001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.imaging.formats.png.scanlinefilters;
018
019import java.io.IOException;
020
021import org.apache.commons.imaging.ImagingException;
022
023public class ScanlineFilterPaeth implements ScanlineFilter {
024    private final int bytesPerPixel;
025
026    public ScanlineFilterPaeth(final int bytesPerPixel) {
027        this.bytesPerPixel = bytesPerPixel;
028    }
029
030    private int paethPredictor(final int a, final int b, final int c) {
031        // ; a = left, b = above, c = upper left
032        final int p = a + b - c; // ; initial estimate
033        final int pa = Math.abs(p - a); // ; distances to a, b, c
034        final int pb = Math.abs(p - b);
035        final int pc = Math.abs(p - c);
036        // ; return nearest of a,b,c,
037        // ; breaking ties in order a,b,c.
038        if (pa <= pb && pa <= pc) {
039            return a;
040        }
041        if (pb <= pc) {
042            return b;
043        }
044        return c;
045    }
046
047    @Override
048    public void unfilter(final byte[] src, final byte[] dst, final byte[] up) throws ImagingException, IOException {
049        for (int i = 0; i < src.length; i++) {
050            int left = 0;
051            final int prevIndex = i - bytesPerPixel;
052            if (prevIndex >= 0) {
053                left = dst[prevIndex];
054            }
055
056            int above = 0;
057            if (up != null) {
058                above = up[i];
059            }
060            // above = 255;
061
062            int upperLeft = 0;
063            if (prevIndex >= 0 && up != null) {
064                upperLeft = up[prevIndex];
065            }
066            // upperLeft = 255;
067
068            final int paethPredictor = paethPredictor(0xff & left, 0xff & above, 0xff & upperLeft);
069
070            dst[i] = (byte) ((src[i] + paethPredictor) % 256);
071            // dst[i] = (byte) ((src[i] + paethPredictor) );
072            // dst[i] = src[i];
073
074            // dst[i] = (byte) 0;
075        }
076    }
077}