L28 — Third-person follow camera
Goal
Drive a follow camera with its own yaw and positional lag, and move relative to what you see — not player facing.
In plain English
text
camYaw = camera horizontal angle (C-left/right only — free orbit)
eyeWant = player + offset(camYaw) * distance + up * height
eye/look = lerp toward desired (positional lag)
move basis = direction of (eye → player) on XZ // lagged view, not player yaw
fallback: camYaw if eye sits on the player
player yaw = face the move direction (model only — never drives the camera)Why not couple camera to player yaw?
If movement is relative to player yaw and the camera sits on player yaw, turning (strafe or forward facing updates) changes the move basis every frame → the view spins. Same bug if you soft-lerp camYaw toward playerYaw while running: those angles differ by ~π (camera sits behind, player faces the look direction), so the lerp races around the circle.
Rule:
- camYaw only from C-buttons (desired orbit).
- Move relative to the lagged eye (what is on screen), not player yaw.
- Player yaw turns the model only.
- Never soft-follow
camYaw→ player yaw.
What you will see
bash
make -C lessons/l28-follow-camCamera trails with lag; stick-up = into the scene; C-left/right orbits; strafe does not flip the camera.
Uses skinned player_anim (scale ~0.02 on the course island) with t3d_skeleton_use before t3d_model_draw_skinned, and push/pop for island and player matrices. Snake model yaw is -yaw (mesh faces −Z). Soft wall is a box clamp.
Display: FILTERS_RESAMPLE + opaque clear (no VI AA edge flicker).
Full lesson source
The blocks below are imported from the real repository files at build time (VitePress <<< snippets). They are not hand-copied into this markdown.
lessons/l28-follow-cam/Makefile · lessons/l28-follow-cam/src/main.c
lessons/l28-follow-cam/Makefile
make
ROMNAME := l28_cam
ROM_TITLE := "L28 FollowCam"
USE_T3D := 1
include ../../common/lesson.mklessons/l28-follow-cam/src/main.c
c
/**
* L28 — Third-person follow camera
* ============================================================================
*
* LEARNING GOAL
* -------------
* Camera has its own yaw (C-orbit only — never soft-follows player facing), with
* positional lag so motion feels soft — not glued 1:1 to the character.
*
* RECIPE
* ------
* camYaw = free orbit (C-left/right only)
* desired_eye = player + offset(camYaw)*dist + up*height
* desired_look = player + head_offset
* eye/look = lerp toward desired (positional lag)
* t3d_viewport_look_at(eye, look, up)
*
* Move is relative to the *lagged* camera (eye→player on XZ), not player yaw.
* If move used player yaw and the camera sat on player yaw, pure strafe would
* spin forever. Soft-lerping camYaw toward playerYaw also spins (~π apart).
*
* CONTROLS: Stick move, C-left/right orbit camera
* DOCS: docs/guide/m4/l28-follow-cam.md
* Also see: capstone/starshard-cove/src/main.c (CAMERA section)
*/
#include <libdragon.h>
#include <t3d/t3d.h>
#include <t3d/t3dmodel.h>
#include <t3d/t3dskeleton.h>
#include <t3d/t3danim.h>
#include <stdio.h>
#include "ng_game.h"
#define FB_COUNT 3
int main(void)
{
debug_init_isviewer();
debug_init_usblog();
asset_init_compression(2);
dfs_init(DFS_DEFAULT_LOCATION);
/* FILTERS_RESAMPLE only — avoid RESAMPLE_ANTIALIAS (1px top-edge flicker). */
display_init(RESOLUTION_320x240, DEPTH_16_BPP, FB_COUNT, GAMMA_NONE,
FILTERS_RESAMPLE);
rdpq_init();
joypad_init();
rdpq_text_register_font(1, rdpq_font_load_builtin(FONT_BUILTIN_DEBUG_VAR));
t3d_init((T3DInitParams){});
T3DViewport viewport = t3d_viewport_create_buffered(FB_COUNT);
T3DMat4FP *playerMat = malloc_uncached(sizeof(T3DMat4FP) * FB_COUNT);
T3DMat4FP *islandMat = malloc_uncached(sizeof(T3DMat4FP));
t3d_mat4fp_from_srt_euler(islandMat,
(float[3]){ 0.032f, 0.032f, 0.032f }, (float[3]){ 0, 0, 0 }, (float[3]){ 0, 0, 0 });
T3DModel *island = t3d_model_load("rom:/island.t3dm");
T3DModel *player = t3d_model_load("rom:/player_anim.t3dm");
/* Fail loudly (like L27) if the DFS models are missing — otherwise the
* island/player silently drop out and you just get the clear color. */
assertf(island && player, "L28: failed to load island/player models from ROM (DFS)");
T3DSkeleton skel = t3d_skeleton_create_buffered(player, FB_COUNT);
T3DSkeleton skelBlend = t3d_skeleton_clone(&skel, false);
T3DAnim animIdle = t3d_anim_create(player, "Snake_Idle");
T3DAnim animWalk = t3d_anim_create(player, "Snake_Walk");
t3d_anim_attach(&animIdle, &skel);
t3d_anim_attach(&animWalk, &skelBlend);
fm_vec3_t pos = {{ 0, 0.15f, 0 }};
float yaw = 0.f; /* player facing (model) */
float camYaw = 0.f; /* camera horizontal angle — NOT the same as player yaw */
fm_vec3_t eye = {{ 0, 8, 14 }};
fm_vec3_t look = {{ 0, 1, 0 }};
float last = ng_time_s();
int frame = 0;
uint8_t amb[4] = { 85, 90, 100, 255 };
uint8_t dirC[4] = { 255, 220, 200, 255 };
fm_vec3_t ldir = {{ 0.4f, 1.f, 0.3f }};
fm_vec3_norm(&ldir, &ldir);
for (;;) {
joypad_poll();
joypad_inputs_t in = joypad_get_inputs(JOYPAD_PORT_1);
float dt = ng_time_s() - last;
last = ng_time_s();
if (dt < 0.f) {
dt = 0.f;
}
if (dt > 0.1f) {
dt = 0.1f;
}
frame = (frame + 1) % FB_COUNT;
/* Orbit camera with C-buttons (changes camYaw only). */
if (in.btn.c_left) {
camYaw -= 1.2f * dt;
}
if (in.btn.c_right) {
camYaw += 1.2f * dt;
}
float sx = (float)ng_dz(in.stick_x) / 80.f;
float sy = (float)ng_dz(in.stick_y) / 80.f;
float speed = sqrtf(sx * sx + sy * sy);
if (speed > 1.f) {
sx /= speed;
sy /= speed;
speed = 1.f;
}
/* Move relative to lagged camera (eye→player), not player yaw / camYaw alone. */
float edx = eye.v[0] - pos.v[0];
float edz = eye.v[2] - pos.v[2];
float elen = sqrtf(edx * edx + edz * edz);
float c, s;
if (elen > 0.001f) {
float backNow = atan2f(edx, edz);
c = fm_cosf(backNow);
s = fm_sinf(backNow);
} else {
c = fm_cosf(camYaw);
s = fm_sinf(camYaw);
}
float mx = sx * c - sy * s;
float mz = -sx * s - sy * c;
float blend = 0.f;
if (speed > 0.15f) {
pos.v[0] += mx * 7.5f * dt;
pos.v[2] += mz * 7.5f * dt;
yaw = ng_lerp_angle(yaw, atan2f(mx, mz), 0.22f);
blend = speed;
}
pos.v[0] = ng_clamp(pos.v[0], -5.5f, 5.5f);
pos.v[2] = ng_clamp(pos.v[2], -5.5f, 5.5f);
t3d_anim_update(&animIdle, dt);
t3d_anim_set_speed(&animWalk, 0.2f + blend);
t3d_anim_update(&animWalk, dt);
t3d_skeleton_blend(&skel, &skel, &skelBlend, blend);
t3d_skeleton_update(&skel);
/* Desired camera: sit along camYaw (behind relative to camera basis). */
fm_vec3_t eyeWant = {{
pos.v[0] + fm_sinf(camYaw) * 12.f,
pos.v[1] + 6.5f,
pos.v[2] + fm_cosf(camYaw) * 12.f,
}};
fm_vec3_t lookWant = {{ pos.v[0], pos.v[1] + 1.4f, pos.v[2] }};
float lag = ng_clamp(10.f * dt, 0.08f, 0.4f);
eye.v[0] = ng_lerp(eye.v[0], eyeWant.v[0], lag);
eye.v[1] = ng_lerp(eye.v[1], eyeWant.v[1], lag);
eye.v[2] = ng_lerp(eye.v[2], eyeWant.v[2], lag);
look.v[0] = ng_lerp(look.v[0], lookWant.v[0], lag);
look.v[1] = ng_lerp(look.v[1], lookWant.v[1], lag);
look.v[2] = ng_lerp(look.v[2], lookWant.v[2], lag);
t3d_viewport_set_projection(&viewport, T3D_DEG_TO_RAD(58.f), 1.f, 200.f);
t3d_viewport_look_at(&viewport, &eye, &look, &(fm_vec3_t){{ 0, 1, 0 }});
t3d_mat4fp_from_srt_euler(&playerMat[frame],
(float[3]){ 0.02f, 0.02f, 0.02f },
(float[3]){ 0.f, -yaw, 0.f },
(float[3]){ pos.v[0], pos.v[1], pos.v[2] });
rdpq_attach(display_get(), display_get_zbuf());
t3d_frame_start();
t3d_viewport_attach(&viewport);
t3d_screen_clear_color(RGBA32(30, 60, 100, 0xFF));
t3d_screen_clear_depth();
t3d_light_set_ambient(amb);
t3d_light_set_directional(0, dirC, &ldir);
t3d_light_set_count(1);
t3d_matrix_push(islandMat);
if (island) {
t3d_model_draw(island);
}
t3d_matrix_pop(1);
t3d_skeleton_use(&skel);
t3d_matrix_push(&playerMat[frame]);
t3d_model_draw_skinned(player, &skel);
t3d_matrix_pop(1);
rdpq_set_mode_standard();
rdpq_text_print(NULL, 1, 10, 12, "L28 — Follow camera");
rdpq_text_print(NULL, 1, 10, 28, "Stick move C-left/right orbit lag cam");
rdpq_detach_show();
}
}