    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}