Implement step-wise animation locked to time series

animation.c

@@ -44,19 +44,43 @@ void animation_cleanup(animation_state_t *state)
 
 void animation_start(animation_state_t *state, const model_t *model)
 {
+	state->current_keyframe = model->animation.keyframe_count - 1;
+	state->next_keyframe = 0;
+
 	state->current = model->bones;
+	state->next = model->animation.keyframes;
+
+	state->duration_us = model->animation.times_us[0];
+	state->elapsed_us = 0;
 }
 
-void animation_update(animation_state_t *state, const model_t *model)
+void animation_update(
+    animation_state_t *state, const model_t *model, uint32_t dt_us)
 {
-	const size_t offset = state->keyframe * model->bone_count;
-	state->current = model->animation.keyframes + offset;
+	state->elapsed_us += dt_us;
+	if (state->elapsed_us >= state->duration_us) { // Move to next keyframe
+		state->elapsed_us -= state->duration_us;
+
+		state->current_keyframe = state->next_keyframe;
+		const uint32_t keyframe_count = model->animation.keyframe_count;
+		if (state->current_keyframe + 1 < keyframe_count) {
+			state->next_keyframe = state->current_keyframe + 1;
+			state->duration_us
+			    = model->animation.times_us[state->next_keyframe]
+			    - model->animation.times_us[state->current_keyframe];
+		} else {
+			// Handle loop-around
+			state->next_keyframe = 0;
+			state->duration_us
+			    = model->animation.times_us[state->next_keyframe];
+		}
+
+		state->current = state->next;
+		const uint32_t offset = state->next_keyframe * model->bone_count;
+		state->next = model->animation.keyframes + offset;
+	}
 
 	compute_bone_matrices(
 	    state->current, model->bone_count, model->bone_parents,
 	    model->inverse_bind_matrices, state->bone_matrices);
-
-	++state->keyframe;
-	if (state->keyframe >= model->animation.keyframe_count)
-		state->keyframe = 0;
 }