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 (at) google.com) 13 14 #include "./filters.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 24 static 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 29 WEBP_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
