/* brushlib - The MyPaint Brush Library
 * Copyright (C) 2007-2011 Martin Renold <martinxyz@gmx.ch>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <stdio.h>
#include <string.h>
#include <glib.h>
#include <math.h>
#include "Python.h"

#include "brushsettings.hpp"
#include "mapping.hpp"

#define ACTUAL_RADIUS_MIN 0.2
#define ACTUAL_RADIUS_MAX 800 // safety guard against radius like 1e20 and against rendering overload with unexpected brush dynamics

/* The Brush class stores two things:
   b) settings: constant during a stroke (eg. size, spacing, dynamics, color selected by the user)
   a) states: modified during a stroke (eg. speed, smudge colors, time/distance to next dab, position filter states)

   FIXME: Actually those are two orthogonal things. Should separate them:
          a) brush settings class that is saved/loaded/selected  (without states)
          b) brush core class to draw the dabs (using an instance of the above)

   In python, there are two kinds of instances from this: a "global
   brush" which does the cursor tracking, and the "brushlist" where
   the states are ignored. When a brush is selected, its settings are
   copied into the global one, leaving the state intact.
 */


class Brush {
public:
  bool print_inputs; // debug menu
  // for stroke splitting (undo/redo)
  double stroke_total_painting_time;
  double stroke_current_idling_time; 

private:
  // see also brushsettings.py

  // the states (get_state, set_state, reset) that change during a stroke
  float states[STATE_COUNT];
  GRand * rng;

  // Those mappings describe how to calculate the current value for each setting.
  // Most of settings will be constant (eg. only their base_value is used).
  Mapping * settings[BRUSH_SETTINGS_COUNT];

  // the current value of all settings (calculated using the current state)
  float settings_value[BRUSH_SETTINGS_COUNT];

  // cached calculation results
  float speed_mapping_gamma[2], speed_mapping_m[2], speed_mapping_q[2];

  bool reset_requested;

public:
  Brush() {
    for (int i=0; i<BRUSH_SETTINGS_COUNT; i++) {
      settings[i] = new Mapping(INPUT_COUNT);
    }
    rng = g_rand_new();
    print_inputs = false;
    
    for (int i=0; i<STATE_COUNT; i++) {
      states[i] = 0;
    }
    new_stroke();

    settings_base_values_have_changed();

    reset_requested = true;
  }

  ~Brush() {
    for (int i=0; i<BRUSH_SETTINGS_COUNT; i++) {
      delete settings[i];
    }
    g_rand_free (rng); rng = NULL;
  }

  void reset()
  {
    reset_requested = true;
  }

  void new_stroke()
  {
    stroke_current_idling_time = 0;
    stroke_total_painting_time = 0;
  }

  void set_base_value (int id, float value) {
    assert (id >= 0 && id < BRUSH_SETTINGS_COUNT);
    settings[id]->base_value = value;

    settings_base_values_have_changed ();
  }

  void set_mapping_n (int id, int input, int n) {
    assert (id >= 0 && id < BRUSH_SETTINGS_COUNT);
    settings[id]->set_n (input, n);
  }

  void set_mapping_point (int id, int input, int index, float x, float y) {
    assert (id >= 0 && id < BRUSH_SETTINGS_COUNT);
    settings[id]->set_point (input, index, x, y);
  }

  float get_state (int i)
  {
    assert (i >= 0 && i < STATE_COUNT);
    return states[i];
  }

  void set_state (int i, float value)
  {
    assert (i >= 0 && i < STATE_COUNT);
    states[i] = value;
  }

private:
  // returns the fraction still left after t seconds
  float exp_decay (float T_const, float t)
  {
    // the argument might not make mathematical sense (whatever.)
    if (T_const <= 0.001) {
      return 0.0;
    } else {
      return exp(- t / T_const);
    }
  }


  void settings_base_values_have_changed ()
  {
    // precalculate stuff that does not change dynamically

    // Precalculate how the physical speed will be mapped to the speed input value.
    // The forumla for this mapping is:
    //
    // y = log(gamma+x)*m + q;
    //
    // x: the physical speed (pixels per basic dab radius)
    // y: the speed input that will be reported
    // gamma: parameter set by ths user (small means a logarithmic mapping, big linear)
    // m, q: parameters to scale and translate the curve
    //
    // The code below calculates m and q given gamma and two hardcoded constraints.
    //
    for (int i=0; i<2; i++) {
      float gamma;
      gamma = settings[(i==0)?BRUSH_SPEED1_GAMMA:BRUSH_SPEED2_GAMMA]->base_value;
      gamma = exp(gamma);

      float fix1_x, fix1_y, fix2_x, fix2_dy;
      fix1_x = 45.0;
      fix1_y = 0.5;
      fix2_x = 45.0;
      fix2_dy = 0.015;

      float m, q;
      float c1;
      c1 = log(fix1_x+gamma);
      m = fix2_dy * (fix2_x + gamma);
      q = fix1_y - m*c1;
    
      speed_mapping_gamma[i] = gamma;
      speed_mapping_m[i] = m;
      speed_mapping_q[i] = q;
    }
  }

  // This function runs a brush "simulation" step. Usually it is
  // called once or twice per dab. In theory the precision of the
  // "simulation" gets better when it is called more often. In
  // practice this only matters if there are some highly nonlinear
  // mappings in critical places or extremely few events per second.
  //
  // note: parameters are is dx/ddab, ..., dtime/ddab (dab is the number, 5.0 = 5th dab)
  void update_states_and_setting_values (float step_dx, float step_dy, float step_dpressure, float step_declination, float step_ascension, float step_dtime)
  {
    float pressure;
    float inputs[INPUT_COUNT];

    if (step_dtime < 0.0) {
      printf("Time is running backwards!\n");
      step_dtime = 0.001;
    } else if (step_dtime == 0.0) {
      // FIXME: happens about every 10th start, workaround (against division by zero)
      step_dtime = 0.001;
    }

    states[STATE_X]        += step_dx;
    states[STATE_Y]        += step_dy;
    states[STATE_PRESSURE] += step_dpressure;

    states[STATE_DECLINATION] += step_declination;
    states[STATE_ASCENSION] += step_ascension;

    float base_radius = expf(settings[BRUSH_RADIUS_LOGARITHMIC]->base_value);

    // FIXME: does happen (interpolation problem?)
    states[STATE_PRESSURE] = CLAMP(states[STATE_PRESSURE], 0.0, 1.0);
    pressure = states[STATE_PRESSURE];

    { // start / end stroke (for "stroke" input only)
      if (!states[STATE_STROKE_STARTED]) {
        if (pressure > settings[BRUSH_STROKE_THRESHOLD]->base_value + 0.0001) {
          // start new stroke
          //printf("stroke start %f\n", pressure);
          states[STATE_STROKE_STARTED] = 1;
          states[STATE_STROKE] = 0.0;
        }
      } else {
        if (pressure <= settings[BRUSH_STROKE_THRESHOLD]->base_value * 0.9 + 0.0001) {
          // end stroke
          //printf("stroke end\n");
          states[STATE_STROKE_STARTED] = 0;
        }
      }
    }

    // now follows input handling

    float norm_dx, norm_dy, norm_dist, norm_speed;
    norm_dx = step_dx / step_dtime / base_radius;
    norm_dy = step_dy / step_dtime / base_radius;
    norm_speed = sqrt(SQR(norm_dx) + SQR(norm_dy));
    norm_dist = norm_speed * step_dtime;

    inputs[INPUT_PRESSURE] = pressure;
    inputs[INPUT_SPEED1] = log(speed_mapping_gamma[0] + states[STATE_NORM_SPEED1_SLOW])*speed_mapping_m[0] + speed_mapping_q[0];
    inputs[INPUT_SPEED2] = log(speed_mapping_gamma[1] + states[STATE_NORM_SPEED2_SLOW])*speed_mapping_m[1] + speed_mapping_q[1];
    inputs[INPUT_RANDOM] = g_rand_double (rng);
    inputs[INPUT_STROKE] = MIN(states[STATE_STROKE], 1.0);
    inputs[INPUT_DIRECTION] = fmodf (atan2f (states[STATE_DIRECTION_DY], states[STATE_DIRECTION_DX])/(2*M_PI)*360 + 180.0, 180.0);
    inputs[INPUT_TILT_DECLINATION] = states[STATE_DECLINATION];
    inputs[INPUT_TILT_ASCENSION] = states[STATE_ASCENSION];
    inputs[INPUT_CUSTOM] = states[STATE_CUSTOM_INPUT];
    if (print_inputs) {
      g_print("press=% 4.3f, speed1=% 4.4f\tspeed2=% 4.4f\tstroke=% 4.3f\tcustom=% 4.3f\n", (double)inputs[INPUT_PRESSURE], (double)inputs[INPUT_SPEED1], (double)inputs[INPUT_SPEED2], (double)inputs[INPUT_STROKE], (double)inputs[INPUT_CUSTOM]);
    }
    // FIXME: this one fails!!!
    //assert(inputs[INPUT_SPEED1] >= 0.0 && inputs[INPUT_SPEED1] < 1e8); // checking for inf

    for (int i=0; i<BRUSH_SETTINGS_COUNT; i++) {
      settings_value[i] = settings[i]->calculate (inputs);
    }

    {
      float fac = 1.0 - exp_decay (settings_value[BRUSH_SLOW_TRACKING_PER_DAB], 1.0);
      states[STATE_ACTUAL_X] += (states[STATE_X] - states[STATE_ACTUAL_X]) * fac; // FIXME: should this depend on base radius?
      states[STATE_ACTUAL_Y] += (states[STATE_Y] - states[STATE_ACTUAL_Y]) * fac;
    }

    { // slow speed
      float fac;
      fac = 1.0 - exp_decay (settings_value[BRUSH_SPEED1_SLOWNESS], step_dtime);
      states[STATE_NORM_SPEED1_SLOW] += (norm_speed - states[STATE_NORM_SPEED1_SLOW]) * fac;
      fac = 1.0 - exp_decay (settings_value[BRUSH_SPEED2_SLOWNESS], step_dtime);
      states[STATE_NORM_SPEED2_SLOW] += (norm_speed - states[STATE_NORM_SPEED2_SLOW]) * fac;
    }
  
    { // slow speed, but as vector this time

      // FIXME: offset_by_speed should be removed.
      //   Is it broken, non-smooth, system-dependent math?!
      //   A replacement could be a directed random offset.

      float time_constant = exp(settings_value[BRUSH_OFFSET_BY_SPEED_SLOWNESS]*0.01)-1.0;
      // Workaround for a bug that happens mainly on Windows, causing
      // individual dabs to be placed far far away. Using the speed
      // with zero filtering is just asking for trouble anyway.
      if (time_constant < 0.002) time_constant = 0.002;
      float fac = 1.0 - exp_decay (time_constant, step_dtime);
      states[STATE_NORM_DX_SLOW] += (norm_dx - states[STATE_NORM_DX_SLOW]) * fac;
      states[STATE_NORM_DY_SLOW] += (norm_dy - states[STATE_NORM_DY_SLOW]) * fac;
    }

    { // orientation (similar lowpass filter as above, but use dabtime instead of wallclock time)
      float dx = step_dx / base_radius;
      float dy = step_dy / base_radius;
      float step_in_dabtime = hypotf(dx, dy); // FIXME: are we recalculating something here that we already have?
      float fac = 1.0 - exp_decay (exp(settings_value[BRUSH_DIRECTION_FILTER]*0.5)-1.0, step_in_dabtime);

      float dx_old = states[STATE_DIRECTION_DX];
      float dy_old = states[STATE_DIRECTION_DY];
      // use the opposite speed vector if it is closer (we don't care about 180 degree turns)
      if (SQR(dx_old-dx) + SQR(dy_old-dy) > SQR(dx_old-(-dx)) + SQR(dy_old-(-dy))) {
        dx = -dx;
        dy = -dy;
      }
      states[STATE_DIRECTION_DX] += (dx - states[STATE_DIRECTION_DX]) * fac;
      states[STATE_DIRECTION_DY] += (dy - states[STATE_DIRECTION_DY]) * fac;
    }

    { // custom input
      float fac;
      fac = 1.0 - exp_decay (settings_value[BRUSH_CUSTOM_INPUT_SLOWNESS], 0.1);
      states[STATE_CUSTOM_INPUT] += (settings_value[BRUSH_CUSTOM_INPUT] - states[STATE_CUSTOM_INPUT]) * fac;
    }

    { // stroke length
      float frequency;
      float wrap;
      frequency = expf(-settings_value[BRUSH_STROKE_DURATION_LOGARITHMIC]);
      states[STATE_STROKE] += norm_dist * frequency;
      // can happen, probably caused by rounding
      if (states[STATE_STROKE] < 0) states[STATE_STROKE] = 0;
      wrap = 1.0 + settings_value[BRUSH_STROKE_HOLDTIME];
      if (states[STATE_STROKE] > wrap) {
        if (wrap > 9.9 + 1.0) {
          // "inifinity", just hold stroke somewhere >= 1.0
          states[STATE_STROKE] = 1.0;
        } else {
          states[STATE_STROKE] = fmodf(states[STATE_STROKE], wrap);
          // just in case
          if (states[STATE_STROKE] < 0) states[STATE_STROKE] = 0;
        }
      }
    }

    // calculate final radius
    float radius_log;
    radius_log = settings_value[BRUSH_RADIUS_LOGARITHMIC];
    states[STATE_ACTUAL_RADIUS] = expf(radius_log);
    if (states[STATE_ACTUAL_RADIUS] < ACTUAL_RADIUS_MIN) states[STATE_ACTUAL_RADIUS] = ACTUAL_RADIUS_MIN;
    if (states[STATE_ACTUAL_RADIUS] > ACTUAL_RADIUS_MAX) states[STATE_ACTUAL_RADIUS] = ACTUAL_RADIUS_MAX;

    // aspect ratio (needs to be caluclated here because it can affect the dab spacing)
    states[STATE_ACTUAL_ELLIPTICAL_DAB_RATIO] = settings_value[BRUSH_ELLIPTICAL_DAB_RATIO];
    states[STATE_ACTUAL_ELLIPTICAL_DAB_ANGLE] = settings_value[BRUSH_ELLIPTICAL_DAB_ANGLE];
  }

  // Called only from stroke_to(). Calculate everything needed to
  // draw the dab, then let the surface do the actual drawing.
  //
  // This is only gets called right after update_states_and_setting_values().
  // Returns true if the surface was modified.
  bool prepare_and_draw_dab (Surface * surface)
  {
    float x, y, opaque;
    float radius;

    // ensure we don't get a positive result with two negative opaque values
    if (settings_value[BRUSH_OPAQUE] < 0) settings_value[BRUSH_OPAQUE] = 0;
    opaque = settings_value[BRUSH_OPAQUE] * settings_value[BRUSH_OPAQUE_MULTIPLY];
    opaque = CLAMP(opaque, 0.0, 1.0);
    //if (opaque == 0.0) return false; <-- cannot do that, since we need to update smudge state.
    if (settings_value[BRUSH_OPAQUE_LINEARIZE]) {
      // OPTIMIZE: no need to recalculate this for each dab
      float alpha, beta, alpha_dab, beta_dab;
      float dabs_per_pixel;
      // dabs_per_pixel is just estimated roughly, I didn't think hard
      // about the case when the radius changes during the stroke
      dabs_per_pixel = (
                        settings[BRUSH_DABS_PER_ACTUAL_RADIUS]->base_value + 
                        settings[BRUSH_DABS_PER_BASIC_RADIUS]->base_value
                        ) * 2.0;

      // the correction is probably not wanted if the dabs don't overlap
      if (dabs_per_pixel < 1.0) dabs_per_pixel = 1.0;

      // interpret the user-setting smoothly
      dabs_per_pixel = 1.0 + settings[BRUSH_OPAQUE_LINEARIZE]->base_value*(dabs_per_pixel-1.0);

      // see doc/brushdab_saturation.png
      //      beta = beta_dab^dabs_per_pixel
      // <==> beta_dab = beta^(1/dabs_per_pixel)
      alpha = opaque;
      beta = 1.0-alpha;
      beta_dab = powf(beta, 1.0/dabs_per_pixel);
      alpha_dab = 1.0-beta_dab;
      opaque = alpha_dab;
    }

    x = states[STATE_ACTUAL_X];
    y = states[STATE_ACTUAL_Y];

    float base_radius = expf(settings[BRUSH_RADIUS_LOGARITHMIC]->base_value);

    if (settings_value[BRUSH_OFFSET_BY_SPEED]) {
      x += states[STATE_NORM_DX_SLOW] * settings_value[BRUSH_OFFSET_BY_SPEED] * 0.1 * base_radius;
      y += states[STATE_NORM_DY_SLOW] * settings_value[BRUSH_OFFSET_BY_SPEED] * 0.1 * base_radius;
    }

    if (settings_value[BRUSH_OFFSET_BY_RANDOM]) {
      float amp = settings_value[BRUSH_OFFSET_BY_RANDOM];
      if (amp < 0.0) amp = 0.0;
      x += rand_gauss (rng) * amp * base_radius;
      y += rand_gauss (rng) * amp * base_radius;
    }

  
    radius = states[STATE_ACTUAL_RADIUS];
    if (settings_value[BRUSH_RADIUS_BY_RANDOM]) {
      float radius_log, alpha_correction;
      // go back to logarithmic radius to add the noise
      radius_log  = settings_value[BRUSH_RADIUS_LOGARITHMIC];
      radius_log += rand_gauss (rng) * settings_value[BRUSH_RADIUS_BY_RANDOM];
      radius = expf(radius_log);
      radius = CLAMP(radius, ACTUAL_RADIUS_MIN, ACTUAL_RADIUS_MAX);
      alpha_correction = states[STATE_ACTUAL_RADIUS] / radius;
      alpha_correction = SQR(alpha_correction);
      if (alpha_correction <= 1.0) {
        opaque *= alpha_correction;
      }
    }

    // color part

    float color_h = settings[BRUSH_COLOR_H]->base_value;
    float color_s = settings[BRUSH_COLOR_S]->base_value;
    float color_v = settings[BRUSH_COLOR_V]->base_value;
    float eraser_target_alpha = 1.0;
    if (settings_value[BRUSH_SMUDGE] > 0.0) {
      // mix (in RGB) the smudge color with the brush color
      hsv_to_rgb_float (&color_h, &color_s, &color_v);
      float fac = settings_value[BRUSH_SMUDGE];
      if (fac > 1.0) fac = 1.0;
      // If the smudge color somewhat transparent, then the resulting
      // dab will do erasing towards that transparency level.
      // see also ../doc/smudge_math.png
      eraser_target_alpha = (1-fac)*1.0 + fac*states[STATE_SMUDGE_A];
      // fix rounding errors (they really seem to happen in the previous line)
      eraser_target_alpha = CLAMP(eraser_target_alpha, 0.0, 1.0);
      if (eraser_target_alpha > 0) {
        color_h = (fac*states[STATE_SMUDGE_RA] + (1-fac)*color_h) / eraser_target_alpha;
        color_s = (fac*states[STATE_SMUDGE_GA] + (1-fac)*color_s) / eraser_target_alpha;
        color_v = (fac*states[STATE_SMUDGE_BA] + (1-fac)*color_v) / eraser_target_alpha;
      } else {
        // we are only erasing; the color does not matter
        color_h = 1.0;
        color_s = 0.0;
        color_v = 0.0;
      }
      rgb_to_hsv_float (&color_h, &color_s, &color_v);
    }

    if (settings_value[BRUSH_SMUDGE_LENGTH] < 1.0 and
        // optimization, since normal brushes have smudge_length == 0.5 without actually smudging
        (settings_value[BRUSH_SMUDGE] != 0.0 or not settings[BRUSH_SMUDGE]->is_constant())) {

      float fac = settings_value[BRUSH_SMUDGE_LENGTH];
      if (fac < 0.01) fac = 0.01;
      int px, py;
      px = ROUND(x);
      py = ROUND(y);

      // Calling get_color() is almost as expensive as rendering a
      // dab. Because of this we use the previous value if it is not
      // expected to hurt quality too much. We call it at most every
      // second dab.
      float r, g, b, a;
      states[STATE_LAST_GETCOLOR_RECENTNESS] *= fac;
      if (states[STATE_LAST_GETCOLOR_RECENTNESS] < 0.5*fac) {
        states[STATE_LAST_GETCOLOR_RECENTNESS] = 1.0;

        float smudge_radius = radius * expf(settings_value[BRUSH_SMUDGE_RADIUS_LOG]);
        smudge_radius = CLAMP(smudge_radius, ACTUAL_RADIUS_MIN, ACTUAL_RADIUS_MAX);
        surface->get_color (px, py, smudge_radius, &r, &g, &b, &a);

        states[STATE_LAST_GETCOLOR_R] = r;
        states[STATE_LAST_GETCOLOR_G] = g;
        states[STATE_LAST_GETCOLOR_B] = b;
        states[STATE_LAST_GETCOLOR_A] = a;
      } else {
        r = states[STATE_LAST_GETCOLOR_R];
        g = states[STATE_LAST_GETCOLOR_G];
        b = states[STATE_LAST_GETCOLOR_B];
        a = states[STATE_LAST_GETCOLOR_A];
      }

      // updated the smudge color (stored with premultiplied alpha)
      states[STATE_SMUDGE_A ] = fac*states[STATE_SMUDGE_A ] + (1-fac)*a;
      // fix rounding errors
      states[STATE_SMUDGE_A ] = CLAMP(states[STATE_SMUDGE_A], 0.0, 1.0);

      states[STATE_SMUDGE_RA] = fac*states[STATE_SMUDGE_RA] + (1-fac)*r*a;
      states[STATE_SMUDGE_GA] = fac*states[STATE_SMUDGE_GA] + (1-fac)*g*a;
      states[STATE_SMUDGE_BA] = fac*states[STATE_SMUDGE_BA] + (1-fac)*b*a;
    }

    // eraser
    if (settings_value[BRUSH_ERASER]) {
      eraser_target_alpha *= (1.0-settings_value[BRUSH_ERASER]);
    }

    // HSV color change
    color_h += settings_value[BRUSH_CHANGE_COLOR_H];
    color_s += settings_value[BRUSH_CHANGE_COLOR_HSV_S];
    color_v += settings_value[BRUSH_CHANGE_COLOR_V];

    // HSL color change
    if (settings_value[BRUSH_CHANGE_COLOR_L] || settings_value[BRUSH_CHANGE_COLOR_HSL_S]) {
      // (calculating way too much here, can be optimized if neccessary)
      // this function will CLAMP the inputs
      hsv_to_rgb_float (&color_h, &color_s, &color_v);
      rgb_to_hsl_float (&color_h, &color_s, &color_v);
      color_v += settings_value[BRUSH_CHANGE_COLOR_L];
      color_s += settings_value[BRUSH_CHANGE_COLOR_HSL_S];
      hsl_to_rgb_float (&color_h, &color_s, &color_v);
      rgb_to_hsv_float (&color_h, &color_s, &color_v);
    }

    float hardness = CLAMP(settings_value[BRUSH_HARDNESS], 0.0, 1.0);

    // anti-aliasing attempt (works surprisingly well for ink brushes)
    float current_fadeout_in_pixels = radius * (1.0 - hardness);
    float min_fadeout_in_pixels = settings_value[BRUSH_ANTI_ALIASING];
    if (current_fadeout_in_pixels < min_fadeout_in_pixels) {
      // need to soften the brush (decrease hardness), but keep optical radius
      // so we tune both radius and hardness, to get the desired fadeout_in_pixels
      float current_optical_radius = radius - (1.0-hardness)*radius/2.0;

      // Equation 1: (new fadeout must be equal to min_fadeout)
      //   min_fadeout_in_pixels = radius_new*(1.0 - hardness_new)
      // Equation 2: (optical radius must remain unchanged)
      //   current_optical_radius = radius_new - (1.0-hardness_new)*radius_new/2.0
      //
      // Solved Equation 1 for hardness_new, using Equation 2: (thanks to mathomatic)
      float hardness_new = ((current_optical_radius - (min_fadeout_in_pixels/2.0))/(current_optical_radius + (min_fadeout_in_pixels/2.0)));
      // Using Equation 1:
      float radius_new = (min_fadeout_in_pixels/(1.0 - hardness_new));

      hardness = hardness_new;
      radius = radius_new;
    }

    // the functions below will CLAMP most inputs
    hsv_to_rgb_float (&color_h, &color_s, &color_v);
    return surface->draw_dab (x, y, radius, color_h, color_s, color_v, opaque, hardness, eraser_target_alpha,
                              states[STATE_ACTUAL_ELLIPTICAL_DAB_RATIO], states[STATE_ACTUAL_ELLIPTICAL_DAB_ANGLE],
                              settings_value[BRUSH_LOCK_ALPHA]);
  }

  // How many dabs will be drawn between the current and the next (x, y, pressure, +dt) position?
  float count_dabs_to (float x, float y, float pressure, float dt)
  {
    float xx, yy;
    float res1, res2, res3;
    float dist;

    if (states[STATE_ACTUAL_RADIUS] == 0.0) states[STATE_ACTUAL_RADIUS] = expf(settings[BRUSH_RADIUS_LOGARITHMIC]->base_value);
    if (states[STATE_ACTUAL_RADIUS] < ACTUAL_RADIUS_MIN) states[STATE_ACTUAL_RADIUS] = ACTUAL_RADIUS_MIN;
    if (states[STATE_ACTUAL_RADIUS] > ACTUAL_RADIUS_MAX) states[STATE_ACTUAL_RADIUS] = ACTUAL_RADIUS_MAX;


    // OPTIMIZE: expf() called too often
    float base_radius = expf(settings[BRUSH_RADIUS_LOGARITHMIC]->base_value);
    if (base_radius < ACTUAL_RADIUS_MIN) base_radius = ACTUAL_RADIUS_MIN;
    if (base_radius > ACTUAL_RADIUS_MAX) base_radius = ACTUAL_RADIUS_MAX;
    //if (base_radius < 0.5) base_radius = 0.5;
    //if (base_radius > 500.0) base_radius = 500.0;

    xx = x - states[STATE_X];
    yy = y - states[STATE_Y];
    //dp = pressure - pressure; // Not useful?
    // TODO: control rate with pressure (dabs per pressure) (dpressure is useless)

    if (states[STATE_ACTUAL_ELLIPTICAL_DAB_RATIO] > 1.0) {
      // code duplication, see tiledsurface::draw_dab()
      float angle_rad=states[STATE_ACTUAL_ELLIPTICAL_DAB_ANGLE]/360*2*M_PI;
      float cs=cos(angle_rad);
      float sn=sin(angle_rad);
      float yyr=(yy*cs-xx*sn)*states[STATE_ACTUAL_ELLIPTICAL_DAB_RATIO];
      float xxr=yy*sn+xx*cs;
      dist = sqrt(yyr*yyr + xxr*xxr);
    } else {
      dist = hypotf(xx, yy);
    }

    // FIXME: no need for base_value or for the range checks above IF always the interpolation
    //        function will be called before this one
    res1 = dist / states[STATE_ACTUAL_RADIUS] * settings[BRUSH_DABS_PER_ACTUAL_RADIUS]->base_value;
    res2 = dist / base_radius   * settings[BRUSH_DABS_PER_BASIC_RADIUS]->base_value;
    res3 = dt * settings[BRUSH_DABS_PER_SECOND]->base_value;
    return res1 + res2 + res3;
  }

public:
  // This function:
  // - is called once for each motion event
  // - does motion event interpolation
  // - paints zero, one or several dabs
  // - decides whether the stroke is finished (for undo/redo)
  // returns true if the stroke is finished or empty
  bool stroke_to (Surface * surface, float x, float y, float pressure, float xtilt, float ytilt, double dtime)
  {
    //printf("%f %f %f %f\n", (double)dtime, (double)x, (double)y, (double)pressure);

    float tilt_ascension = 0.0;
    float tilt_declination = 90.0;
    if (xtilt != 0 || ytilt != 0) {
      // shield us from insane tilt input
      xtilt = CLAMP(xtilt, -1.0, 1.0);
      ytilt = CLAMP(ytilt, -1.0, 1.0);
      assert(isfinite(xtilt) && isfinite(ytilt));

      tilt_ascension = 180.0*atan2(-xtilt, ytilt)/M_PI;
      float e;
      if (abs(xtilt) > abs(ytilt)) {
        e = sqrt(1+ytilt*ytilt);
      } else {
        e = sqrt(1+xtilt*xtilt);
      }
      float rad = hypot(xtilt, ytilt);
      float cos_alpha = rad/e;
      if (cos_alpha >= 1.0) cos_alpha = 1.0; // fix numerical inaccuracy
      tilt_declination = 180.0*acos(cos_alpha)/M_PI;

      assert(isfinite(tilt_ascension));
      assert(isfinite(tilt_declination));
    }

    // printf("xtilt %f, ytilt %f\n", (double)xtilt, (double)ytilt);
    // printf("ascension %f, declination %f\n", (double)tilt_ascension, (double)tilt_declination);
      
    pressure = CLAMP(pressure, 0.0, 1.0);
    if (!isfinite(x) || !isfinite(y) ||
        (x > 1e10 || y > 1e10 || x < -1e10 || y < -1e10)) {
      // workaround attempt for https://gna.org/bugs/?14372
      g_print("Warning: ignoring brush::stroke_to with insane inputs (x = %f, y = %f)\n", (double)x, (double)y);
      x = 0.0;
      y = 0.0;
      pressure = 0.0;
    }
    // the assertion below is better than out-of-memory later at save time
    assert(x < 1e8 && y < 1e8 && x > -1e8 && y > -1e8);

    if (dtime < 0) g_print("Time jumped backwards by dtime=%f seconds!\n", dtime);
    if (dtime <= 0) dtime = 0.0001; // protect against possible division by zero bugs
    
    if (dtime > 0.100 && pressure && states[STATE_PRESSURE] == 0) {
      // Workaround for tablets that don't report motion events without pressure.
      // This is to avoid linear interpolation of the pressure between two events.
      stroke_to (surface, x, y, 0.0, 90.0, 0.0, dtime-0.0001);
      dtime = 0.0001;
    }

    g_rand_set_seed (rng, states[STATE_RNG_SEED]);

    { // calculate the actual "virtual" cursor position

      // noise first
      if (settings[BRUSH_TRACKING_NOISE]->base_value) {
        // OPTIMIZE: expf() called too often
        float base_radius = expf(settings[BRUSH_RADIUS_LOGARITHMIC]->base_value);

        x += rand_gauss (rng) * settings[BRUSH_TRACKING_NOISE]->base_value * base_radius;
        y += rand_gauss (rng) * settings[BRUSH_TRACKING_NOISE]->base_value * base_radius;
      }

      float fac = 1.0 - exp_decay (settings[BRUSH_SLOW_TRACKING]->base_value, 100.0*dtime);
      x = states[STATE_X] + (x - states[STATE_X]) * fac;
      y = states[STATE_Y] + (y - states[STATE_Y]) * fac;
    }

    // draw many (or zero) dabs to the next position

    // see doc/stroke2dabs.png
    float dist_moved = states[STATE_DIST];
    float dist_todo = count_dabs_to (x, y, pressure, dtime);

    //if (dtime > 5 || dist_todo > 300) {
    if (dtime > 5 || reset_requested) {
      reset_requested = false;

      /*
        TODO:
        if (dist_todo > 300) {
        // this happens quite often, eg when moving the cursor back into the window
        // FIXME: bad to hardcode a distance treshold here - might look at zoomed image
        //        better detect leaving/entering the window and reset then.
        g_print ("Warning: NOT drawing %f dabs.\n", dist_todo);
        g_print ("dtime=%f, dx=%f\n", dtime, x-states[STATE_X]);
        //must_reset = 1;
        }
      */

      //printf("Brush reset.\n");
      for (int i=0; i<STATE_COUNT; i++) {
        states[i] = 0;
      }

      states[STATE_X] = x;
      states[STATE_Y] = y;
      states[STATE_PRESSURE] = pressure;

      // not resetting, because they will get overwritten below:
      //dx, dy, dpress, dtime

      states[STATE_ACTUAL_X] = states[STATE_X];
      states[STATE_ACTUAL_Y] = states[STATE_Y];
      states[STATE_STROKE] = 1.0; // start in a state as if the stroke was long finished

      return true;
    }

    //g_print("dist = %f\n", states[STATE_DIST]);
    enum { UNKNOWN, YES, NO } painted = UNKNOWN;
    double dtime_left = dtime;

    float step_dx, step_dy, step_dpressure, step_dtime;
    float step_declination, step_ascension;
    while (dist_moved + dist_todo >= 1.0) { // there are dabs pending
      { // linear interpolation (nonlinear variant was too slow, see SVN log)
        float frac; // fraction of the remaining distance to move
        if (dist_moved > 0) {
          // "move" the brush exactly to the first dab (moving less than one dab)
          frac = (1.0 - dist_moved) / dist_todo;
          dist_moved = 0;
        } else {
          // "move" the brush from one dab to the next
          frac = 1.0 / dist_todo;
        }
        step_dx        = frac * (x - states[STATE_X]);
        step_dy        = frac * (y - states[STATE_Y]);
        step_dpressure = frac * (pressure - states[STATE_PRESSURE]);
        step_dtime     = frac * (dtime_left - 0.0);
        step_declination = frac * (tilt_declination - states[STATE_DECLINATION]);
        step_ascension   = frac * (tilt_ascension - states[STATE_ASCENSION]);
        // Though it looks different, time is interpolated exactly like x/y/pressure.
      }
    
      update_states_and_setting_values (step_dx, step_dy, step_dpressure, step_declination, step_ascension, step_dtime);
      bool painted_now = prepare_and_draw_dab (surface);
      if (painted_now) {
        painted = YES;
      } else if (painted == UNKNOWN) {
        painted = NO;
      }

      dtime_left   -= step_dtime;
      dist_todo  = count_dabs_to (x, y, pressure, dtime_left);
    }

    {
      // "move" the brush to the current time (no more dab will happen)
      // Important to do this at least once every event, because
      // brush_count_dabs_to depends on the radius and the radius can
      // depend on something that changes much faster than only every
      // dab (eg speed).
    
      step_dx        = x - states[STATE_X];
      step_dy        = y - states[STATE_Y];
      step_dpressure = pressure - states[STATE_PRESSURE];
      step_declination = tilt_declination - states[STATE_DECLINATION];
      step_ascension = tilt_ascension - states[STATE_ASCENSION];
      step_dtime     = dtime_left;
    
      //dtime_left = 0; but that value is not used any more

      update_states_and_setting_values (step_dx, step_dy, step_dpressure, step_declination, step_ascension, step_dtime);
    }

    // save the fraction of a dab that is already done now
    states[STATE_DIST] = dist_moved + dist_todo;
    //g_print("dist_final = %f\n", states[STATE_DIST]);

    // next seed for the RNG (GRand has no get_state() and states[] must always contain our full state)
    states[STATE_RNG_SEED] = g_rand_int(rng);

    // stroke separation logic (for undo/redo)

    if (painted == UNKNOWN) {
      if (stroke_current_idling_time > 0 || stroke_total_painting_time == 0) {
        // still idling
        painted = NO;
      } else {
        // probably still painting (we get more events than brushdabs)
        painted = YES;
        //if (pressure == 0) g_print ("info: assuming 'still painting' while there is no pressure\n");
      }
    }
    if (painted == YES) {
      //if (stroke_current_idling_time > 0) g_print ("idling ==> painting\n");
      stroke_total_painting_time += dtime;
      stroke_current_idling_time = 0;
      // force a stroke split after some time
      if (stroke_total_painting_time > 4 + 3*pressure) {
        // but only if pressure is not being released
        // FIXME: use some smoothed state for dpressure, not the output of the interpolation code
        //        (which might easily wrongly give dpressure == 0)
        if (step_dpressure >= 0) {
          return true;
        }
      }
    } else if (painted == NO) {
      //if (stroke_current_idling_time == 0) g_print ("painting ==> idling\n");
      stroke_current_idling_time += dtime;
      if (stroke_total_painting_time == 0) {
        // not yet painted, start a new stroke if we have accumulated a lot of irrelevant motion events
        if (stroke_current_idling_time > 1.0) {
          return true;
        }
      } else {
        // Usually we have pressure==0 here. But some brushes can paint
        // nothing at full pressure (eg gappy lines, or a stroke that
        // fades out). In either case this is the prefered moment to split.
//        if (stroke_total_painting_time+stroke_current_idling_time > 0.9 + 5*pressure) {
          return true;
//        }
      }
    }
    return false;
  }

};
