Change the treatment of min-distance in springs.

Previously, we ensured that the ideal distance was at least
as large as the minimum distance, but it sometimes useful
not to. Now, if a spring has minimum distance larger than
ideal distance, it will remain fixed (at minimum-distance)
for a while as you start to stretch it, but it will stretch
eventually. This helps give us a more uniform vertical
spacing.

3 files changed, 23 insertions, 10 deletions

diff --git a/lily/include/spring.hh b/lily/include/spring.hh
index c379889adb..4288af5884 100644
--- a/lily/include/spring.hh
+++ b/lily/include/spring.hh
@@ -39,6 +39,7 @@ public:
 
   void set_distance (Real);
   void set_min_distance (Real);
+  void ensure_min_distance (Real);
   void set_inverse_stretch_strength (Real);
   void set_inverse_compress_strength (Real);
   void set_blocking_force (Real);
diff --git a/lily/simple-spacer.cc b/lily/simple-spacer.cc
index 1d38d9a70f..bab2a05dcb 100644
--- a/lily/simple-spacer.cc
+++ b/lily/simple-spacer.cc
@@ -216,7 +216,9 @@ Simple_spacer::compress_line ()
     {
       Spring sp = sorted_springs[i];
 
-      assert (sp.blocking_force () <= cur_force);
+      if (sp.blocking_force () > cur_force)
+	continue;
+
       if (isinf (sp.blocking_force ()))
 	break;
 
@@ -234,7 +236,7 @@ Simple_spacer::compress_line ()
 	}
       
       cur_len -= block_dist;
-      inv_hooke -= sp.inverse_compress_strength ();
+      inv_hooke -= compressed ? sp.inverse_compress_strength () : sp.inverse_stretch_strength ();
       cur_force = sp.blocking_force ();
     }
 
diff --git a/lily/spring.cc b/lily/spring.cc
index 67d41c9c39..fd8e0147cb 100644
--- a/lily/spring.cc
+++ b/lily/spring.cc
@@ -32,10 +32,16 @@ Spring::Spring (Real dist, Real min_dist)
 void
 Spring::update_blocking_force ()
 {
-  if (distance_ == min_distance_)
-    blocking_force_ = 0.0;
+  if (min_distance_ > distance_)
+    blocking_force_ = (min_distance_ - distance_) / inverse_stretch_strength_;
   else
     blocking_force_ = (min_distance_ - distance_) / inverse_compress_strength_;
+
+  if (isnan (blocking_force_) || blocking_force_ == infinity_f)
+    blocking_force_ = 0;
+
+  if (blocking_force_ >= 0)
+    inverse_compress_strength_ = 0;
 }
 
 /* scale a spring, but in a way that doesn't violate min_distance */
@@ -43,7 +49,7 @@ void
 Spring::operator*= (Real r)
 {
   distance_ = max (min_distance_, distance_ * r);
-  inverse_compress_strength_ = distance_ - min_distance_;
+  inverse_compress_strength_ = max (0.0, distance_ - min_distance_);
   inverse_stretch_strength_ *= 0.8;
 }
 
@@ -90,7 +96,6 @@ Spring::set_distance (Real d)
     programming_error ("insane spring distance requested, ignoring it");
   else
     {
-      min_distance_ = min (min_distance_, d);
       distance_ = d;
       update_blocking_force ();
     }
@@ -104,12 +109,17 @@ Spring::set_min_distance (Real d)
   else
     {
       min_distance_ = d;
-      distance_ = max (distance_, min_distance_);
       update_blocking_force ();
     }
 }
 
 void
+Spring::ensure_min_distance (Real d)
+{
+  set_min_distance (max (d, min_distance_));
+}
+
+void
 Spring::set_inverse_stretch_strength (Real f)
 {
   if (isinf (f) || isnan (f) || f < 0)
@@ -147,8 +157,8 @@ Spring::set_blocking_force (Real f)
 void
 Spring::set_default_strength ()
 {
-  inverse_compress_strength_ = distance_ - min_distance_;
-  inverse_stretch_strength_ = distance_ - min_distance_;
+  inverse_compress_strength_ = (distance_ >= min_distance_) ? distance_ - min_distance_ : 0;
+  inverse_stretch_strength_ = distance_;
 }
 
 Real
@@ -163,5 +173,5 @@ Spring::length (Real f) const
       force = 0.0;
     }
 
-  return distance_ + force * inv_k;
+  return max (min_distance_, distance_ + force * inv_k);
 }