util_math_cdf.cpp

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/*
 * Copyright 2011-2015 Blender Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 
#include "util/util_math_cdf.h"
 
#include "util/util_algorithm.h"
#include "util/util_math.h"
 
CCL_NAMESPACE_BEGIN
 
/* Invert pre-calculated CDF function. */
void util_cdf_invert(const int resolution,
                     const float from,
                     const float to,
                     const vector<float> &cdf,
                     const bool make_symmetric,
                     vector<float> &inv_cdf)
{
  const float inv_resolution = 1.0f / (float)resolution;
  const float range = to - from;
  inv_cdf.resize(resolution);
  if (make_symmetric) {
    const int half_size = (resolution - 1) / 2;
    for (int i = 0; i <= half_size; i++) {
      float x = i / (float)half_size;
      int index = upper_bound(cdf.begin(), cdf.end(), x) - cdf.begin();
      float t;
      if (index < cdf.size() - 1) {
        t = (x - cdf[index]) / (cdf[index + 1] - cdf[index]);
      }
      else {
        t = 0.0f;
        index = cdf.size() - 1;
      }
      float y = ((index + t) / (resolution - 1)) * (2.0f * range);
      inv_cdf[half_size + i] = 0.5f * (1.0f + y);
      inv_cdf[half_size - i] = 0.5f * (1.0f - y);
    }
  }
  else {
    for (int i = 0; i < resolution; i++) {
      float x = from + range * (float)i * inv_resolution;
      int index = upper_bound(cdf.begin(), cdf.end(), x) - cdf.begin();
      float t;
      if (index < cdf.size() - 1) {
        t = (x - cdf[index]) / (cdf[index + 1] - cdf[index]);
      }
      else {
        t = 0.0f;
        index = resolution;
      }
      inv_cdf[i] = (index + t) * inv_resolution;
    }
  }
}
 
CCL_NAMESPACE_END