brush_profile.h

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/*
    This file is part of the Ansel project.
    Copyright (C) 2026 Aurélien PIERRE.
 
    Ansel 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.
 
    Ansel 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 Ansel.  If not, see <http://www.gnu.org/licenses/>.
*/
 
#pragma once
 
#include "iop/drawlayer/brush.h"
 
#include <math.h>
 
static inline float dt_drawlayer_brush_profile_clamp01(const float v)
{
  return fminf(fmaxf(v, 0.0f), 1.0f);
}
 
static inline float dt_drawlayer_brush_transition_profile_eval(const int shape, const float t, const float inv_t)
{
  switch(shape)
  {
    case DT_DRAWLAYER_BRUSH_SHAPE_QUADRATIC:
      return inv_t * inv_t;
    case DT_DRAWLAYER_BRUSH_SHAPE_SIGMOIDAL:
    {
      const float smooth = t * t * (3.0f - 2.0f * t);
      return 1.0f - smooth;
    }
    case DT_DRAWLAYER_BRUSH_SHAPE_LINEAR:
    default:
      return inv_t;
  }
}
 
static inline float dt_drawlayer_brush_transition_mass_primitive_eval(const int shape, const float u,
                                                                      const float inner, const float w,
                                                                      const float base)
{
  switch(shape)
  {
    case DT_DRAWLAYER_BRUSH_SHAPE_QUADRATIC:
    {
      const float q_u = 0.5f * u * u - (2.0f / 3.0f) * u * u * u + 0.25f * u * u * u * u;
      const float q_i = 0.5f * inner * inner - (2.0f / 3.0f) * inner * inner * inner
                        + 0.25f * inner * inner * inner * inner;
      return base + (q_u - q_i) / (w * w);
    }
    case DT_DRAWLAYER_BRUSH_SHAPE_SIGMOIDAL:
    {
      const float s = dt_drawlayer_brush_profile_clamp01((u - inner) / w);
      const float s2 = s * s;
      const float s3 = s2 * s;
      const float s4 = s2 * s2;
      const float s5 = s4 * s;
      const float delta = w * (inner * (s - s3 + 0.5f * s4)
                               + w * (0.5f * s2 - 0.75f * s4 + 0.4f * s5));
      return base + delta;
    }
    case DT_DRAWLAYER_BRUSH_SHAPE_LINEAR:
    default:
    {
      const float l_u = 0.5f * u * u - (1.0f / 3.0f) * u * u * u;
      const float l_i = 0.5f * inner * inner - (1.0f / 3.0f) * inner * inner * inner;
      return base + (l_u - l_i) / w;
    }
  }
}
 
static inline float dt_drawlayer_brush_profile_eval(const dt_drawlayer_brush_dab_t *dab, const float norm2)
{
  if(!dab || norm2 >= 1.0f) return 0.0f;
 
  if(dab->shape == DT_DRAWLAYER_BRUSH_SHAPE_GAUSSIAN)
  {
    const float radius = sqrtf(norm2);
    if(radius < 0.5f) return 1.0f - 6.0f * norm2 + 6.0f * norm2 * radius;
    const float inv_r = 1.0f - radius;
    return 2.0f * inv_r * inv_r * inv_r;
  }
 
  const float hardness = dt_drawlayer_brush_profile_clamp01(dab->hardness);
  if(hardness >= 1.0f - 1e-6f) return 1.0f;
 
  const float min_inner = 0.5f / fmaxf(dab->radius, 0.5f);
  const float inner = fmaxf(hardness, dt_drawlayer_brush_profile_clamp01(min_inner));
  const float radius = sqrtf(norm2);
  if(radius <= inner) return 1.0f;
 
  const float t = dt_drawlayer_brush_profile_clamp01((radius - inner) / fmaxf(1.0f - inner, 1e-6f));
  return dt_drawlayer_brush_transition_profile_eval(dab->shape, t, 1.0f - t);
}
 
static inline float dt_drawlayer_brush_mass_primitive_eval(const dt_drawlayer_brush_dab_t *dab, const float u_in)
{
  if(!dab) return 0.0f;
  const float u = dt_drawlayer_brush_profile_clamp01(u_in);
  if(u <= 0.0f) return 0.0f;
 
  if(dab->shape == DT_DRAWLAYER_BRUSH_SHAPE_GAUSSIAN)
  {
    if(u <= 0.5f) return 0.5f * u * u - 1.5f * u * u * u * u + 1.2f * u * u * u * u * u;
    const float u2 = u * u;
    const float u3 = u2 * u;
    const float u4 = u2 * u2;
    const float u5 = u4 * u;
    return u2 - 2.0f * u3 + 1.5f * u4 - 0.4f * u5 - 0.0125f;
  }
 
  const float hardness = dt_drawlayer_brush_profile_clamp01(dab->hardness);
  if(hardness >= 1.0f - 1e-6f) return 0.5f * u * u;
  const float min_inner = 0.5f / fmaxf(dab->radius, 0.5f);
  const float inner = fmaxf(hardness, dt_drawlayer_brush_profile_clamp01(min_inner));
  if(u <= inner) return 0.5f * u * u;
  const float w = fmaxf(1.0f - inner, 1e-6f);
  const float base = 0.5f * inner * inner;
  return dt_drawlayer_brush_transition_mass_primitive_eval(dab->shape, u, inner, w, base);
}