)) {
// XXX Warning: bad data type
} else if (tiffTag.isIFDPointer() && obj != null) {
stream.mark();
stream.seek(((long[])obj)[0]);
List tagSets = new ArrayList(1);
tagSets.add(tiffTag.getTagSet());
TIFFIFD subIFD = new TIFFIFD(tagSets);
// XXX Use same ignore policy for sub-IFD fields?
subIFD.initialize(stream, ignoreUnknownFields);
obj = subIFD;
stream.reset();
}
if (tiffTag == null) {
tiffTag = new TIFFTag(null, tag, 1 << type, null);
}
// Add the field if its contents have been initialized which
// will not be the case if an EOF was ignored above.
if(obj != null) {
TIFFField f = new TIFFField(tiffTag, type, count, obj);
addTIFFField(f);
}
stream.seek(nextTagOffset);
}
this.lastPosition = stream.getStreamPosition();
}
View Code
Tiff常用的Tag标签类有 BaseLineTiffTagSet、FaxTiffTagSet、GeoTiffTagSet、EXIFPTiffTagSet、 PrivateTIFFTagSet等。
其中的GeoTiffTagSet用于geotiff的额外存储信息，在这里说明下，Geotiff是Tiff格式对Gis数据的一种存储支持，而 PrivateTIFFTagSet是对gdal的支持，增加了NODATA、MEATADATA的信息。
对于文章开头提的关于bandOffsets.length is wrong!，主要原因出现在getImageTypes(int imageIndex)这个方法的下面这个实现中。
ImageTypeSpecifier itsRaw =
TIFFDecompressor.getRawImageTypeSpecifier
(photometricInterpretation,
compression,
samplesPerPixel,
bitsPerSample,
sampleFormat,
extraSamples,
colorMap);
最终我们在ImageTypeSpecifier这个类的Interleaved(ColorSpace colorSpace,int[] bandOffsets,int dataType,boolean hasAlpha,boolean isAlphaPremultiplied) 方法中发现问题。
public Interleaved(ColorSpace colorSpace,
int[] bandOffsets,
int dataType,
boolean hasAlpha,
boolean isAlphaPremultiplied) {
if (colorSpace == null) {
throw new IllegalArgumentException("colorSpace == null!");
}
if (bandOffsets == null) {
throw new IllegalArgumentException("bandOffsets == null!");
}
int numBands = colorSpace.getNumComponents() +
(hasAlpha ? 1 : 0);
if (bandOffsets.length != numBands) {
throw new IllegalArgumentException
("bandOffsets.length is wrong!");
}
我们发现只有当我们的图像偏移数量和我们的通道数不一致的时候，就会报这个错误！
总结
通过研究这个问题，基本上梳理了Java基于ImageIO-ext读取tiff的过程，基本跟tiff的数据结构对应起来。
