Implement linear interpolation between animation keyframes

animation.c

@@ -1,9 +1,44 @@
 #include "animation.h"
 
 #include <assert.h>
+#include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
 
+static vec3_t lerp(vec3_t v, vec3_t u, double t)
+{
+	return vec3_add(vec3_scale(1 - t, v), vec3_scale(t, u));
+}
+
+static vec4_t slerp(vec4_t v, vec4_t u, double t)
+{
+	float dot = vec4_dot(v, u);
+	if (dot < 0) {
+		dot = -dot;
+		u = vec4_scale(-1, u);
+	}
+
+	if (dot < 0.95) {
+		const float a = acosf(dot);
+		return vec4_scale(
+		    1 / sin(a),
+		    vec4_add(
+		        vec4_scale(sinf(a * (1 - t)), v),
+		        vec4_scale(sinf(a * t), u)));
+	} else {
+		return vec4_add(vec4_scale(1 - t, v), vec4_scale(t, u));
+	}
+}
+
+static void interpolate(
+    bone_t *dst, const bone_t *as, const bone_t *bs, size_t n, double t)
+{
+	for (unsigned i = 0; i < n; ++i) {
+		dst[i].translation = lerp(as[i].translation, bs[i].translation, t);
+		dst[i].rotation = slerp(as[i].rotation, bs[i].rotation, t);
+	}
+}
+
 static mat4_t bone_transform(bone_t bone)
 {
 	return mat4_mul(
@@ -27,6 +62,27 @@ static void compute_bone_matrices(
 		matrices[i] = mat4_mul(matrices[i], inverse_bind_matrices[i]);
 }
 
+static void load_keyframe(animation_state_t *state, const model_t *model)
+{
+	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.length_us
+		    - model->animation.times_us[state->current_keyframe]
+		    + model->animation.times_us[state->next_keyframe];
+	}
+
+	state->current = model->animation.keyframes
+	    + state->current_keyframe * model->bone_count;
+	state->next = model->animation.keyframes
+	    + state->next_keyframe * model->bone_count;
+}
+
 animation_state_t animation_init(const model_t *model)
 {
 	mat4_t *bone_matrices = malloc(model->bone_count * sizeof(mat4_t));
@@ -42,16 +98,12 @@ void animation_cleanup(animation_state_t *state)
 	}
 }
 
-void animation_start(animation_state_t *state, const model_t *model)
+void animation_start(
+    animation_state_t *state, const model_t *model, uint32_t start_keyframe)
 {
-	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;
+	state->current_keyframe = start_keyframe;
+	load_keyframe(state, model);
 }
 
 void animation_update(
@@ -60,27 +112,16 @@ void animation_update(
 	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;
+		load_keyframe(state, model);
 	}
 
+	bone_t bones[MAX_BONE_COUNT];
+	interpolate(
+	    bones, state->current, state->next, model->bone_count,
+	    (double)state->elapsed_us / state->duration_us);
+
 	compute_bone_matrices(
-	    state->current, model->bone_count, model->bone_parents,
+	    bones, model->bone_count, model->bone_parents,
 	    model->inverse_bind_matrices, state->bone_matrices);
 }