diff --git a/include/hexagram/anim.h b/include/hexagram/anim.h
index f081263..910b010 100644
--- a/include/hexagram/anim.h
+++ b/include/hexagram/anim.h
@@ -13,6 +13,10 @@ typedef enum {
     HEXAGRAM_ANIM_RADIUS = 1 << 2
 } hexagram_anim_flags;
 
+typedef struct _hexagram_anim_stop hexagram_anim_stop;
+
+typedef double (hexagram_anim_progress_fn)(hexagram_anim_stop *, double);
+
 typedef struct _hexagram_anim_action {
     int flags;
 
@@ -31,11 +35,12 @@ typedef struct _hexagram_anim_action {
     } from, to;
 } hexagram_anim_action;
 
-typedef struct _hexagram_anim_stop {
+struct _hexagram_anim_stop {
     hexagram_anim_action *actions;
     size_t count;
     double duration;
-} hexagram_anim_stop;
+    hexagram_anim_progress_fn *fn;
+};
 
 typedef struct _hexagram_anim {
     hexagram_anim_stop *stops;
@@ -43,11 +48,9 @@ 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, hexagram_anim_progress_fn *fn);
+int hexagram_anim_step(hexagram_anim *anim);
 
 double hexagram_anim_duration(hexagram_anim *anim);
 
diff --git a/src/anim.c b/src/anim.c
index 408fd62..978cbb7 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, hexagram_anim_progress_fn *fn) {
+int hexagram_anim_step(hexagram_anim *anim) {
     struct timeval tv;
     double offset = 0.0, interval, progress;
     size_t i, a;
@@ -53,8 +53,8 @@ int hexagram_anim_step(hexagram_anim *anim, hexagram_anim_progress_fn *fn) {
     if (i == anim->count) {
         i--;
         progress = 1.0;
-    } else if (fn) {
-        progress = fn(&anim->stops[i],
+    } else if (anim->stops[i].fn) {
+        progress = anim->stops[i].fn(&anim->stops[i],
                       (interval - offset) / anim->stops[i].duration);
     } else {
         progress = (interval - offset) / anim->stops[i].duration;