/* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright ownership. * The ASF licenses this file to You 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. */ package org.apache.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: * * <code>Paeth(i,j) = Raw(i,j) - PaethPredictor(Raw(i-1,j), Raw(i,j-1), Raw(i-1,j-1))</code> * * To decode the Paeth filter * * <code>Raw(i,j) = Paeth(i,j) - PaethPredictor(Raw(i-1,j), Raw(i,j-1), Raw(i-1,j-1))</code> * * @author xylifyx@yahoo.co.uk * @version $Revision: 1.3 $ */ 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; } } /** * {@inheritDoc} */ 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))); } } /** * {@inheritDoc} */ 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))); } } }