hazeremoval.c

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/*
    This file is part of darktable,
    Copyright (C) 2017-2020 Heiko Bauke.
    Copyright (C) 2017 luzpaz.
    Copyright (C) 2017, 2019 Tobias Ellinghaus.
    Copyright (C) 2018, 2020, 2023, 2025-2026 Aurélien PIERRE.
    Copyright (C) 2018 Edgardo Hoszowski.
    Copyright (C) 2018 Maurizio Paglia.
    Copyright (C) 2018, 2020, 2022 Pascal Obry.
    Copyright (C) 2018 rawfiner.
    Copyright (C) 2019 Andreas Schneider.
    Copyright (C) 2019 Diederik ter Rahe.
    Copyright (C) 2020 Aldric Renaudin.
    Copyright (C) 2020, 2022 Diederik Ter Rahe.
    Copyright (C) 2020, 2022 Ralf Brown.
    Copyright (C) 2022 Hanno Schwalm.
    Copyright (C) 2022 Martin Bařinka.
    Copyright (C) 2022 Philipp Lutz.
    Copyright (C) 2024 Alban Gruin.
    Copyright (C) 2024 Alynx Zhou.
    
    darktable 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.
    
    darktable 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 darktable.  If not, see <http://www.gnu.org/licenses/>.
*/
 
/*
    This module implements automatic single-image haze removal as
    described by K. He et al. in
 
    * Kaiming He, Jian Sun, and Xiaoou Tang, "Single Image Haze
      Removal Using Dark Channel Prior," IEEE Transactions on Pattern
      Analysis and Machine Intelligence, vol. 33, no. 12,
      pp. 2341-2353, Dec. 2011. DOI: 10.1109/TPAMI.2010.168
 
    * K. He, J. Sun, and X. Tang, "Guided Image Filtering," Lecture
      Notes in Computer Science, pp. 1-14, 2010. DOI:
      10.1007/978-3-642-15549-9_1
*/
 
 
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
 
#include "widgets/bauhaus.h"
#include "pixel/box_filters.h"
#include "system/macros.h"
#include "system/openmp.h"
#include "system/target_clones.h"
#include "system/mem_alloc.h"
#include "common/logging.h"
#include "common/module_versioning.h"
#include "caches/pixelpipe_cache_alloc.h"
#include "pixel/guided_filter.h"
#include "control/signal.h"
#include "develop/imageop.h"
#include "develop/imageop_math.h"
#include "develop/imageop_gui.h"
 
#include "develop/tiling.h"
#include "iop/iop_api.h"
 
#ifdef HAVE_OPENCL
#include "common/opencl.h"
#endif
 
#include <float.h>
#include <gtk/gtk.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
 
//----------------------------------------------------------------------
// implement the module api
//----------------------------------------------------------------------
 
DT_MODULE_INTROSPECTION(1, dt_iop_hazeremoval_params_t)
 
typedef float rgb_pixel[3];
 
typedef struct dt_iop_hazeremoval_params_t
{
  float strength; // $MIN: -1.0 $MAX: 1.0 $DEFAULT: 0.2
  float distance; // $MIN:  0.0 $MAX: 1.0 $DEFAULT: 0.2
} dt_iop_hazeremoval_params_t;
 
// types  dt_iop_hazeremoval_params_t and dt_iop_hazeremoval_data_t are
// equal, thus no commit_params function needs to be implemented
typedef dt_iop_hazeremoval_params_t dt_iop_hazeremoval_data_t;
 
void commit_params(struct dt_iop_module_t *self, dt_iop_params_t *params,
                  dt_dev_pixelpipe_t *pipe, dt_dev_pixelpipe_iop_t *piece)
{
  memcpy(piece->data, params, self->params_size);
  piece->cache_output_on_ram = TRUE;
}
 
typedef struct dt_iop_hazeremoval_gui_data_t
{
  GtkWidget *strength;
  GtkWidget *distance;
  rgb_pixel A0;
  float distance_max;
  uint64_t expected_preview_hash;
  uint64_t hash;
} dt_iop_hazeremoval_gui_data_t;
 
typedef struct dt_iop_hazeremoval_global_data_t
{
  int kernel_hazeremoval_transision_map;
  int kernel_hazeremoval_box_min_x;
  int kernel_hazeremoval_box_min_y;
  int kernel_hazeremoval_box_max_x;
  int kernel_hazeremoval_box_max_y;
  int kernel_hazeremoval_dehaze;
} dt_iop_hazeremoval_global_data_t;
 
 
const char *name()
{
  return _("haze removal");
}
 
 
const char *aliases()
{
  return _("dehaze|defog|smoke|smog");
}
 
const char **description(struct dt_iop_module_t *self)
{
  return dt_iop_set_description(self, _("remove fog and atmospheric hazing from pictures"),
                                      _("corrective"),
                                      _("linear, RGB, scene-referred"),
                                      _("frequential, RGB"),
                                      _("linear, RGB, scene-referred"));
}
 
int flags()
{
  return IOP_FLAGS_INCLUDE_IN_STYLES | IOP_FLAGS_SUPPORTS_BLENDING;
}
 
 
int default_group()
{
  return IOP_GROUP_REPAIR;
}
 
 
int default_colorspace(dt_iop_module_t *self, dt_dev_pixelpipe_t *pipe, const dt_dev_pixelpipe_iop_t *piece)
{
  return IOP_CS_RGB;
}
 
 
void init_pipe(struct dt_iop_module_t *self, dt_dev_pixelpipe_t *pipe, dt_dev_pixelpipe_iop_t *piece)
{
  piece->data = dt_calloc_align(sizeof(dt_iop_hazeremoval_data_t));
  piece->data_size = sizeof(dt_iop_hazeremoval_data_t);
}
 
 
void cleanup_pipe(struct dt_iop_module_t *self, dt_dev_pixelpipe_t *pipe, dt_dev_pixelpipe_iop_t *piece)
{
  dt_free_align(piece->data);
  piece->data = NULL;
}
 
 
void init_global(dt_iop_module_so_t *self)
{
  dt_iop_hazeremoval_global_data_t *gd = malloc(sizeof(*gd));
  const int program = 27; // hazeremoval.cl, from programs.conf
  gd->kernel_hazeremoval_transision_map = dt_opencl_create_kernel(program, "hazeremoval_transision_map");
  gd->kernel_hazeremoval_box_min_x = dt_opencl_create_kernel(program, "hazeremoval_box_min_x");
  gd->kernel_hazeremoval_box_min_y = dt_opencl_create_kernel(program, "hazeremoval_box_min_y");
  gd->kernel_hazeremoval_box_max_x = dt_opencl_create_kernel(program, "hazeremoval_box_max_x");
  gd->kernel_hazeremoval_box_max_y = dt_opencl_create_kernel(program, "hazeremoval_box_max_y");
  gd->kernel_hazeremoval_dehaze = dt_opencl_create_kernel(program, "hazeremoval_dehaze");
  self->data = gd;
}
 
 
void cleanup_global(dt_iop_module_so_t *self)
{
  dt_iop_hazeremoval_global_data_t *gd = self->data;
  dt_opencl_free_kernel(gd->kernel_hazeremoval_transision_map);
  dt_opencl_free_kernel(gd->kernel_hazeremoval_box_min_x);
  dt_opencl_free_kernel(gd->kernel_hazeremoval_box_min_y);
  dt_opencl_free_kernel(gd->kernel_hazeremoval_box_max_x);
  dt_opencl_free_kernel(gd->kernel_hazeremoval_box_max_y);
  dt_opencl_free_kernel(gd->kernel_hazeremoval_dehaze);
  dt_free(self->data);
}
 
 
void gui_update(struct dt_iop_module_t *self)
{
  dt_iop_hazeremoval_gui_data_t *g = (dt_iop_hazeremoval_gui_data_t *)self->gui_data;
 
  dt_iop_gui_enter_critical_section(self);
  g->distance_max = NAN;
  g->A0[0] = NAN;
  g->A0[1] = NAN;
  g->A0[2] = NAN;
  g->expected_preview_hash = DT_PIXELPIPE_CACHE_HASH_INVALID;
  g->hash = DT_PIXELPIPE_CACHE_HASH_INVALID;
  dt_iop_gui_leave_critical_section(self);
}
 
static uint64_t _current_preview_hash(dt_iop_module_t *self)
{
  if(IS_NULL_PTR(self) || IS_NULL_PTR(self->dev) || IS_NULL_PTR(self->dev->preview_pipe)) return DT_PIXELPIPE_CACHE_HASH_INVALID;
 
  dt_dev_pixelpipe_iop_t *piece = dt_dev_distort_get_iop_pipe(self->dev->preview_pipe, self);
  if(IS_NULL_PTR(piece) || !piece->enabled || piece->roi_in.width <= 0 || piece->roi_in.height <= 0)
    return DT_PIXELPIPE_CACHE_HASH_INVALID;
 
  return piece->global_hash;
}
 
static void _history_resync_callback(gpointer instance, gpointer user_data)
{
  (void)instance;
  dt_iop_module_t *self = (dt_iop_module_t *)user_data;
  dt_iop_hazeremoval_gui_data_t *g = (dt_iop_hazeremoval_gui_data_t *)self->gui_data;
  if(IS_NULL_PTR(g)) return;
 
  const uint64_t preview_hash = _current_preview_hash(self);
  dt_iop_gui_enter_critical_section(self);
  g->expected_preview_hash = preview_hash;
  dt_iop_gui_leave_critical_section(self);
}
 
 
void gui_init(dt_iop_module_t *self)
{
  dt_iop_hazeremoval_gui_data_t *g = IOP_GUI_ALLOC(hazeremoval);
 
  g->distance_max = NAN;
  g->A0[0] = NAN;
  g->A0[1] = NAN;
  g->A0[2] = NAN;
  g->expected_preview_hash = DT_PIXELPIPE_CACHE_HASH_INVALID;
  g->hash = DT_PIXELPIPE_CACHE_HASH_INVALID;
 
  g->strength = dt_bauhaus_slider_from_params(self, N_("strength"));
  gtk_widget_set_tooltip_text(g->strength, _("amount of haze reduction"));
 
  g->distance = dt_bauhaus_slider_from_params(self, N_("distance"));
  dt_bauhaus_slider_set_digits(g->distance, 3);
  gtk_widget_set_tooltip_text(g->distance, _("limit haze removal up to a specific spatial depth"));
 
  DT_DEBUG_CONTROL_SIGNAL_CONNECT(dt_control_signal_get_global(), DT_SIGNAL_HISTORY_RESYNC,
                                  G_CALLBACK(_history_resync_callback), self);
}
 
 
void gui_cleanup(dt_iop_module_t *self)
{
  DT_DEBUG_CONTROL_SIGNAL_DISCONNECT(dt_control_signal_get_global(), G_CALLBACK(_history_resync_callback), self);
  IOP_GUI_FREE;
}
 
//----------------------------------------------------------------------
// module local functions and structures required by process function
//----------------------------------------------------------------------
 
typedef struct tile
{
  int left, right, lower, upper;
} tile;
 
 
typedef struct rgb_image
{
  float *data;
  int width, height, stride;
} rgb_image;
 
 
typedef struct const_rgb_image
{
  const float *data;
  int width, height, stride;
} const_rgb_image;
 
 
 
 
// swap the two floats that the pointers point to
static inline void pointer_swap_f(float *a, float *b)
{
  float t = *a;
  *a = *b;
  *b = t;
}
 
 
// calculate the dark channel (minimal color component over a box of size (2*w+1) x (2*w+1) )
__DT_CLONE_TARGETS__
static int dark_channel(const const_rgb_image img1, const gray_image img2, const int w)
{
  const size_t size = (size_t)img1.height * img1.width;
  __OMP_PARALLEL_FOR__()
  for(size_t i = 0; i < size; i++)
  {
    const float *pixel = img1.data + i * img1.stride;
    float m = pixel[0];
    m = fminf(pixel[1], m);
    m = fminf(pixel[2], m);
    img2.data[i] = m;
  }
  if(dt_box_min(img2.data, img2.height, img2.width, 1, w) != 0)
  {
    return 1;
  }
  return 0;
}
 
 
// calculate the transition map
__DT_CLONE_TARGETS__
static int transition_map(const const_rgb_image img1, const gray_image img2, const int w, const float *const A0,
                          const float strength)
{
  const size_t size = (size_t)img1.height * img1.width;
  __OMP_PARALLEL_FOR__()
  for(size_t i = 0; i < size; i++)
  {
    const float *pixel = img1.data + i * img1.stride;
    float m = pixel[0] / A0[0];
    m = fminf(pixel[1] / A0[1], m);
    m = fminf(pixel[2] / A0[2], m);
    img2.data[i] = 1.f - m * strength;
  }
  if(dt_box_max(img2.data, img2.height, img2.width, 1, w) != 0)
  {
    return 1;
  }
  return 0;
}
 
 
// partition the array [first, last) using the pivot value val, i.e.,
// reorder the elements in the range [first, last) in such a way that
// all elements that are less than the pivot precede the elements
// which are larger or equal the pivot
static float *partition(float *first, float *last, float val)
{
  for(; first != last; ++first)
  {
    if(!((*first) < val)) break;
  }
  if(first == last) return first;
  for(float *i = first + 1; i != last; ++i)
  {
    if((*i) < val)
    {
      pointer_swap_f(i, first);
      ++first;
    }
  }
  return first;
}
 
 
// quick select algorithm, arranges the range [first, last) such that
// the element pointed to by nth is the same as the element that would
// be in that position if the entire range [first, last) had been
// sorted, additionally, none of the elements in the range [nth, last)
// is less than any of the elements in the range [first, nth)
__DT_CLONE_TARGETS__
void quick_select(float *first, float *nth, float *last)
{
  if(first == last) return;
  for(;;)
  {
    // select pivot by median of three heuristic for better performance
    float *p1 = first;
    float *pivot = first + (last - first) / 2;
    float *p3 = last - 1;
    if(!(*p1 < *pivot)) pointer_swap_f(p1, pivot);
    if(!(*p1 < *p3)) pointer_swap_f(p1, p3);
    if(!(*pivot < *p3)) pointer_swap_f(pivot, p3);
    pointer_swap_f(pivot, last - 1); // move pivot to end
    partition(first, last - 1, *(last - 1));
    pointer_swap_f(last - 1, pivot); // move pivot to its final place
    if(nth == pivot)
      break;
    else if(nth < pivot)
      last = pivot;
    else
      first = pivot + 1;
  }
}
 
 
// calculate diffusive ambient light and the maximal depth in the image
// depth is estimated by the local amount of haze and given in units of the
// characteristic haze depth, i.e., the distance over which object light is
// reduced by the factor exp(-1)
__DT_CLONE_TARGETS__
static int ambient_light(const const_rgb_image img, int w1, rgb_pixel *pA0, float *max_depth_out)
{
  const float dark_channel_quantil = 0.95f; // quantil for determining the most hazy pixels
  const float bright_quantil = 0.95f; // quantil for determining the brightest pixels among the most hazy pixels
  const int width = img.width;
  const int height = img.height;
  const size_t size = (size_t)width * height;
  // calculate dark channel, which is an estimate for local amount of haze
  gray_image dark_ch = { 0 };
  gray_image bright_hazy = { 0 };
  if(new_gray_image(&dark_ch, width, height)) return 1;
  if(dark_channel(img, dark_ch, w1)) goto error;
  // determine the brightest pixels among the most hazy pixels
  if(new_gray_image(&bright_hazy, width, height)) goto error;
  // first determine the most hazy pixels
  copy_gray_image(dark_ch, bright_hazy);
  size_t p = (size_t)(size * dark_channel_quantil);
  quick_select(bright_hazy.data, bright_hazy.data + p, bright_hazy.data + size);
  const float crit_haze_level = bright_hazy.data[p];
  size_t N_most_hazy = 0;
  for(size_t i = 0; i < size; i++)
    if(dark_ch.data[i] >= crit_haze_level)
    {
      const float *pixel_in = img.data + i * img.stride;
      // next line prevents parallelization via OpenMP
      bright_hazy.data[N_most_hazy] = pixel_in[0] + pixel_in[1] + pixel_in[2];
      N_most_hazy++;
    }
  p = (size_t)(N_most_hazy * bright_quantil);
  quick_select(bright_hazy.data, bright_hazy.data + p, bright_hazy.data + N_most_hazy);
  const float crit_brightness = bright_hazy.data[p];
  free_gray_image(&bright_hazy);
  // average over the brightest pixels among the most hazy pixels to
  // estimate the diffusive ambient light
  float A0_r = 0, A0_g = 0, A0_b = 0;
  size_t N_bright_hazy = 0;
  const float *const data = dark_ch.data;
  __OMP_PARALLEL_FOR__(reduction(+ : N_bright_hazy, A0_r, A0_g, A0_b))
  for(size_t i = 0; i < size; i++)
  {
    const float *pixel_in = img.data + i * img.stride;
    if((data[i] >= crit_haze_level) && (pixel_in[0] + pixel_in[1] + pixel_in[2] >= crit_brightness))
    {
      A0_r += pixel_in[0];
      A0_g += pixel_in[1];
      A0_b += pixel_in[2];
      N_bright_hazy++;
    }
  }
  if(N_bright_hazy > 0)
  {
    A0_r /= N_bright_hazy;
    A0_g /= N_bright_hazy;
    A0_b /= N_bright_hazy;
  }
  (*pA0)[0] = A0_r;
  (*pA0)[1] = A0_g;
  (*pA0)[2] = A0_b;
  free_gray_image(&dark_ch);
  // for almost haze free images it may happen that crit_haze_level=0, this means
  // there is a very large image depth, in this case a large number is returned, that
  // is small enough to avoid overflow in later processing
  // the critical haze level is at dark_channel_quantil (not 100%) to be insensitive
  // to extreme outliners, compensate for that by some factor slightly larger than
  // unity when calculating the maximal image depth
  if(max_depth_out)
    *max_depth_out = crit_haze_level > 0 ? -1.125f * logf(crit_haze_level) : logf(FLT_MAX) / 2;
  return 0;
 
error:
  if(bright_hazy.data) free_gray_image(&bright_hazy);
  if(dark_ch.data) free_gray_image(&dark_ch);
  return 1;
}
 
 
__DT_CLONE_TARGETS__
int process(struct dt_iop_module_t *self, const dt_dev_pixelpipe_t *pipe, const dt_dev_pixelpipe_iop_t *piece, const void *const ivoid,
             void *const ovoid)
{
  const dt_iop_roi_t *const roi_in = &piece->roi_in;
  const dt_iop_roi_t *const roi_out = &piece->roi_out;
  dt_iop_hazeremoval_gui_data_t *const g = (dt_iop_hazeremoval_gui_data_t*)self->gui_data;
  dt_iop_hazeremoval_params_t *d = piece->data;
  int err = 0;
  gray_image trans_map = (gray_image){ 0 };
  gray_image trans_map_filtered = (gray_image){ 0 };
 
  const int ch = piece->dsc_in.channels;
  const int width = roi_in->width;
  const int height = roi_in->height;
  const size_t size = (size_t)width * height;
  const int w1 = 6; // window size (positive integer) for determining the dark channel and the transition map
  const int w2 = 9; // window size (positive integer) for the guided filter
 
  // module parameters
  const float strength = d->strength; // strength of haze removal
  const float distance = d->distance; // maximal distance from camera to remove haze
  const float eps = sqrtf(0.025f);    // regularization parameter for guided filter
 
  const const_rgb_image img_in = (const_rgb_image){ ivoid, width, height, ch };
  const rgb_image img_out = (rgb_image){ ovoid, width, height, ch };
 
  // estimate diffusive ambient light and image depth
  rgb_pixel A0;
  A0[0] = NAN;
  A0[1] = NAN;
  A0[2] = NAN;
  float distance_max = NAN;
 
  // hazeremoval module needs the color and the haziness (which yields
  // distance_max) of the most hazy region of the image.  In pixelpipe
  // FULL we can not reliably get this value as the pixelpipe might
  // only see part of the image (region of interest).  Therefore, we
  // try to get A0 and distance_max from the PREVIEW pixelpipe which
  // luckily stores it for us.
  if(self->dev->gui_attached && !IS_NULL_PTR(g) && !dt_dev_pixelpipe_has_preview_output(self->dev, pipe, roi_out))
  {
    dt_iop_gui_enter_critical_section(self);
    const uint64_t hash = g->hash;
    const uint64_t expected_preview_hash = g->expected_preview_hash;
    dt_iop_gui_leave_critical_section(self);
    /* Full-pipe dehazing needs the preview statistics only when they belong to the
     * currently resynchronized preview graph. HISTORY_RESYNC publishes that expected
     * preview hash before either pipe starts processing, so a mismatch here means
     * preview has not produced the new full-image reading yet and we must recompute
     * locally instead of reusing stale GUI state. */
    if(hash != DT_PIXELPIPE_CACHE_HASH_INVALID
       && hash == expected_preview_hash)
    {
      dt_iop_gui_enter_critical_section(self);
      A0[0] = g->A0[0];
      A0[1] = g->A0[1];
      A0[2] = g->A0[2];
      distance_max = g->distance_max;
      dt_iop_gui_leave_critical_section(self);
    }
  }
  // In all other cases we calculate distance_max and A0 here.
  if(isnan(distance_max))
  {
    if(ambient_light(img_in, w1, &A0, &distance_max) != 0)
    {
      err = 1;
      goto error;
    }
  }
  // PREVIEW pixelpipe stores values.
  if(self->dev->gui_attached && !IS_NULL_PTR(g) && dt_dev_pixelpipe_has_preview_output(self->dev, pipe, roi_out))
  {
    uint64_t hash = piece->global_hash;
    dt_iop_gui_enter_critical_section(self);
    g->A0[0] = A0[0];
    g->A0[1] = A0[1];
    g->A0[2] = A0[2];
    g->distance_max = distance_max;
    g->expected_preview_hash = hash;
    g->hash = hash;
    dt_iop_gui_leave_critical_section(self);
  }
 
  // calculate the transition map
  if(new_gray_image(&trans_map, width, height))
  {
    err = 1;
    goto error;
  }
  if(transition_map(img_in, trans_map, w1, A0, strength))
  {
    err = 1;
    goto error;
  }
 
  // refine the transition map
  if(dt_box_min(trans_map.data, trans_map.height, trans_map.width, 1, w1))
  {
    err = 1;
    goto error;
  }
  if(new_gray_image(&trans_map_filtered, width, height))
  {
    err = 1;
    goto error;
  }
  // apply guided filter with no clipping
  if(guided_filter(img_in.data, trans_map.data, trans_map_filtered.data, width, height, ch, w2, eps, 1.f, -FLT_MAX,
                   FLT_MAX))
  {
    err = 1;
    goto error;
  }
 
  // finally, calculate the haze-free image
  const float t_min
      = fminf(fmaxf(expf(-distance * distance_max), 1.f / 1024), 1.f); // minimum allowed value for transition map
  const float *const c_A0 = A0;
  const gray_image c_trans_map_filtered = trans_map_filtered;
  __OMP_PARALLEL_FOR__()
  for(size_t i = 0; i < size; i++)
  {
    float t = fmaxf(c_trans_map_filtered.data[i], t_min);
    const float *pixel_in = img_in.data + i * img_in.stride;
    float *pixel_out = img_out.data + i * img_out.stride;
    pixel_out[0] = (pixel_in[0] - c_A0[0]) / t + c_A0[0];
    pixel_out[1] = (pixel_in[1] - c_A0[1]) / t + c_A0[1];
    pixel_out[2] = (pixel_in[2] - c_A0[2]) / t + c_A0[2];
  }
 
  free_gray_image(&trans_map);
  free_gray_image(&trans_map_filtered);
 
  if(pipe->mask_display & DT_DEV_PIXELPIPE_DISPLAY_MASK)
    dt_iop_alpha_copy(ivoid, ovoid, roi_out->width, roi_out->height);
  return 0;
 
error:
  if(trans_map.data) free_gray_image(&trans_map);
  if(trans_map_filtered.data) free_gray_image(&trans_map_filtered);
  return err;
}
 
#ifdef HAVE_OPENCL
 
// calculate diffusive ambient light and the maximal depth in the image
// depth is estimated by the local amount of haze and given in units of the
// characteristic haze depth, i.e., the distance over which object light is
// reduced by the factor exp(-1)
// some parts of the calculation are not suitable for a parallel implementation,
// thus we copy data to host memory fall back to a cpu routine
static int ambient_light_cl(struct dt_iop_module_t *self, int devid, cl_mem img, int w1, rgb_pixel *pA0,
                            float *max_depth_out)
{
  const int width = dt_opencl_get_image_width(img);
  const int height = dt_opencl_get_image_height(img);
  const int element_size = dt_opencl_get_image_element_size(img);
  float *in = dt_pixelpipe_cache_alloc_align_float_cache(
      (size_t)width * height * element_size,
      0);
  if(IS_NULL_PTR(in)) goto error;
 
  int err = dt_opencl_read_host_from_device(devid, in, img, width, height, element_size);
  if(err != CL_SUCCESS) goto error;
  const const_rgb_image img_in = (const_rgb_image){ in, width, height, element_size / sizeof(float) };
  if(ambient_light(img_in, w1, pA0, max_depth_out) != 0)
  {
    err = CL_MEM_OBJECT_ALLOCATION_FAILURE;
    goto error;
  }
  dt_pixelpipe_cache_free_align(in);
  return 0;
 
error:
  dt_pixelpipe_cache_free_align(in);
  return 1;
}
 
 
static int box_min_cl(struct dt_iop_module_t *self, int devid, cl_mem in, cl_mem out, const int w)
{
  dt_iop_hazeremoval_global_data_t *gd = self->global_data;
  const int width = dt_opencl_get_image_width(in);
  const int height = dt_opencl_get_image_height(in);
  void *temp = dt_opencl_alloc_device(devid, width, height, (int)sizeof(float));
 
  const int kernel_x = gd->kernel_hazeremoval_box_min_x;
  dt_opencl_set_kernel_arg(devid, kernel_x, 0, sizeof(int), &width);
  dt_opencl_set_kernel_arg(devid, kernel_x, 1, sizeof(int), &height);
  dt_opencl_set_kernel_arg(devid, kernel_x, 2, sizeof(cl_mem), &in);
  dt_opencl_set_kernel_arg(devid, kernel_x, 3, sizeof(cl_mem), &temp);
  dt_opencl_set_kernel_arg(devid, kernel_x, 4, sizeof(int), &w);
  const size_t sizes_x[] = { 1, ROUNDUPDHT(height, devid), 1 };
  int err = dt_opencl_enqueue_kernel_2d(devid, kernel_x, sizes_x);
  if(err != CL_SUCCESS) goto error;
 
  const int kernel_y = gd->kernel_hazeremoval_box_min_y;
  dt_opencl_set_kernel_arg(devid, kernel_y, 0, sizeof(int), &width);
  dt_opencl_set_kernel_arg(devid, kernel_y, 1, sizeof(int), &height);
  dt_opencl_set_kernel_arg(devid, kernel_y, 2, sizeof(cl_mem), &temp);
  dt_opencl_set_kernel_arg(devid, kernel_y, 3, sizeof(cl_mem), &out);
  dt_opencl_set_kernel_arg(devid, kernel_y, 4, sizeof(int), &w);
  const size_t sizes_y[] = { ROUNDUPDWD(width, devid), 1, 1 };
  err = dt_opencl_enqueue_kernel_2d(devid, kernel_y, sizes_y);
 
error:
  if(err != CL_SUCCESS) dt_print(DT_DEBUG_OPENCL, "[hazeremoval, box_min_cl] unknown error: %d\n", err);
  dt_opencl_release_mem_object(temp);
  return err;
}
 
 
static int box_max_cl(struct dt_iop_module_t *self, int devid, cl_mem in, cl_mem out, const int w)
{
  dt_iop_hazeremoval_global_data_t *gd = self->global_data;
  const int width = dt_opencl_get_image_width(in);
  const int height = dt_opencl_get_image_height(in);
  void *temp = dt_opencl_alloc_device(devid, width, height, (int)sizeof(float));
 
  const int kernel_x = gd->kernel_hazeremoval_box_max_x;
  dt_opencl_set_kernel_arg(devid, kernel_x, 0, sizeof(int), &width);
  dt_opencl_set_kernel_arg(devid, kernel_x, 1, sizeof(int), &height);
  dt_opencl_set_kernel_arg(devid, kernel_x, 2, sizeof(cl_mem), &in);
  dt_opencl_set_kernel_arg(devid, kernel_x, 3, sizeof(cl_mem), &temp);
  dt_opencl_set_kernel_arg(devid, kernel_x, 4, sizeof(int), &w);
  const size_t sizes_x[] = { 1, ROUNDUPDHT(height, devid), 1 };
  int err = dt_opencl_enqueue_kernel_2d(devid, kernel_x, sizes_x);
  if(err != CL_SUCCESS) goto error;
 
  const int kernel_y = gd->kernel_hazeremoval_box_max_y;
  dt_opencl_set_kernel_arg(devid, kernel_y, 0, sizeof(int), &width);
  dt_opencl_set_kernel_arg(devid, kernel_y, 1, sizeof(int), &height);
  dt_opencl_set_kernel_arg(devid, kernel_y, 2, sizeof(cl_mem), &temp);
  dt_opencl_set_kernel_arg(devid, kernel_y, 3, sizeof(cl_mem), &out);
  dt_opencl_set_kernel_arg(devid, kernel_y, 4, sizeof(int), &w);
  const size_t sizes_y[] = { ROUNDUPDWD(width, devid), 1, 1 };
  err = dt_opencl_enqueue_kernel_2d(devid, kernel_y, sizes_y);
 
error:
  if(err != CL_SUCCESS) dt_print(DT_DEBUG_OPENCL, "[hazeremoval, box_max_cl] unknown error: %d\n", err);
  dt_opencl_release_mem_object(temp);
  return err;
}
 
 
static int transition_map_cl(struct dt_iop_module_t *self, int devid, cl_mem img1, cl_mem img2, const int w1,
                             const float strength, const float *const A0)
{
  dt_iop_hazeremoval_global_data_t *gd = self->global_data;
  const int width = dt_opencl_get_image_width(img1);
  const int height = dt_opencl_get_image_height(img1);
 
  const int kernel = gd->kernel_hazeremoval_transision_map;
  dt_opencl_set_kernel_arg(devid, kernel, 0, sizeof(int), &width);
  dt_opencl_set_kernel_arg(devid, kernel, 1, sizeof(int), &height);
  dt_opencl_set_kernel_arg(devid, kernel, 2, sizeof(cl_mem), &img1);
  dt_opencl_set_kernel_arg(devid, kernel, 3, sizeof(cl_mem), &img2);
  dt_opencl_set_kernel_arg(devid, kernel, 4, sizeof(float), &strength);
  dt_opencl_set_kernel_arg(devid, kernel, 5, sizeof(float), &A0[0]);
  dt_opencl_set_kernel_arg(devid, kernel, 6, sizeof(float), &A0[1]);
  dt_opencl_set_kernel_arg(devid, kernel, 7, sizeof(float), &A0[2]);
  size_t sizes[] = { ROUNDUPDWD(width, devid), ROUNDUPDHT(height, devid), 1 };
  int err = dt_opencl_enqueue_kernel_2d(devid, kernel, sizes);
  if(err != CL_SUCCESS)
  {
    dt_print(DT_DEBUG_OPENCL, "[hazeremoval, transition_map_cl] unknown error: %d\n", err);
    return err;
  }
  err = box_max_cl(self, devid, img2, img2, w1);
 
  return err;
}
 
 
static int dehaze_cl(struct dt_iop_module_t *self, int devid, cl_mem img_in, cl_mem trans_map, cl_mem img_out,
                     const float t_min, const float *const A0)
{
  dt_iop_hazeremoval_global_data_t *gd = self->global_data;
  const int width = dt_opencl_get_image_width(img_in);
  const int height = dt_opencl_get_image_height(img_in);
 
  const int kernel = gd->kernel_hazeremoval_dehaze;
  dt_opencl_set_kernel_arg(devid, kernel, 0, sizeof(int), &width);
  dt_opencl_set_kernel_arg(devid, kernel, 1, sizeof(int), &height);
  dt_opencl_set_kernel_arg(devid, kernel, 2, sizeof(cl_mem), &img_in);
  dt_opencl_set_kernel_arg(devid, kernel, 3, sizeof(cl_mem), &trans_map);
  dt_opencl_set_kernel_arg(devid, kernel, 4, sizeof(cl_mem), &img_out);
  dt_opencl_set_kernel_arg(devid, kernel, 5, sizeof(float), &t_min);
  dt_opencl_set_kernel_arg(devid, kernel, 6, sizeof(float), &A0[0]);
  dt_opencl_set_kernel_arg(devid, kernel, 7, sizeof(float), &A0[1]);
  dt_opencl_set_kernel_arg(devid, kernel, 8, sizeof(float), &A0[2]);
  size_t sizes[] = { ROUNDUPDWD(width, devid), ROUNDUPDHT(height, devid), 1 };
  int err = dt_opencl_enqueue_kernel_2d(devid, kernel, sizes);
  if(err != CL_SUCCESS) dt_print(DT_DEBUG_OPENCL, "[hazeremoval, dehaze_cl] unknown error: %d\n", err);
  return err;
}
 
void tiling_callback(struct dt_iop_module_t *self, const struct dt_dev_pixelpipe_t *pipe, const struct dt_dev_pixelpipe_iop_t *piece, struct dt_develop_tiling_t *tiling)
{
  tiling->factor = 2.5f;  // in + out + two single-channel temp buffers
  tiling->factor_cl = 5.0f;
  tiling->maxbuf = 1.0f;
  tiling->maxbuf_cl = 1.0f;
  tiling->overhead = 0;
  tiling->overlap = 0;
  tiling->xalign = 1;
  tiling->yalign = 1;
}
 
int process_cl(struct dt_iop_module_t *self, const dt_dev_pixelpipe_t *pipe, const dt_dev_pixelpipe_iop_t *piece, cl_mem img_in, cl_mem img_out)
{
  const dt_iop_roi_t *const roi_in = &piece->roi_in;
  const dt_iop_roi_t *const roi_out = &piece->roi_out;
  dt_iop_hazeremoval_gui_data_t *const g = (dt_iop_hazeremoval_gui_data_t*)self->gui_data;
  dt_iop_hazeremoval_params_t *d = piece->data;
 
  const int ch = piece->dsc_in.channels;
  const int devid = pipe->devid;
  const int width = roi_in->width;
  const int height = roi_in->height;
  const int w1 = 6; // window size (positive integer) for determining the dark channel and the transition map
  const int w2 = 9; // window size (positive integer) for the guided filter
 
  // module parameters
  const float strength = d->strength; // strength of haze removal
  const float distance = d->distance; // maximal distance from camera to remove haze
  const float eps = sqrtf(0.025f);    // regularization parameter for guided filter
 
  // estimate diffusive ambient light and image depth
  rgb_pixel A0;
  A0[0] = NAN;
  A0[1] = NAN;
  A0[2] = NAN;
  float distance_max = NAN;
 
  // hazeremoval module needs the color and the haziness (which yields
  // distance_max) of the most hazy region of the image.  In pixelpipe
  // FULL we can not reliably get this value as the pixelpipe might
  // only see part of the image (region of interest).  Therefore, we
  // try to get A0 and distance_max from the PREVIEW pixelpipe which
  // luckily stores it for us.
  if(self->dev->gui_attached && g && !dt_dev_pixelpipe_has_preview_output(self->dev, pipe, roi_out))
  {
    dt_iop_gui_enter_critical_section(self);
    const uint64_t hash = g->hash;
    const uint64_t expected_preview_hash = g->expected_preview_hash;
    dt_iop_gui_leave_critical_section(self);
    if(hash != DT_PIXELPIPE_CACHE_HASH_INVALID
       && hash == expected_preview_hash)
    {
      dt_iop_gui_enter_critical_section(self);
      A0[0] = g->A0[0];
      A0[1] = g->A0[1];
      A0[2] = g->A0[2];
      distance_max = g->distance_max;
      dt_iop_gui_leave_critical_section(self);
    }
  }
  // In all other cases we calculate distance_max and A0 here.
  if(isnan(distance_max))
  {
    float max_depth = 0.f;
    if(ambient_light_cl(self, devid, img_in, w1, &A0, &max_depth)) return FALSE;
    distance_max = max_depth;
  }
  // PREVIEW pixelpipe stores values.
  if(self->dev->gui_attached && g && dt_dev_pixelpipe_has_preview_output(self->dev, pipe, roi_out))
  {
    uint64_t hash = piece->global_hash;
    dt_iop_gui_enter_critical_section(self);
    g->A0[0] = A0[0];
    g->A0[1] = A0[1];
    g->A0[2] = A0[2];
    g->distance_max = distance_max;
    g->expected_preview_hash = hash;
    g->hash = hash;
    dt_iop_gui_leave_critical_section(self);
  }
 
  // calculate the transition map
  void *trans_map = dt_opencl_alloc_device(devid, width, height, (int)sizeof(float));
  transition_map_cl(self, devid, img_in, trans_map, w1, strength, A0);
  // refine the transition map
  box_min_cl(self, devid, trans_map, trans_map, w1);
  void *trans_map_filtered = dt_opencl_alloc_device(devid, width, height, (int)sizeof(float));
  // apply guided filter with no clipping
  if(guided_filter_cl(devid, img_in, trans_map, trans_map_filtered, width, height, ch, w2, eps, 1.f, -CL_FLT_MAX,
                      CL_FLT_MAX) != 0)
  {
    dt_opencl_release_mem_object(trans_map);
    dt_opencl_release_mem_object(trans_map_filtered);
    return FALSE;
  }
 
  // finally, calculate the haze-free image
  const float t_min
      = fminf(fmaxf(expf(-distance * distance_max), 1.f / 1024), 1.f); // minimum allowed value for transition map
  dehaze_cl(self, devid, img_in, trans_map_filtered, img_out, t_min, A0);
 
  dt_opencl_release_mem_object(trans_map);
  dt_opencl_release_mem_object(trans_map_filtered);
 
  return TRUE;
}
#endif
 
// clang-format off
// modelines: These editor modelines have been set for all relevant files by tools/update_modelines.py
// vim: shiftwidth=2 expandtab tabstop=2 cindent
// kate: tab-indents: off; indent-width 2; replace-tabs on; indent-mode cstyle; remove-trailing-spaces modified;
// clang-format on