1// Copyright 2011 Google Inc. All Rights Reserved.
2//
3// Use of this source code is governed by a BSD-style license
4// that can be found in the COPYING file in the root of the source
5// tree. An additional intellectual property rights grant can be found
6// in the file PATENTS. All contributing project authors may
7// be found in the AUTHORS file in the root of the source tree.
8// -----------------------------------------------------------------------------
9//
10// filter estimation
11//
12// Author: Urvang (urvang@google.com)
13
14#include "src/utils/filters_utils.h"
15#include <stdlib.h>
16#include <string.h>
17
18// -----------------------------------------------------------------------------
19// Quick estimate of a potentially interesting filter mode to try.
20
21#define SMAX 16
22#define SDIFF(a, b) (abs((a) - (b)) >> 4) // Scoring diff, in [0..SMAX)
23
24static WEBP_INLINE int GradientPredictor(uint8_t a, uint8_t b, uint8_t c) {
25 const int g = a + b - c;
26 return ((g & ~0xff) == 0) ? g : (g < 0) ? 0 : 255; // clip to 8bit
27}
28
29WEBP_FILTER_TYPE WebPEstimateBestFilter(const uint8_t* data,
30 int width, int height, int stride) {
31 int i, j;
32 int bins[WEBP_FILTER_LAST][SMAX];
33 memset(bins, 0, sizeof(bins));
34
35 // We only sample every other pixels. That's enough.
36 for (j = 2; j < height - 1; j += 2) {
37 const uint8_t* const p = data + j * stride;
38 int mean = p[0];
39 for (i = 2; i < width - 1; i += 2) {
40 const int diff0 = SDIFF(p[i], mean);
41 const int diff1 = SDIFF(p[i], p[i - 1]);
42 const int diff2 = SDIFF(p[i], p[i - width]);
43 const int grad_pred =
44 GradientPredictor(p[i - 1], p[i - width], p[i - width - 1]);
45 const int diff3 = SDIFF(p[i], grad_pred);
46 bins[WEBP_FILTER_NONE][diff0] = 1;
47 bins[WEBP_FILTER_HORIZONTAL][diff1] = 1;
48 bins[WEBP_FILTER_VERTICAL][diff2] = 1;
49 bins[WEBP_FILTER_GRADIENT][diff3] = 1;
50 mean = (3 * mean + p[i] + 2) >> 2;
51 }
52 }
53 {
54 int filter;
55 WEBP_FILTER_TYPE best_filter = WEBP_FILTER_NONE;
56 int best_score = 0x7fffffff;
57 for (filter = WEBP_FILTER_NONE; filter < WEBP_FILTER_LAST; ++filter) {
58 int score = 0;
59 for (i = 0; i < SMAX; ++i) {
60 if (bins[filter][i] > 0) {
61 score += i;
62 }
63 }
64 if (score < best_score) {
65 best_score = score;
66 best_filter = (WEBP_FILTER_TYPE)filter;
67 }
68 }
69 return best_filter;
70 }
71}
72
73#undef SMAX
74#undef SDIFF
75
76//------------------------------------------------------------------------------
77