e a PNG "IHDR" chunk into the pngBytes array.
*/
protected void writeHeader()
{
int startPos;

startPos = bytePos = writeInt4( 13, bytePos );
bytePos = writeString( "IHDR", bytePos );
width = image.getWidth( null );
height = image.getHeight( null );
bytePos = writeInt4( width, bytePos );
bytePos = writeInt4( height, bytePos );
bytePos = writeByte( 8, bytePos ); // bit depth
bytePos = writeByte( (encodeAlpha) ? 6 : 2, bytePos ); // direct model
bytePos = writeByte( 0, bytePos ); // compression method
bytePos = writeByte( 0, bytePos ); // filter method
bytePos = writeByte( 0, bytePos ); // no interlace
crc.reset();
crc.update( pngBytes, startPos, bytePos-startPos );
crcValue = crc.getValue();
bytePos = writeInt4( (int) crcValue, bytePos );
}

/**
* Perform "sub" filtering on the given row.
* Uses temporary array leftBytes to store the original values
* of the previous pixels. The array is 16 bytes long, which
* will easily hold two-byte samples plus two-byte alpha.
*
* @param pixels The array holding the scan lines being built
* @param startPos Starting position within pixels of bytes to be filtered.
* @param width Width of a scanline in pixels.
*/
protected void filterSub( byte[] pixels, int startPos, int width )
{
int i;
int offset = bytesPerPixel;
int actualStart = startPos + offset;
int nBytes = width * bytesPerPixel;
int leftInsert = offset;
int leftExtract = 0;
byte current_byte;

for (i=actualStart; i < startPos + nBytes; i++)
{
leftBytes[leftInsert] = pixels[i];
pixels[i] = (byte) ((pixels[i] - leftBytes[leftExtract]) % 256);
leftInsert = (leftInsert+1) % 0x0f;
leftExtract = (leftExtract + 1) % 0x0f;
}
}

/**
* Perform "up" filtering on the given row.
* Side effect: refills the prior row with current row
*
* @param pixels The array holding the scan lines being built
* @param startPos Starting position within pixels of bytes to be filtered.
* @param width Width of a scanline in pixels.
*/
protected void filterUp( byte[] pixels, int startPos, int width )
{
int i, nBytes;
byte current_byte;

nBytes = width * bytesPerPixel;

for (i=0; i < nBytes; i++)
{
current_byte = pixels[startPos + i];
pixels[startPos + i] = (byte) ((pixels[startPos + i] - priorRow[i]) % 256);
priorRow[i] = current_byte;
}
}

/**
* Write the image data into the pngBytes array.
* This will write one or more PNG "IDAT" chunks. In order
* to conserve memory, this method grabs as many rows as will
* fit into 32K bytes, or the whole image; whichever is less.
*
*
* @return true if no errors; false if error grabbing pixels
*/
protected boolean writeImageData()
{
int rowsLeft = height; // number of rows remaining to write
int startRow = 0; // starting row to process this time through
int nRows; // how many rows to grab at a time

byte[] scanLines; // the scan lines to be compressed
int scanPos; // where we are in the scan lines
int startPos; // where this line's actual pixels start (used for filtering)

byte[] compressedLines; // the resultant compressed lines
int nCompressed; // how big is the compressed area?

int depth; // color depth ( handle only 8 or 32 )

PixelGrabber pg;

bytesPerPixel = (encodeAlpha) ? 4 : 3;

Deflater scrunch = new Deflater( compressionLevel );
ByteArrayOutputStream outBytes =
new ByteArrayOutputStream(1024);

DeflaterOutputStream compBytes =
new DeflaterOutputStream( outBytes, scrunch );
try
{
while (rowsLeft > 0)
{
nRows = Math.min( 32767 / (width*(bytesPerPixel+1)), rowsLeft );
// nRows = rowsLeft;

int[] pixels = new int[width * nRows];

pg = new PixelGrabber(image, 0, startRow,
width, nRows, pixels, 0, width);
try {
pg.grabPixels();
}
catch (Exception e) {
System.err.println("interrupted waiting for pixels!");
return false;
}
if ((pg.getStatus() & ImageObserver.ABORT) != 0) {
System.err.println("image fetch aborted or errored");
return false;
}

/*
* Create a data chunk. scanLines adds "nRows" for
* the filter bytes.
*/
scanLines = new byte[width * nRows * bytesPerPixel + nRows];

if (filter == FILTER_SUB)
{
leftBytes = new byte[16];
}
if (filter == FILTER_UP)
{
priorRow = new byte[width*bytesPerPixel];
}

scanPos = 0;
startPos = 1;
for (int i=0; i {
if (i % width == 0)
{
scanLines[scanPos++] = (byte) filter;
startPos = scanPos;
}
scanLines[scanPos++] = (byte) ((pixels[i] >> 16) & 0xff);
scanLines[scanPos++] = (byte) ((pixels[i] >> 8) & 0xff);
scanLines[scanPos++] = (byte) ((pixels[i] ) & 0xff);
if (encodeAlpha)
{
scanLines[scanPos++] = (byte) ((pixels[i] >> 24) & 0xff );
}
if ((i % width == width-1) && (filter != FILTER_NONE))
{
if (filter == FILTER_SUB)
{
filterSub( scanLines, startPos, width );
}
if (filter == FILTER_UP)
{
filterUp( scanLines, startPos, width );
}
}
}

/*
* Write these lines to the output area
*/
compBytes.write( scanLines, 0, scanPos );


startRow += nRows;
rowsLeft -= nRows;
}
compBytes.close();

/*
* Write the compressed bytes
*/
compressedLines = outBytes.toByteArray();
nCompressed = compressedLines.length;

crc.reset();
bytePos = writeInt4( nCompressed, bytePos );
bytePos = writeString("IDAT", bytePos );
cr