diff --git a/include/hexagram/anim.h b/include/hexagram/anim.h
index b97dd66..ca2a6ba 100644
--- a/include/hexagram/anim.h
+++ b/include/hexagram/anim.h
@@ -43,9 +43,11 @@ typedef struct _hexagram_anim {
     struct timeval start, now;
 } hexagram_anim;
 
+typedef double (hexagram_anim_progress_fn)(hexagram_anim_stop *, double);
+
 void hexagram_anim_init(hexagram_anim *anim);
 
-int hexagram_anim_step(hexagram_anim *anim);
+int hexagram_anim_step(hexagram_anim *anim, hexagram_anim_progress_fn *fn);
 
 double hexagram_anim_duration(hexagram_anim *anim);
 
diff --git a/src/anim.c b/src/anim.c
index 9f402d4..6184bcf 100644
--- a/src/anim.c
+++ b/src/anim.c
@@ -22,7 +22,7 @@ static inline double stop_offset(hexagram_anim *anim, size_t index) {
     return offset;
 }
 
-int hexagram_anim_step(hexagram_anim *anim) {
+int hexagram_anim_step(hexagram_anim *anim, hexagram_anim_progress_fn *fn) {
     struct timeval tv;
     double offset = 0.0, interval, progress;
     size_t i, a;
@@ -55,7 +55,12 @@ int hexagram_anim_step(hexagram_anim *anim) {
     }
 
     /* Determine current progress through current stop */
-    progress = (interval - offset) / anim->stops[i].duration;
+    if (fn) {
+        progress = fn(&anim->stops[i],
+                      (interval - offset) / anim->stops[i].duration);
+    } else {
+        progress = (interval - offset) / anim->stops[i].duration;
+    }
 
     for (a=0; a<anim->stops[i].actionc; a++) {
         hexagram_anim_action *action = &anim->stops[i].actions[a];