JDK8/Java8源码在线阅读

/*
 * Copyright (c) 2000, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package com.sun.imageio.plugins.png;

public class RowFilter {

    private static final int abs(int x) {
        return (x < 0) ? -x : x;
    }

    // Returns the sum of absolute differences
    protected static int subFilter(byte[] currRow,
                                   byte[] subFilteredRow,
                                   int bytesPerPixel,
                                   int bytesPerRow) {
        int badness = 0;
        for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
            int curr = currRow[i] & 0xff;
            int left = currRow[i - bytesPerPixel] & 0xff;
            int difference = curr - left;
            subFilteredRow[i] = (byte)difference;

            badness += abs(difference);
        }

        return badness;
    }

    // Returns the sum of absolute differences
    protected static int upFilter(byte[] currRow,
                                  byte[] prevRow,
                                  byte[] upFilteredRow,
                                  int bytesPerPixel,
                                  int bytesPerRow) {
        int badness = 0;
        for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
            int curr = currRow[i] & 0xff;
            int up = prevRow[i] & 0xff;
            int difference = curr - up;
            upFilteredRow[i] = (byte)difference;

            badness += abs(difference);
        }

        return badness;
    }

    protected final int paethPredictor(int a, int b, int c) {
        int p = a + b - c;
        int pa = abs(p - a);
        int pb = abs(p - b);
        int pc = abs(p - c);

        if ((pa <= pb) && (pa <= pc)) {
            return a;
        } else if (pb <= pc) {
            return b;
        } else {
            return c;
        }
    }

    public int filterRow(int colorType,
                         byte[] currRow,
                         byte[] prevRow,
                         byte[][] scratchRows,
                         int bytesPerRow,
                         int bytesPerPixel) {

        // Use type 0 for palette images
        if (colorType != PNGImageReader.PNG_COLOR_PALETTE) {
            System.arraycopy(currRow, bytesPerPixel,
                             scratchRows[0], bytesPerPixel,
                             bytesPerRow);
            return 0;
        }

        int[] filterBadness = new int[5];
        for (int i = 0; i < 5; i++) {
            filterBadness[i] = Integer.MAX_VALUE;
        }

        {
            int badness = 0;

            for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
                int curr = currRow[i] & 0xff;
                badness += curr;
            }

            filterBadness[0] = badness;
        }

        {
            byte[] subFilteredRow = scratchRows[1];
            int badness = subFilter(currRow,
                                    subFilteredRow,
                                    bytesPerPixel,
                                    bytesPerRow);

            filterBadness[1] = badness;
        }

        {
            byte[] upFilteredRow = scratchRows[2];
            int badness = upFilter(currRow,
                                   prevRow,
                                   upFilteredRow,
                                   bytesPerPixel,
                                   bytesPerRow);

            filterBadness[2] = badness;
        }

        {
            byte[] averageFilteredRow = scratchRows[3];
            int badness = 0;

            for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
                int curr = currRow[i] & 0xff;
                int left = currRow[i - bytesPerPixel] & 0xff;
                int up = prevRow[i] & 0xff;
                int difference = curr - (left + up)/2;;
                averageFilteredRow[i] = (byte)difference;

                badness += abs(difference);
            }

            filterBadness[3] = badness;
        }

        {
            byte[] paethFilteredRow = scratchRows[4];
            int badness = 0;

            for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
                int curr = currRow[i] & 0xff;
                int left = currRow[i - bytesPerPixel] & 0xff;
                int up = prevRow[i] & 0xff;
                int upleft = prevRow[i - bytesPerPixel] & 0xff;
                int predictor = paethPredictor(left, up, upleft);
                int difference = curr - predictor;

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