// GUI / widget creation
#include "develop/imageop.h"          // IOP infrastructure, copy_pixel_nontemporal
#include "develop/imageop_gui.h"      // Widget creation helpers (_from_params)
#include "bauhaus/bauhaus.h"          // Slider/combobox configuration
#include "gui/gtk.h"                  // Notebooks, sections, collapsibles
#include "dtgtk/paint.h"             // Icon paint functions for buttons
#include "gui/color_picker_proxy.h"   // Color picker attachment

// Processing
#include "common/darktable.h"         // DT_OMP_FOR, copy_pixel, dt_alloc_align
#include "common/imagebuf.h"          // dt_iop_alloc_image_buffers, dt_iop_copy_image_roi
#include "develop/tiling.h"           // dt_develop_tiling_t (for tiling_callback)

// Validate input channels (copies input→output and sets trouble message on mismatch)
if(!dt_iop_have_required_input_format(4, self, piece->colors, ivoid, ovoid, roi_in, roi_out))
  return;

// Parallel pixel loop with safe defaults
DT_OMP_FOR()
for(int j = 0; j < roi_out->height; j++)
{
  const float *in = ((float *)ivoid) + (size_t)4 * roi_in->width * j;
  float *out = ((float *)ovoid) + (size_t)4 * roi_out->width * j;
  for(int i = 0; i < roi_out->width; i++)
  {
    for_each_channel(c, aligned(in, out : 16))
      out[c] = in[c] * factor;
    in += 4; out += 4;
  }
}

// Allocate a temporary buffer (auto trouble message on failure)
float *tmp = NULL;
if(!dt_iop_alloc_image_buffers(self, roi_in, roi_out,
                                4 | DT_IMGSZ_OUTPUT, &tmp, NULL))
  return;
// ... use tmp ...
dt_free_align(tmp);

// Scale spatial parameters for current zoom level
const float sigma = user_radius * roi_out->scale / piece->iscale;

// Report trouble to the user (icon on module header)
dt_iop_set_module_trouble_message(self, _("warning text"), _("tooltip"), NULL);

// Check pipe type
if(piece->pipe->type & DT_DEV_PIXELPIPE_FULL) { /* full view only */ }

gui_init()    →  Create widgets, configure ranges/formats
                 (do NOT call gui_update here - params may not be ready)
                      ↓
gui_update()  ←  Called when params change (image switch, history)
                      ↓
gui_changed() ←  Always call gui_changed(self, NULL, NULL) at end of gui_update
                 to apply any UI adjustments (show/hide, sensitivity, etc.)
                      ↓
User interacts with widget
                      ↓
Auto-callback (from_params) or manual callback
                      ↓
self->params updated → gui_changed() called automatically
                      ↓
dt_dev_add_history_item() → commit_params() → process()
         │                        │                │
    records params         transforms params    reads piece->data
    to history stack       into piece->data     (data_t or params_t)