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4.4 Writing Writer Plug-ins
MyFormatImageWriterSpi TheMyFormatImageWriterSpi call plays a similar role to
the MyFormatImageReaderSpi class discussed in the
previous section. However, instead of being responsible for
determining whether a given stream can be read, it must deterine
whether an image in memory can be written. Rather than inspecting
the image itself, an ImageTypeSpecifier is used so
that writers may be selected before an actual image is available.
package com.mycompany.imageio;
import java.io.IOException;
import java.util.Locale;
import javax.imageio.ImageWriter;
import javax.imageio.ImageTypeSpecifier;
import javax.imageio.spi.ImageWriterSpi;
import javax.imageio.stream.ImageInputStream;
public class MyFormatImageWriterSpi extends ImageWriterSpi {
static final String vendorName = "My Company";
static final String version = "1.0_beta33_build9467";
static final String writerClassName =
"com.mycompany.imageio.MyFormatImageWriter";
static final String[] names = { "myformat" };
static final String[] suffixes = { "myf" };
static final String[] MIMETypes = { "image/x-myformat" };
static final String[] readerSpiNames = {
"com.mycompany.imageio.MyFormatImageReaderSpi" };
static final boolean supportsStandardStreamMetadataFormat = false;
static final String nativeStreamMetadataFormatName = null;
static final String nativeStreamMetadataFormatClassName = null;
static final String[] extraStreamMetadataFormatNames = null;
static final String[] extraStreamMetadataFormatClassNames = null;
static final boolean supportsStandardImageMetadataFormat = false;
static final String nativeImageMetadataFormatName =
"com.mycompany.imageio.MyFormatMetadata_1.0";
static final String nativeImageMetadataFormatClassName =
"com.mycompany.imageio.MyFormatMetadata";
static final String[] extraImageMetadataFormatNames = null;
static final String[] extraImageMetadataFormatClassNames = null;
public MyFormatImageWriterSpi() {
super(vendorName, version,
names, suffixes, MIMETypes,
writerClassName,
STANDARD_OUTPUT_TYPE, // Write to ImageOutputStreams
readerSpiNames,
supportsStandardStreamMetadataFormat,
nativeStreamMetadataFormatName,
nativeStreamMetadataFormatClassName,
extraStreamMetadataFormatNames,
extraStreamMetadataFormatClassNames,
supportsStandardImageMetadataFormat,
nativeImageMetadataFormatName,
nativeImageMetadataFormatClassName,
extraImageMetadataFormatNames,
extraImageMetadataFormatClassNames);
}
public boolean canEncodeImage(ImageTypeSpecifier imageType) {
int bands = imageType.getNumBands();
return bands == 1 || bands == 3;
}
public String getDescription(Locale locale) {
// Localize as appropriate
return "Description goes here";
}
public ImageWriter createWriterInstance(Object extension) {
return new MyFormatImageWriter(this);
}
}
MyFormatImageWriter
package com.mycompany.imageio;
import java.awt.Rectangle;
import java.awt.image.Raster;
import java.awt.image.RenderedImage;
import java.io.IOException;
import java.util.Iterator;
import javax.imageio.IIOException;
import javax.imageio.IIOImage;
import javax.imageio.ImageTypeSpecifier;
import javax.imageio.ImageWriteParam;
import javax.imageio.ImageWriter;
import javax.imageio.metadata.IIOMetadata;
import javax.imageio.spi.ImageWriterSpi;
import javax.imageio.stream.ImageOutputStream;
public class MyFormatImageWriter extends ImageWriter {
ImageOutputStream stream = null;
public MyFormatImageWriter(ImageWriterSpi originatingProvider) {
super(originatingProvider);
}
public void setOutput(Object output) {
super.setOutput(output);
if (output != null) {
if (!(output instanceof ImageOutputStream)) {
throw new IllegalArgumentException
("output not an ImageOutputStream!");
}
this.stream = (ImageOutputStream)output;
} else {
this.stream = null;
}
}
The ImageWriteParam
returned by getDefaultWriteParam must be customized
based on the writer's capabilities. Since this writer does not
support tiling, progessive encoding, or compression, we pass in
values of false or null as appropriate:
// Tiling, progressive encoding, compression are disabled by default
public ImageWriteParam getDefaultWriteParam() {
return new ImageWriteParam(getLocale());
}
The format only handles image
metadata. The convertImageMetadata method does very little; it
could be defined to interpret the metadata classes used by other
plug-ins.
public IIOMetadata getDefaultStreamMetadata(ImageWriteParam param) {
return null;
}
public IIOMetadata
getDefaultImageMetadata(ImageTypeSpecifier imageType,
ImageWriteParam param) {
return new MyFormatMetadata();
}
public IIOMetadata convertStreamMetadata(IIOMetadata inData,
ImageWriteParam param) {
return null;
}
public IIOMetadata convertImageMetadata(IIOMetadata inData,
ImageTypeSpecifier imageType,
ImageWriteParam param) {
// We only understand our own metadata
if (inData instanceof MyFormatMetadata) {
return inData;
} else {
return null;
}
}
The actual writing of the image
requires first applying the source region, source bands, and
subsampling factors from the ImageWriteParam. The
source region and source bands may be handled by creating a child
Raster. For simplicity, we extract a single
Raster from the source image. If the source image is
tiled, we can save memory by extracting smaller
Rasters as needed.
public void write(IIOMetadata streamMetadata,
IIOImage image,
ImageWriteParam param) throws IIOException {
RenderedImage im = image.getRenderedImage();
Rectangle sourceRegion =
new Rectangle(0, 0, im.getWidth(), im.getHeight());
int sourceXSubsampling = 1;
int sourceYSubsampling = 1;
int[] sourceBands = null;
if (param != null) {
sourceRegion =
sourceRegion.intersection(param.getSourceRegion());
sourceXSubsampling = param.getSourceXSubsampling();
sourceYSubsampling = param.getSourceYSubsampling();
sourceBands = param.getSourceBands();
int subsampleXOffset = param.getSubsamplingXOffset();
int subsampleYOffset = param.getSubsamplingYOffset();
sourceRegion.x += subsampleXOffset;
sourceRegion.y += subsampleYOffset;
sourceRegion.width -= subsampleXOffset;
sourceRegion.height -= subsampleYOffset;
}
// Grab a Raster containing the region of interest
int width = sourceRegion.width;
int height = sourceRegion.height;
Raster imRas = im.getData(sourceRegion);
int numBands = imRas.getNumBands();
// Check that sourceBands values are in range
if (sourceBands != null) {
for (int i = 0; i < sourceBands.length; i++) {
if (sourceBands[i] >= numBands) {
throw new IllegalArgumentException("bad band!");
}
}
}
// Translate imRas to start at (0, 0) and subset the bands
imRas = imRas.createChild(sourceRegion.x, sourceRegion.y,
width, height,
0, 0,
sourceBands);
// Reduce width and height according to subsampling factors
width = (width + sourceXSubsampling - 1)/sourceXSubsampling;
height = (height + sourceYSubsampling - 1)/sourceYSubsampling;
// Assume 1 band image is grayscale, 3 band image is RGB
int colorType;
if (numBands == 1) {
colorType = MyFormatImageReader.COLOR_TYPE_GRAY;
} else if (numBands == 3) {
colorType = MyFormatImageReader.COLOR_TYPE_RGB;
} else {
throw new IIOException("Image must have 1 or 3 bands!");
}
Once the image dimensions and color
type of the image have been ascertained, the plug-in is ready to
write the file header:
try {
byte[] signature = {
(byte)'m', (byte)'y', (byte)'f', (byte)'o',
(byte)'r', (byte)'m', (byte)'a', (byte)'t'
};
// Output header information
stream.write(signature);
stream.write(`\n');
stream.writeInt(width);
stream.writeInt(height);
stream.writeByte(colorType);
stream.write(`\n');
Next, the plug-in extracts the image
metadata from the write method's
IIOImage argument, and attempts to convert it into a
MyFormatMetadata object by calling
convertImageMetadata. If the result is
non-null, the keywords and values are extracted from
the metadata and written to the output:
// Attempt to convert metadata, if present
IIOMetadata imd = image.getMetadata();
MyFormatMetadata metadata = null;
if (imd != null) {
ImageTypeSpecifier type =
ImageTypeSpecifier.createFromRenderedImage(im);
metadata =
(MyFormatMetadata)convertImageMetadata(imd,
type,
null);
}
// Output metadata if present
if (metadata != null) {
Iterator keywordIter = metadata.keywords.iterator();
Iterator valueIter = metadata.values.iterator();
while (keywordIter.hasNext()) {
String keyword = (String)keywordIter.next();
String value = (String)valueIter.next();
stream.writeUTF(keyword);
stream.write(`\n');
stream.writeUTF(value);
stream.write(`\n');
}
}
stream.writeUTF("END");
stream.write(`\n');
Finally, the plug-in is ready to
begin writing the pixel data. The image Raster is
copied into an int array, one row at a time using the
getPixels method. Then these values are subsampled
using the horizontal subsampling factor, and copied into a byte
array, which is written to the output with a single write call. The
source row is then incremented by the vertical subsampling factor
until the end of the source region is reached, and the output
stream is flushed:
// Output (subsampled) pixel values
int rowLength = width*numBands;
int xSkip = sourceXSubsampling*numBands;
int[] rowPixels = imRas.getPixels(0, 0, width, 1,
(int[])null);
byte[] rowSamples = new byte[rowLength];
// Output every (sourceYSubsampling)^th row
for (int y = 0; y < height; y += sourceYSubsampling) {
imRas.getPixels(0, y, width, 1, rowPixels);
// Subsample horizontally and convert to bytes
int count = 0;
for (int x = 0; x < width; x += xSkip) {
if (colorType ==
MyFormatImageReader.COLOR_TYPE_GRAY) {
rowSamples[count++] = (byte)rowPixels[x];
} else {
rowSamples[count++] = (byte)rowPixels[x];
rowSamples[count++] =
(byte)rowPixels[x + 1];
rowSamples[count++] =
(byte)rowPixels[x + 2];
}
}
// Output a row's worth of bytes
stream.write(rowSamples, 0, width*numBands);
}
stream.flush();
} catch (IOException e) {
throw new IIOException("I/O error!", e);
}
}
}
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