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// Copyright 2011 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// filter estimation
//
// Author: Urvang (urvang@google.com)
#include "src/utils/filters_utils.h"
#include <stdlib.h>
#include <string.h>
// -----------------------------------------------------------------------------
// Quick estimate of a potentially interesting filter mode to try.
#define SMAX 16
#define SDIFF(a, b) (abs((a) - (b)) >> 4) // Scoring diff, in [0..SMAX)
static WEBP_INLINE int GradientPredictor(uint8_t a, uint8_t b, uint8_t c) {
const int g = a + b - c;
return ((g & ~0xff) == 0) ? g : (g < 0) ? 0 : 255; // clip to 8bit
}
WEBP_FILTER_TYPE WebPEstimateBestFilter(const uint8_t* data,
int width, int height, int stride) {
int i, j;
int bins[WEBP_FILTER_LAST][SMAX];
memset(bins, 0, sizeof(bins));
// We only sample every other pixels. That's enough.
for (j = 2; j < height - 1; j += 2) {
const uint8_t* const p = data + j * stride;
int mean = p[0];
for (i = 2; i < width - 1; i += 2) {
const int diff0 = SDIFF(p[i], mean);
const int diff1 = SDIFF(p[i], p[i - 1]);
const int diff2 = SDIFF(p[i], p[i - width]);
const int grad_pred =
GradientPredictor(p[i - 1], p[i - width], p[i - width - 1]);
const int diff3 = SDIFF(p[i], grad_pred);
bins[WEBP_FILTER_NONE][diff0] = 1;
bins[WEBP_FILTER_HORIZONTAL][diff1] = 1;
bins[WEBP_FILTER_VERTICAL][diff2] = 1;
bins[WEBP_FILTER_GRADIENT][diff3] = 1;
mean = (3 * mean + p[i] + 2) >> 2;
}
}
{
int filter;
WEBP_FILTER_TYPE best_filter = WEBP_FILTER_NONE;
int best_score = 0x7fffffff;
for (filter = WEBP_FILTER_NONE; filter < WEBP_FILTER_LAST; ++filter) {
int score = 0;
for (i = 0; i < SMAX; ++i) {
if (bins[filter][i] > 0) {
score += i;
}
}
if (score < best_score) {
best_score = score;
best_filter = (WEBP_FILTER_TYPE)filter;
}
}
return best_filter;
}
}
#undef SMAX
#undef SDIFF
//------------------------------------------------------------------------------