libwebp/src/utils/filters.c

// 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 "./filters.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

//------------------------------------------------------------------------------