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package org.pdfbox.pdmodel.graphics.predictor;
/**
* From http://www.w3.org/TR/PNG-Filters.html: The Paeth filter computes a
* simple linear function of the three neighboring pixels (left, above, upper
* left), then chooses as predictor the neighboring pixel closest to the
* computed value. This technique is due to Alan W. Paeth [PAETH].
*
* To compute the Paeth filter, apply the following formula to each byte of the
* scanline:
*
* Paeth(i,j) = Raw(i,j) - PaethPredictor(Raw(i-1,j), Raw(i,j-1), Raw(i-1,j-1))
*
* To decode the Paeth filter
*
* Raw(i,j) = Paeth(i,j) - PaethPredictor(Raw(i-1,j), Raw(i,j-1), Raw(i-1,j-1))
*
* @author xylifyx@yahoo.co.uk
* @version $Revision: 1.2 $
*/
public class Paeth extends PredictorAlgorithm
{
/**
* The paeth predictor function.
*
* This function is taken almost directly from the PNG definition on
* http://www.w3.org/TR/PNG-Filters.html
*
* @param a
* left
* @param b
* above
* @param c
* upper left
* @return The result of the paeth predictor.
*/
public int paethPredictor(int a, int b, int c)
{
int p = a + b - c; // initial estimate
int pa = Math.abs(p - a); // distances to a, b, c
int pb = Math.abs(p - b);
int pc = Math.abs(p - c);
// return nearest of a,b,c,
// breaking ties in order a,b,c.
if (pa <= pb && pa <= pc)
{
return a;
}
else if (pb <= pc)
{
return b;
}
else
{
return c;
}
}
/**
* @see org.pdfbox.pdmodel.graphics.predictor.PredictorAlgorithm#encodeLine(byte[], byte[],
* int, int, int, int)
*/
public void encodeLine(byte[] src, byte[] dest, int srcDy, int srcOffset,
int destDy, int destOffset)
{
int bpl = getWidth() * getBpp();
for (int x = 0; x < bpl; x++)
{
dest[x + destOffset] = (byte) (src[x + srcOffset] - paethPredictor(
leftPixel(src, srcOffset, srcDy, x), abovePixel(src,
srcOffset, srcDy, x), aboveLeftPixel(src,
srcOffset, srcDy, x)));
}
}
/**
* @see org.pdfbox.pdmodel.graphics.predictor.PredictorAlgorithm#decodeLine(byte[], byte[],
* int, int, int, int)
*/
public void decodeLine(byte[] src, byte[] dest, int srcDy, int srcOffset,
int destDy, int destOffset)
{
int bpl = getWidth() * getBpp();
for (int x = 0; x < bpl; x++)
{
dest[x + destOffset] = (byte) (src[x + srcOffset] + paethPredictor(
leftPixel(dest, destOffset, destDy, x), abovePixel(dest,
destOffset, destDy, x), aboveLeftPixel(dest,
destOffset, destDy, x)));
}
}
}