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my part
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@dario-loi
dario-loi committed Dec 12, 2022
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218 changes: 138 additions & 80 deletions hdr.h
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#pragma once

#include "vendor/CImg.h"
using namespace cimg_library;

/**
* @brief Map LDR images to HDR
*
* Implements the Exposure Fusion algorithm described in Mertens et al. combines
* the information of multiple Low Definition Range (LDR) images to reconstruct
* a High Definition image (HDR).
*
* @see https://web.stanford.edu/class/cs231m/project-1/exposure-fusion.pdf
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the HDR image
*/
template<typename T>
CImg<T> LDR_to_HDR(CImg<T> const& LDR_images);

/**
* @brief Compute the weights for each LDR image
*
* Computes the normalized weight matrix for a stack of LDR images.
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the weights for each LDR image
*/
template<typename T>
CImg<T> compute_W(CImg<T> const& LDR_images);

/**
* @brief Compute the Laplacian pyramid of the LDR images
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the Laplacian pyramid of the LDR images
*/
template<typename T>
CImg<T> compute_contrast(CImg<T> const& LDR_images);

/**
* @brief Compute the saturation of the LDR images
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the saturation of the LDR images
*/
template<typename T>
CImg<T> compute_saturation(CImg<T> const& LDR_images);

/**
* @brief Compute the exposure of the LDR images
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the exposure of the LDR images
*/
template<typename T>
CImg<T> compute_wexp(CImg<T> const& LDR_images);
namespace hdr
{

constexpr const float EXPOSITION_VARIANCE = 0.2f;
using namespace cimg_library;

/**
* @brief Fold expression for multiplication
*
* @note non l'ho mai fatto, ma l'ho sempre sognato.
*
* @tparam Args a variadic template parameter pack
* @tparam T a type that supports multiplication
* @param args a parameter pack, containing the values to be multiplied
* @return constexpr T the result of the multiplication
*/
template <typename... Args, typename T>
constexpr inline T mul_fold(Args&&... args)
{
return 1 * ... * args;
}

/**
* @brief Map LDR images to HDR
*
* Implements the Exposure Fusion algorithm described in Mertens et al. combines
* the information of multiple Low Definition Range (LDR) images to reconstruct
* a High Definition image (HDR).
*
* @see https://web.stanford.edu/class/cs231m/project-1/exposure-fusion.pdf
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the HDR image
*/
template<typename T>
CImg<T> LDR_to_HDR(CImg<T> const& LDR_images);

/**
* @brief Compute the weights for each LDR image
*
* Computes the normalized weight matrix for a stack of LDR images.
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the weights for each LDR image
*/
template<typename T>
CImg<T> compute_W(CImg<T> const& LDR_images)
{

auto C = compute_contrast(LDR_images);
auto S = compute_saturation(LDR_images);
auto E = compute_wexp(LDR_images);

auto W = C.get_mul(S).get_mul(E);

auto& W_normalized; // alias

//W also changes, but more legible
W_normalized = W.cumulate(axis = 2).get_invert() * W;

return W_normalized;
}

/**
* @brief Compute the Contrast of the LDR images
*
* Computes a contrast metric by performing a Laplacian convolution
* on the LDR images.
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the contrast metric of the LDR images
*/
template<typename T>
CImg<T> compute_contrast(CImg<T> const& LDR_images);

/**
* @brief Compute the saturation of the LDR images
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the saturation of the LDR images
*/
template<typename T>
CImg<T> compute_saturation(CImg<T> const& LDR_images);

/**
* @brief Compute the exposure of the LDR images
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @return CImg<T> a CImg<T> object containing the exposure of the LDR images
*/
template<typename T>
CImg<T> compute_wexp(CImg<T> const& LDR_images)
{
auto E = CImg<T>{LDR_images, "x,y,z,1"};

cimg_forXYZ(img, x, y, z)
{
T buf[E.spectrum]; //should be of tiny size (three in most cases)
cimg_forC(img, c)
{
auto& i = img(x, y, z, c);
buffer[c] = exp(-pow(i - 0.5f, 2) / (2 * EXPOSITION_VARIANCE * EXPOSITION_VARIANCE));
}

E(x, y, z, 1) = mul_fold(buf[0], buf[1], buf[2]);
}

return E;
}


/**
* @brief Fuse the LDR images
*
*
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @param weights a CImg<T> object containing the weights for each LDR image
* @return CImg<T>
*/
template<typename T>
CImg<T> fuse_LDR(CImg<T> const& LDR_images, CImg<T> const& weights);

/**
* @brief Reconstruction of the HDR image from the Laplacian pyramid
*
* @tparam T the integral type of the pixel
* @param fused_laplacian a CImg<T> object containing the Laplacian pyramid of the fused LDR images
* @return CImg<T> a CImg<T> object containing the final HDR image
*/
template<typename T>
CImg<T> reconstruct_pyramid(CImg<T> const& fused_laplacian);

/**
* @brief Fuse the LDR images
*
*
*
* @tparam T the integral type of the pixel
* @param LDR_images a CImg<T> object containing the LDR images
* @param weights a CImg<T> object containing the weights for each LDR image
* @return CImg<T>
*/
template<typename T>
CImg<T> fuse_LDR(CImg<T> const& LDR_images, CImg<T> const& weights);

/**
* @brief Reconstruction of the HDR image from the Laplacian pyramid
*
* @tparam T the integral type of the pixel
* @param fused_laplacian a CImg<T> object containing the Laplacian pyramid of the fused LDR images
* @return CImg<T> a CImg<T> object containing the final HDR image
*/
template<typename T>
CImg<T> reconstruct_pyramid(CImg<T> const& fused_laplacian);

}

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