rawTherapee/rtengine/rt_algo.cc

/*
 *  This file is part of RawTherapee.
 *
 *  Copyright (c) 2017 Ingo Weyrich <heckflosse67@gmx.de>
 *
 *  RawTherapee is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  RawTherapee is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with RawTherapee.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <cstddef>
#include <cmath>
#include <cassert>
#include <algorithm>
#include <vector>
#include <cstdint>
#ifdef _OPENMP
#include <omp.h>
#endif

namespace rtengine
{
void findMinMaxPercentile (const float *data, size_t size, float minPrct, float& minOut, float maxPrct, float& maxOut, bool multithread)
{
    // we need to find the (minPrct*size) smallest value and the (maxPrct*size) smallest value in data
    // We use a histogram based search for speed and to reduce memory usage
    // memory usage of this method is histoSize * sizeof(uint32_t) * (t + 1) byte,
    // where t is the number of threads and histoSize is in [1;65536]
    // The current implementation is not guaranteed to work correctly if size > 2^32 (4294967296)
    assert (minPrct <= maxPrct);

    if(size == 0) {
        return;
    }

    size_t numThreads = 1;
#ifdef _OPENMP
    // Because we have an overhead in the critical region of the main loop for each thread
    // we make a rough calculation to reduce the number of threads for small data size
    // This also works fine for the minmax loop
    if(multithread) {
        size_t maxThreads = omp_get_max_threads();
        while (size > numThreads * numThreads * 16384 && numThreads < maxThreads) {
            ++numThreads;
        }
    }
#endif

    // We need min and max value of data to calculate the scale factor for the histogram
    float minVal = data[0];
    float maxVal = data[0];
#ifdef _OPENMP
    #pragma omp parallel for reduction(min:minVal) reduction(max:maxVal) num_threads(numThreads)
#endif
    for (size_t i = 1; i < size; ++i) {
        minVal = std::min(minVal, data[i]);
        maxVal = std::max(maxVal, data[i]);
    }

    if(std::fabs(maxVal - minVal) == 0.f) { // fast exit, also avoids division by zero in calculation of scale factor
        minOut = maxOut = minVal;
        return;
    }

    // caution: currently this works correctly only for histoSize in range[1;65536]
    // for small data size (i.e. thumbnails) we reduce the size of the histogram to the size of data
    const unsigned int histoSize = std::min(static_cast<size_t>(65536), size);

    // calculate scale factor to use full range of histogram
    const float scale = (histoSize - 1) / (maxVal - minVal);

    // We need one main histogram
    std::vector<uint32_t> histo(histoSize, 0);

    if(numThreads == 1) {
        // just one thread => use main histogram
        for (size_t i = 0; i < size; ++i) {
            // we have to subtract minVal and multiply with scale to get the data in [0;histosize] range
            histo[ (uint16_t) (scale * (data[i] - minVal))]++;
        }
    } else {
#ifdef _OPENMP
    #pragma omp parallel num_threads(numThreads)
#endif
        {
            // We need one histogram per thread
            std::vector<uint32_t> histothr(histoSize, 0);

#ifdef _OPENMP
            #pragma omp for nowait
#endif

            for (size_t i = 0; i < size; ++i) {
                // we have to subtract minVal and multiply with scale to get the data in [0;histosize] range
                histothr[ (uint16_t) (scale * (data[i] - minVal))]++;
            }

#ifdef _OPENMP
            #pragma omp critical
#endif
            {
                // add per thread histogram to main histogram
#ifdef _OPENMP
                #pragma omp simd
#endif

                for(size_t i = 0; i < histoSize; ++i) {
                    histo[i] += histothr[i];
                }
            }
        }
    }

    size_t k = 0;
    size_t count = 0;

    // find (minPrct*size) smallest value
    const float threshmin = minPrct * size;
    while (count < threshmin) {
        count += histo[k++];
    }

    if (k > 0) { // interpolate
        size_t count_ = count - histo[k - 1];
        float c0 = count - threshmin;
        float c1 = threshmin - count_;
        minOut = (c1 * k + c0 * (k - 1)) / (c0 + c1);
    } else {
        minOut = k;
    }
    // go back to original range
    minOut /= scale;
    minOut += minVal;

    // find (maxPrct*size) smallest value
    const float threshmax = maxPrct * size;
    while (count < threshmax) {
        count += histo[k++];
    }

    if (k > 0) { // interpolate
        size_t count_ = count - histo[k - 1];
        float c0 = count - threshmax;
        float c1 = threshmax - count_;
        maxOut = (c1 * k + c0 * (k - 1)) / (c0 + c1);
    } else {
        maxOut = k;
    }
    // go back to original range
    maxOut /= scale;
    maxOut += minVal;

}
}