CLogOddsGridMapLUT.h

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/* +------------------------------------------------------------------------+
   |                     Mobile Robot Programming Toolkit (MRPT)            |
   |                          https://www.mrpt.org/                         |
   |                                                                        |
   | Copyright (c) 2005-2020, Individual contributors, see AUTHORS file     |
   | See: https://www.mrpt.org/Authors - All rights reserved.               |
   | Released under BSD License. See: https://www.mrpt.org/License          |
   +------------------------------------------------------------------------+ */

#pragma once

#include <mrpt/maps/logoddscell_traits.h>
#include <cmath>

namespace mrpt::maps
{
/** One static instance of this struct should exist in any class implementing
 * CLogOddsGridMap2D to hold the Look-up-tables (LUTs) for log-odss Bayesian
 *update. Map cells must be type TCELL, which can be only:
 *      - int8_t or
 *      - int16_t
 *
 *  \sa CLogOddsGridMap2D, see derived classes for usage examples.
 * \ingroup mrpt_maps_grp
 */
template <typename TCELL>
struct CLogOddsGridMapLUT : public detail::logoddscell_traits<TCELL>
{
    /** The type of */
    using cell_t = TCELL;
    using traits_t = detail::logoddscell_traits<TCELL>;

    /** A lookup table to compute occupancy probabilities in [0,1] from integer
     * log-odds values in the cells, using \f$ p(m_{xy}) =
     * \frac{1}{1+exp(-log_odd)} \f$.
     */
    std::vector<float> logoddsTable;

    /** A lookup table to compute occupancy probabilities in the range [0,255]
     * from integer log-odds values in the cells, using \f$ p(m_{xy}) =
     * \frac{1}{1+exp(-log_odd)} \f$.
     *  This is used to speed-up conversions to grayscale images.
     */
    std::vector<uint8_t> logoddsTable_255;

    /** A lookup table for passing from float to log-odds as cell_t. */
    std::vector<cell_t> p2lTable;

    /** Constructor: computes all the required stuff. */
    CLogOddsGridMapLUT()
    {
        // The factor for converting log2-odds into integers:
        static const double LOGODD_K = 16;
        static const double LOGODD_K_INV = 1.0 / LOGODD_K;

        logoddsTable.resize(traits_t::LOGODDS_LUT_ENTRIES);
        logoddsTable_255.resize(traits_t::LOGODDS_LUT_ENTRIES);
        for (int i = traits_t::CELLTYPE_MIN; i <= traits_t::CELLTYPE_MAX; i++)
        {
            float f = 1.0f / (1.0f + std::exp(-i * LOGODD_K_INV));
            unsigned int idx = -traits_t::CELLTYPE_MIN + i;
            logoddsTable[idx] = f;
            logoddsTable_255[idx] = (uint8_t)(f * 255.0f);
        }

        // Build the p2lTable as well:
        p2lTable.resize(traits_t::P2LTABLE_SIZE + 1);
        const double K = 1.0 / traits_t::P2LTABLE_SIZE;
        for (int j = 0; j <= traits_t::P2LTABLE_SIZE; j++)
        {
            const double p = std::min(1.0 - 1e-14, std::max(1e-14, j * K));
            const double logodd = log(p) - log(1 - p);
            int L = round(logodd * LOGODD_K);
            if (L > traits_t::CELLTYPE_MAX)
                L = traits_t::CELLTYPE_MAX;
            else if (L < traits_t::CELLTYPE_MIN)
                L = traits_t::CELLTYPE_MIN;
            p2lTable[j] = L;
        }
    }

    /** Scales an integer representation of the log-odd into a real valued
     * probability in [0,1], using p=exp(l)/(1+exp(l))
     */
    inline float l2p(const cell_t l)
    {
        if (l < traits_t::CELLTYPE_MIN)
            // This is needed since min can be -127 and int8_t can be -128.
            return logoddsTable[0];
        else
            return logoddsTable[-traits_t::CELLTYPE_MIN + l];
    }

    /** Scales an integer representation of the log-odd into a linear scale
     * [0,255], using p=exp(l)/(1+exp(l))
     */
    inline uint8_t l2p_255(const cell_t l)
    {
        if (l < traits_t::CELLTYPE_MIN)
            // This is needed since min can be -127 and int8_t can be -128.
            return logoddsTable_255[0];
        else
            return logoddsTable_255[-traits_t::CELLTYPE_MIN + l];
    }

    /** Scales a real valued probability in [0,1] to an integer representation
     * of: log(p)-log(1-p)  in the valid range of cell_t.
     */
    inline cell_t p2l(const float p)
    {
        return p2lTable[static_cast<unsigned int>(p * traits_t::P2LTABLE_SIZE)];
    }
};

}  // namespace mrpt::maps