lily/spring.cc

   1 /*
   2   This file is part of LilyPond, the GNU music typesetter.
   3
   4   Copyright (C) 2007--2010 Joe Neeman <joeneeman@gmail.com>
   5
   6   LilyPond is free software: you can redistribute it and/or modify
   7   it under the terms of the GNU General Public License as published by
   8   the Free Software Foundation, either version 3 of the License, or
   9   (at your option) any later version.
  10
  11   LilyPond is distributed in the hope that it will be useful,
  12   but WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14   GNU General Public License for more details.
  15
  16   You should have received a copy of the GNU General Public License
  17   along with LilyPond.  If not, see <http://www.gnu.org/licenses/>.
  18 */
  19
  20 #include "spring.hh"
  21
  22 Spring::Spring ()
  23 {
  24   distance_ = 1.0;
  25   min_distance_ = 1.0;
  26   inverse_stretch_strength_ = 1.0;
  27   inverse_compress_strength_ = 1.0;
  28
  29   update_blocking_force ();
  30 }
  31
  32 Spring::Spring (Real dist, Real min_dist)
  33 {
  34   distance_ = 1.0;
  35   min_distance_ = 1.0;
  36   inverse_stretch_strength_ = 1.0;
  37   inverse_compress_strength_ = 1.0;
  38
  39   set_distance (dist);
  40   set_min_distance (min_dist);
  41   set_default_strength ();
  42   update_blocking_force ();
  43 }
  44
  45 void
  46 Spring::update_blocking_force ()
  47 {
  48   if (min_distance_ > distance_)
  49     blocking_force_ = (min_distance_ - distance_) / inverse_stretch_strength_;
  50   else
  51     blocking_force_ = (min_distance_ - distance_) / inverse_compress_strength_;
  52
  53   // If the spring is fixed, it's not clear what the natural value
  54   // of blocking_force_ would be. -infinity_f works fine for now.
  55   if (isnan (blocking_force_) || blocking_force_ == infinity_f)
  56     blocking_force_ = -infinity_f;
  57
  58   if (blocking_force_ >= 0)
  59     inverse_compress_strength_ = 0;
  60 }
  61
  62 /* scale a spring, but in a way that doesn't violate min_distance */
  63 void
  64 Spring::operator*= (Real r)
  65 {
  66   distance_ = max (min_distance_, distance_ * r);
  67   inverse_compress_strength_ = max (0.0, distance_ - min_distance_);
  68   inverse_stretch_strength_ *= 0.8;
  69   update_blocking_force ();
  70 }
  71
  72 bool
  73 Spring::operator> (Spring const &other) const
  74 {
  75   return blocking_force_ > other.blocking_force_;
  76 }
  77
  78 /* merge springs, basically by averaging them, but leave a little headroom
  79    above the largest minimum distance so that things don't get too cramped */
  80 Spring
  81 merge_springs (vector<Spring> const &springs)
  82 {
  83   Real avg_distance = 0;
  84   Real min_distance = 0;
  85   Real avg_stretch = 0;
  86   Real avg_compress = 0;
  87
  88   for (vsize i = 0; i < springs.size (); i++)
  89     {
  90       avg_distance += springs[i].distance ();
  91       avg_stretch += springs[i].inverse_stretch_strength ();
  92       avg_compress += 1 / springs[i].inverse_compress_strength ();
  93       min_distance = max (springs[i].min_distance (), min_distance);
  94     }
  95
  96   avg_stretch /= springs.size ();
  97   avg_compress /= springs.size ();
  98   avg_distance /= springs.size ();
  99   avg_distance = max (min_distance + 0.3, avg_distance);
 100
 101   Spring ret = Spring (avg_distance, min_distance);
 102   ret.set_inverse_stretch_strength (avg_stretch);
 103   ret.set_inverse_compress_strength (1 / avg_compress);
 104
 105   return ret;
 106 }
 107
 108 void
 109 Spring::set_distance (Real d)
 110 {
 111   if (d < 0 || isinf (d) || isnan (d))
 112     programming_error ("insane spring distance requested, ignoring it");
 113   else
 114     {
 115       distance_ = d;
 116       update_blocking_force ();
 117     }
 118 }
 119
 120 void
 121 Spring::set_min_distance (Real d)
 122 {
 123   if (d < 0 || isinf (d) || isnan (d))
 124     programming_error ("insane spring min_distance requested, ignoring it");
 125   else
 126     {
 127       min_distance_ = d;
 128       update_blocking_force ();
 129     }
 130 }
 131
 132 void
 133 Spring::ensure_min_distance (Real d)
 134 {
 135   set_min_distance (max (d, min_distance_));
 136 }
 137
 138 void
 139 Spring::set_inverse_stretch_strength (Real f)
 140 {
 141   if (isinf (f) || isnan (f) || f < 0)
 142     programming_error ("insane spring constant");
 143   else
 144     inverse_stretch_strength_ = f;
 145
 146   update_blocking_force ();
 147 }
 148
 149 void
 150 Spring::set_inverse_compress_strength (Real f)
 151 {
 152   if (isinf (f) || isnan (f) || f < 0)
 153     programming_error ("insane spring constant");
 154   else
 155     inverse_compress_strength_ = f;
 156
 157   update_blocking_force ();
 158 }
 159
 160 void
 161 Spring::set_blocking_force (Real f)
 162 {
 163   if (isinf (f) || isnan (f))
 164     {
 165       programming_error ("insane blocking force");
 166       return;
 167     }
 168
 169   blocking_force_ = -infinity_f;
 170   min_distance_ = length (f);
 171   distance_ = max (distance_, min_distance_);
 172   update_blocking_force ();
 173 }
 174
 175 void
 176 Spring::set_default_strength ()
 177 {
 178   inverse_compress_strength_ = (distance_ >= min_distance_) ? distance_ - min_distance_ : 0;
 179   inverse_stretch_strength_ = distance_;
 180   update_blocking_force ();
 181 }
 182
 183 Real
 184 Spring::length (Real f) const
 185 {
 186   Real force = max (f, blocking_force_);
 187   Real inv_k = force < 0.0 ? inverse_compress_strength_ : inverse_stretch_strength_;
 188
 189   if (force == infinity_f)
 190     {
 191       programming_error ("cruelty to springs");
 192       force = 0.0;
 193     }
 194
 195   // There is a corner case here: if min_distance_ is larger than
 196   // distance_ but the spring is fixed, then inv_k will be zero
 197   // and we need to make sure that we return min_distance_.
 198   return max (min_distance_, distance_ + force * inv_k);
 199 }
 200