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25 
26 package com.sun.imageio.plugins.png;
27 
28 public class RowFilter {
29 
abs(int x)30     private static final int abs(int x) {
31         return (x < 0) ? -x : x;
32     }
33 
34     // Returns the sum of absolute differences
subFilter(byte[] currRow, byte[] subFilteredRow, int bytesPerPixel, int bytesPerRow)35     protected static int subFilter(byte[] currRow,
36                                    byte[] subFilteredRow,
37                                    int bytesPerPixel,
38                                    int bytesPerRow) {
39         int badness = 0;
40         for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
41             int curr = currRow[i] & 0xff;
42             int left = currRow[i - bytesPerPixel] & 0xff;
43             int difference = curr - left;
44             subFilteredRow[i] = (byte)difference;
45 
46             badness += abs(difference);
47         }
48 
49         return badness;
50     }
51 
52     // Returns the sum of absolute differences
upFilter(byte[] currRow, byte[] prevRow, byte[] upFilteredRow, int bytesPerPixel, int bytesPerRow)53     protected static int upFilter(byte[] currRow,
54                                   byte[] prevRow,
55                                   byte[] upFilteredRow,
56                                   int bytesPerPixel,
57                                   int bytesPerRow) {
58         int badness = 0;
59         for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
60             int curr = currRow[i] & 0xff;
61             int up = prevRow[i] & 0xff;
62             int difference = curr - up;
63             upFilteredRow[i] = (byte)difference;
64 
65             badness += abs(difference);
66         }
67 
68         return badness;
69     }
70 
paethPredictor(int a, int b, int c)71     protected final int paethPredictor(int a, int b, int c) {
72         int p = a + b - c;
73         int pa = abs(p - a);
74         int pb = abs(p - b);
75         int pc = abs(p - c);
76 
77         if ((pa <= pb) && (pa <= pc)) {
78             return a;
79         } else if (pb <= pc) {
80             return b;
81         } else {
82             return c;
83         }
84     }
85 
filterRow(int colorType, byte[] currRow, byte[] prevRow, byte[][] scratchRows, int bytesPerRow, int bytesPerPixel)86     public int filterRow(int colorType,
87                          byte[] currRow,
88                          byte[] prevRow,
89                          byte[][] scratchRows,
90                          int bytesPerRow,
91                          int bytesPerPixel) {
92 
93         // Use type 0 for palette images
94         if (colorType != PNGImageReader.PNG_COLOR_PALETTE) {
95             System.arraycopy(currRow, bytesPerPixel,
96                              scratchRows[0], bytesPerPixel,
97                              bytesPerRow);
98             return 0;
99         }
100 
101         int[] filterBadness = new int[5];
102         for (int i = 0; i < 5; i++) {
103             filterBadness[i] = Integer.MAX_VALUE;
104         }
105 
106         {
107             int badness = 0;
108 
109             for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
110                 int curr = currRow[i] & 0xff;
111                 badness += curr;
112             }
113 
114             filterBadness[0] = badness;
115         }
116 
117         {
118             byte[] subFilteredRow = scratchRows[1];
119             int badness = subFilter(currRow,
120                                     subFilteredRow,
121                                     bytesPerPixel,
122                                     bytesPerRow);
123 
124             filterBadness[1] = badness;
125         }
126 
127         {
128             byte[] upFilteredRow = scratchRows[2];
129             int badness = upFilter(currRow,
130                                    prevRow,
131                                    upFilteredRow,
132                                    bytesPerPixel,
133                                    bytesPerRow);
134 
135             filterBadness[2] = badness;
136         }
137 
138         {
139             byte[] averageFilteredRow = scratchRows[3];
140             int badness = 0;
141 
142             for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
143                 int curr = currRow[i] & 0xff;
144                 int left = currRow[i - bytesPerPixel] & 0xff;
145                 int up = prevRow[i] & 0xff;
146                 int difference = curr - (left + up)/2;;
147                 averageFilteredRow[i] = (byte)difference;
148 
149                 badness += abs(difference);
150             }
151 
152             filterBadness[3] = badness;
153         }
154 
155         {
156             byte[] paethFilteredRow = scratchRows[4];
157             int badness = 0;
158 
159             for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
160                 int curr = currRow[i] & 0xff;
161                 int left = currRow[i - bytesPerPixel] & 0xff;
162                 int up = prevRow[i] & 0xff;
163                 int upleft = prevRow[i - bytesPerPixel] & 0xff;
164                 int predictor = paethPredictor(left, up, upleft);
165                 int difference = curr - predictor;
166                 paethFilteredRow[i] = (byte)difference;
167 
168                 badness += abs(difference);
169             }
170 
171             filterBadness[4] = badness;
172         }
173 
174         int minBadness = filterBadness[0];
175         int filterType = 0;
176 
177         for (int i = 1; i < 5; i++) {
178             if (filterBadness[i] < minBadness) {
179                 minBadness = filterBadness[i];
180                 filterType = i;
181             }
182         }
183 
184         if (filterType == 0) {
185             System.arraycopy(currRow, bytesPerPixel,
186                              scratchRows[0], bytesPerPixel,
187                              bytesPerRow);
188         }
189 
190         return filterType;
191     }
192 }
193