correct bounce math?

diff --git a/sprite.c b/sprite.c
index abe9fb3..e63fd68 100644 (file)
--- a/sprite.c
+++ b/sprite.c
@@ -340,8 +340,8 @@ void
 bounce(Sprite *a, Sprite *b)
 {
        float x, y, n;
-       float va, vb;
-       float ma, mb, mr;
+       float va, vb, vc;
+       float ma, mb;
 
        // (x, y) is unit vector pointing from A's center to B's center.
        x = (b->x + b->w / 2) - (a->x + a->w / 2);
@@ -352,10 +352,9 @@ bounce(Sprite *a, Sprite *b)
        va = max(x*a->dx + y*a->dy, 0);
        vb = min(x*b->dx + y*b->dy, 0);
 
-       // mass ratio
        ma = sprite_mass(a); mb = sprite_mass(b);
-       if(ma && mb) mr = mb/ma; else mr = 1;
+       vc = (va*ma + vb*mb) / (ma+mb);
 
-       a->dx += x*(mb*vb - ma*va)/ma; a->dy += y*(mb*vb - ma*va)/ma;
-       b->dx += x*(ma*va - mb*vb)/mb; b->dy += y*(ma*va - mb*vb)/mb;
+       a->dx += 2*x*(vc-va); a->dy += 2*y*(vc-va);
+       b->dx += 2*x*(vc-vb); b->dy += 2*y*(vc-vb);
 }