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];
+		load_keyframe(state, model);
 	}
 
-		state->current = state->next;
-		const uint32_t offset = state->next_keyframe * model->bone_count;
-		state->next = model->animation.keyframes + offset;
-	}
+	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);
 }

animation.h

@@ -13,7 +13,8 @@ typedef struct {
 animation_state_t animation_init(const model_t *model);
 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);
 void animation_update(
     animation_state_t *state, const model_t *model, uint32_t dt_us);
 

linear_alg.c

@@ -2,11 +2,21 @@
 
 #include <math.h>
 
+vec3_t vec3_add(vec3_t v, vec3_t u)
+{
+	return (vec3_t) { v.x + u.x, v.y + u.y, v.z + u.z };
+}
+
 vec3_t vec3_sub(vec3_t v, vec3_t u)
 {
 	return (vec3_t) { v.x - u.x, v.y - u.y, v.z - u.z };
 }
 
+vec3_t vec3_scale(float s, vec3_t v)
+{
+	return (vec3_t) { s * v.x, s * v.y, s * v.z };
+}
+
 float vec3_len(vec3_t v)
 {
 	return sqrtf(v.x * v.x + v.y * v.y + v.z * v.z);
@@ -15,7 +25,7 @@ float vec3_len(vec3_t v)
 vec3_t vec3_norm(vec3_t v)
 {
 	const float l = vec3_len(v);
-	return (vec3_t) { v.x / l, v.y / l, v.z / l };
+	return vec3_scale(1 / l, v);
 }
 
 float vec3_dot(vec3_t v, vec3_t u)
@@ -32,6 +42,21 @@ vec3_t vec3_cross(vec3_t v, vec3_t u)
 	};
 }
 
+vec4_t vec4_add(vec4_t v, vec4_t u)
+{
+	return (vec4_t) { v.x + u.x, v.y + u.y, v.z + u.z, v.w + u.w };
+}
+
+vec4_t vec4_scale(float s, vec4_t v)
+{
+	return (vec4_t) { s * v.x, s * v.y, s * v.z, s * v.w };
+}
+
+float vec4_dot(vec4_t v, vec4_t u)
+{
+	return v.x * u.x + v.y * u.y + v.z * u.z + v.w * u.w;
+}
+
 mat4_t mat4_mul(mat4_t m, mat4_t n)
 {
 #define ELEM(i, j) m.x.i *n.j.x + m.y.i *n.j.y + m.z.i *n.j.z + m.w.i *n.j.w

linear_alg.h

@@ -71,12 +71,18 @@ typedef struct {
 	}
 // clang-format on
 
+vec3_t vec3_add(vec3_t v, vec3_t u);
 vec3_t vec3_sub(vec3_t v, vec3_t u);
+vec3_t vec3_scale(float s, vec3_t v);
 float vec3_len(vec3_t v);
 vec3_t vec3_norm(vec3_t v);
 float vec3_dot(vec3_t v, vec3_t u);
 vec3_t vec3_cross(vec3_t v, vec3_t u);
 
+vec4_t vec4_add(vec4_t v, vec4_t u);
+vec4_t vec4_scale(float s, vec4_t v);
+float vec4_dot(vec4_t v, vec4_t u);
+
 mat4_t mat4_mul(mat4_t m, mat4_t n);
 mat4_t mat4_lookat(vec3_t camera, vec3_t target, vec3_t up);
 mat4_t mat4_perspective(float aspect, float fov, float near, float far);

model.h

@@ -6,6 +6,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#define MAX_BONE_COUNT 64
+
 typedef uint16_t index_t;
 
 typedef struct {

model_format_glb.c

@@ -29,7 +29,6 @@ static_assert(
 #define VERSION 2
 
 #define MAX_STRING_SIZE 128
-#define MAX_BONE_COUNT 32
 
 enum {
 	CHUNK_TYPE_JSON = 0x4e4f534a, // "JSON"