src/main/java/org/apache/commons/imaging/formats/png/scanlinefilters/ScanlineFilterPaeth.java

/*
 * 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.commons.imaging.formats.png.scanlinefilters;

import java.io.IOException;

import org.apache.commons.imaging.ImageReadException;

public class ScanlineFilterPaeth extends ScanlineFilter
{
    private final int BytesPerPixel;

    public ScanlineFilterPaeth(int BytesPerPixel)
    {
        this.BytesPerPixel = BytesPerPixel;
    }

    private int PaethPredictor(int a, int b, int c)
    {
        //                ; a = left, b = above, c = upper left
        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;
    }

    @Override
    public void unfilter(byte src[], byte dst[], byte up[])
            throws ImageReadException, IOException
    {
        for (int i = 0; i < src.length; i++)
        {
            int left = 0;
            int prev_index = i - BytesPerPixel;
            if (prev_index >= 0)
                left = dst[prev_index];

            int above = 0;
            if (up != null)
                above = up[i];
            //                above = 255;

            int upperleft = 0;
            if ((prev_index >= 0) && (up != null))
                upperleft = up[prev_index];
            //                upperleft = 255;

            int PaethPredictor = PaethPredictor(0xff & left, 0xff & above,
                    0xff & upperleft);

            dst[i] = (byte) ((src[i] + PaethPredictor) % 256);
            //                dst[i] = (byte) ((src[i] + PaethPredictor) );
            //                dst[i] = src[i];

            //                dst[i] = (byte) 0;
        }
    }
}