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Replace GDScript BallSystem with GPU compute implementation #87

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cammymoop merged 36 commits from feature/gpu-compute-poc into master 2026-03-06 15:48:36 -06:00
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claude commented 2026-03-06 11:07:02 -06:00
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Summary

  • Adds GpuBallSystem (systems/gpu_ball_system.gd) replacing the GDScript BallSystem with a Vulkan compute shader pipeline while keeping the same public API (spawn_ball, apply_explosion, get_alive_count)
  • systems/ball_sim_v2.glsl: compute shader with 13 bindings (render texture, ball state, explosions, params, rocks, detonation/contact/death outputs, armed list, type config, compact state flags, player-near candidate snapshots). Handles physics, wall/rock bounce, speed cap, bomb skip/bounce, chain fuse countdown, explosion kill/arm, bomb proximity detonation, contact damage detection, and GPU-side color from hp/flags/type config
  • systems/ball_render_v2.gdshader: canvas item shader reading ball pos/color from GPU texture via INSTANCE_CUSTOM.r ball index
  • GPU -> CPU event readback: DetonationOut (bomb/chain fuse -> explosion effects + new explosion events), ContactOut (ball-player overlap -> damage), DeathOut (explosion kills -> shard spawns)
  • CPU-side melee detection remains authoritative, but the old full-ball shadow readback was replaced with a player-centered GPU candidate pass: CPU now reads compact per-ball flags for reconciliation and detailed snapshots only for nearby balls used by melee checks
  • GpuBallSystem capacity now grows dynamically instead of dropping spawns when the initial 512 slots are exhausted
  • Resize path updated so render-texture rebinding happens before the old texture RID is freed
  • test_scene_1.tscn, wave_spawner.gd, player.gd updated to use GpuBallSystem
  • Old ball_system.gd (CPU) kept intact as reference

Since PR Opened

  • Removed the hard 512-ball runtime ceiling by adding dynamic GPU buffer / render resource growth
  • Reduced CPU readback cost by replacing full SSBO shadow sync with player-near candidate snapshots
  • Fixed an invalid RID error that occurred when GPU resources resized during gameplay
  • test_scene_1.tscn currently uses shorter waves for stress testing the growth path

Remaining Validation / Risks

  • Live gameplay validation is still required for melee feel and the player-centered candidate radius tuning
  • DetonationOut and DeathOut remain capped event buffers; current design assumes bomb/chain fan-out stays modest
  • Full per-ball flags are still read back each frame for alive/armed reconciliation, so readback cost is reduced but not eliminated
  • Old CPU ball_system.gd has not been removed yet

Test Plan

  • Headless run exits 0 with only the expected no RenderingDevice warning
  • In-game run: balls spawn, bounce, explode, and dynamic growth works past the original 512-ball ceiling
  • Validate melee behavior with the player-centered candidate capture path
  • Validate bomb proximity detonation and chain reactions under stress
  • Validate shard drops and explosion effects during heavy resize/load
  • Confirm armed/bombed balls still render on top after multiple growth events
## Summary - Adds `GpuBallSystem` (`systems/gpu_ball_system.gd`) replacing the GDScript `BallSystem` with a Vulkan compute shader pipeline while keeping the same public API (`spawn_ball`, `apply_explosion`, `get_alive_count`) - `systems/ball_sim_v2.glsl`: compute shader with 13 bindings (render texture, ball state, explosions, params, rocks, detonation/contact/death outputs, armed list, type config, compact state flags, player-near candidate snapshots). Handles physics, wall/rock bounce, speed cap, bomb skip/bounce, chain fuse countdown, explosion kill/arm, bomb proximity detonation, contact damage detection, and GPU-side color from hp/flags/type config - `systems/ball_render_v2.gdshader`: canvas item shader reading ball pos/color from GPU texture via `INSTANCE_CUSTOM.r` ball index - GPU -> CPU event readback: `DetonationOut` (bomb/chain fuse -> explosion effects + new explosion events), `ContactOut` (ball-player overlap -> damage), `DeathOut` (explosion kills -> shard spawns) - CPU-side melee detection remains authoritative, but the old full-ball shadow readback was replaced with a player-centered GPU candidate pass: CPU now reads compact per-ball flags for reconciliation and detailed snapshots only for nearby balls used by melee checks - `GpuBallSystem` capacity now grows dynamically instead of dropping spawns when the initial 512 slots are exhausted - Resize path updated so render-texture rebinding happens before the old texture RID is freed - `test_scene_1.tscn`, `wave_spawner.gd`, `player.gd` updated to use `GpuBallSystem` - Old `ball_system.gd` (CPU) kept intact as reference ## Since PR Opened - Removed the hard 512-ball runtime ceiling by adding dynamic GPU buffer / render resource growth - Reduced CPU readback cost by replacing full SSBO shadow sync with player-near candidate snapshots - Fixed an invalid RID error that occurred when GPU resources resized during gameplay - `test_scene_1.tscn` currently uses shorter waves for stress testing the growth path ## Remaining Validation / Risks - Live gameplay validation is still required for melee feel and the player-centered candidate radius tuning - `DetonationOut` and `DeathOut` remain capped event buffers; current design assumes bomb/chain fan-out stays modest - Full per-ball flags are still read back each frame for alive/armed reconciliation, so readback cost is reduced but not eliminated - Old CPU `ball_system.gd` has not been removed yet ## Test Plan - [x] Headless run exits 0 with only the expected `no RenderingDevice` warning - [x] In-game run: balls spawn, bounce, explode, and dynamic growth works past the original 512-ball ceiling - [ ] Validate melee behavior with the player-centered candidate capture path - [ ] Validate bomb proximity detonation and chain reactions under stress - [ ] Validate shard drops and explosion effects during heavy resize/load - [ ] Confirm armed/bombed balls still render on top after multiple growth events
Proof-of-concept for rendering large ball counts via GPU pipeline:
- ball_sim.glsl: compute shader animates 2000 balls with Lissajous
  motion, writes pos/radius/alive and color into a 2×N RGBA32F texture
  via imageStore
- ball_render.gdshader: canvas_item shader reads ball_data texture via
  texelFetch(ball_data, ivec2(0/1, INSTANCE_ID), 0), overrides VERTEX
  position and COLOR in the vertex stage, clips to circle in fragment
- gpu_poc.gd: RenderingDevice setup (compute pipeline, Texture2DRD
  bridge, params SSBO, MultiMesh with identity transforms), per-frame
  buffer_update + compute dispatch + barrier(); gracefully skips setup
  when _rd == null (headless)
- gpu_poc.tscn: minimal standalone scene

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
- Remove #[compute] pragma from ball_sim.glsl — only valid when loaded
  via Godot's import system (RDShaderFile), not raw FileAccess text reads
- Replace early return in ball_render.gdshader vertex() with if/else —
  canvas_item vertex functions do not support return statements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
- canvas_item shaders write COLOR directly; ALBEDO/ALPHA are spatial-only
- _rd.barrier() is deprecated in Godot 4.6, RenderingDevice adds barriers
  automatically between compute and rendering passes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Replaces the Lissajous animation POC with a real simulation:

Compute shader (ball_sim.glsl):
- Reads/writes a flat ball state SSBO (pos, vel, radius, alive per ball)
- Each frame: checks explosion events, integrates velocity, bounces off
  arena walls, writes updated state back + render texture
- Colors balls by velocity direction so motion is visible

GDScript (gpu_poc.gd):
- Initializes BALL_COUNT (10000) balls at random positions/velocities
- Ball state SSBO, explosion events SSBO (max 16 per frame), params SSBO
- Timer fires a random explosion every 1.5s; events are uploaded and
  cleared each frame so the SSBO slot is reused
- BALL_COUNT const at the top to tune for benchmarking

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Replace 2×BALL_COUNT texture (exceeded device max ~8192) with a
TEX_ROW_SIZE×H texture where H = 2*ceil(BALL_COUNT/TEX_ROW_SIZE):

  ball idx → col = idx % 4096,  block = idx / 4096
  row block*2+0 : pos.x, pos.y, radius, alive
  row block*2+1 : color rgba

At 10k balls: 4096×6. At 500k balls: 4096×246.
Updated imageStore in ball_sim.glsl, texelFetch in ball_render.gdshader,
and texture_create dimensions in gpu_poc.gd.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Compute shader (ball_sim.glsl):
- Two new SSBOs: RockData (center.x, center.y, radius per rock) and
  RockParams (count)
- Rock bounce: push-out overlap correction + velocity reflection off
  surface normal; only applies when dot(v, normal) < 0 to avoid sticking
- Color by ball type (inferred from radius match to 4 type radii) using
  the game's actual healthy_color values: white / light-blue / purple /
  salmon for standard / small / big / chain

GDScript (gpu_poc.gd):
- BALL_TYPES const with radius, speed range, and spawn weight per type;
  weighted-random assignment at init
- 8 rocks placed in a ring, sizes cycling 1x/1.75x/2.5x base radius 64
- Rock SSBOs initialized once and never updated (static obstacles)
- _draw_rocks(): fan-triangulated MeshInstance2D circles at rock positions
  so they're visible; z_index=-1 so balls render on top

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
CPU arms random balls; GPU tracks fuse and reports explosions back:

Compute shader (ball_sim.glsl):
- Ball state extended to 7 floats (adds fuse_time at index 6)
- New DetonationOut SSBO at binding 6: int count (atomic) + vec4 events[]
- When fuse_time expires: atomicAdd claims a slot, writes blast center +
  radius (br*2+40), marks ball dead
- Armed balls (fuse_time > 0 at frame start) render red

GDScript (gpu_poc.gd):
- BALL_STRIDE 6 → 7; ball init sets fuse_time = 0
- _arm_random_ball(): patches only the 4-byte fuse_time field of a random
  ball via buffer_update at the exact SSBO byte offset — no full re-upload
- _process(): reads DetonationOut via buffer_get_data (GPU stall / readback
  cost measurement), parses count + events, converts to _pending_explosions
  for the next frame's explosion SSBO upload, then resets count to 0 before
  the next dispatch
- Explosion events SSBO is now driven entirely by detonation readback
  (no more CPU-side random position explosions)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Add use_custom_data=true to the MultiMesh. Each instance stores its ball
index in INSTANCE_CUSTOM.r so the vertex shader can look up the correct
texture row independently of instance slot order.

CPU maintains two parallel PackedInt32Arrays (_instance_to_ball,
_ball_to_instance). When a ball is armed, it is swapped into the last
available normal slot (_armed_start-1 and decrements _armed_start).
Armed instances occupy [_armed_start, BALL_COUNT), which renders on top
because MultiMesh uses painter's order (later = in front).

_arm_random_ball now picks ball_idx from _instance_to_ball[randi()%_armed_start]
so it always picks from the normal (non-armed) region, then does the swap
with 2 set_instance_custom_data calls and updates both index arrays.

Vertex shader: replaces INSTANCE_ID lookup with int(round(INSTANCE_CUSTOM.r)).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Fuse countdown disabled — armed balls accumulate indefinitely.
Each armed ball checks against the CPU-uploaded armed list for overlap;
if any two armed balls touch, both detonate via atomicAdd.

Compute shader (ball_sim.glsl):
- New ArmedList SSBO at binding 7: float[0]=count, float[1..N]=ball indices
- After physics, each armed ball iterates the armed list, skips self and
  dead balls, checks circle-circle overlap; on hit calls detonate() which
  atomicAdds a det slot and writes (px,py,blast_r,ball_idx) as vec4
- detonate() extracted to a helper function for clarity
- Fuse countdown block present but commented out in spirit (replaced by
  intersection check as the sole detonation trigger)

GDScript (gpu_poc.gd):
- MAX_ARMED = 256; _armed_list_buffer created at startup
- _upload_armed_list(): builds [count, idx0..N] from _instance_to_ball
  [_armed_start..BALL_COUNT) and uploads to SSBO each frame (tiny buffer)
- _arm_random_ball(): sets fuse_time=1.0 (armed marker, no countdown)
- det event .w now carries ball_idx; _remove_from_armed_region() swaps
  the detonated ball back into the normal region and increments _armed_start
- Armed list buffer freed in _exit_tree()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds GpuBallSystem which replaces systems/ball_system.gd with a
Vulkan compute shader pipeline while preserving the same public API
(spawn_ball, apply_explosion, get_alive_count).

GPU handles: physics integration, wall/rock bounce, speed cap,
bomb skip/bounce counters, chain fuse countdown, explosion kill/arm
(including can_bomb arming), bomb proximity detonation, contact damage
detection, and per-ball color from hp/flags/type config.

CPU handles: melee detection (1-frame lag shadow positions), melee
outcomes (velocity redirect, hp damage, arm/split/kill), GPU→CPU
event readback (detonations, contacts, deaths), shard/explosion pools,
slot management, armed region z-ordering.

Key files:
- systems/ball_sim_v2.glsl: compute shader with 12-float ball state,
  11 bindings (render tex, ball SSBO, explosions, params, rocks×2,
  detonation/contact/death outputs, armed list, type config)
- systems/ball_render_v2.gdshader: canvas shader reading GPU texture
  via INSTANCE_CUSTOM.r ball index for pos/color
- systems/gpu_ball_system.gd: GpuBallSystem with full shadow arrays
  (pos/vel/flags/hp/type/contact_cd), slot management, reconciliation
- test_scene_1.tscn: switched to gpu_ball_system.gd script
- wave_spawner.gd, player.gd: updated type annotations to GpuBallSystem

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
codex force-pushed feature/gpu-compute-poc from 10c746dac5 to 9bd88518e6 2026-03-06 15:47:57 -06:00 Compare
cammymoop merged commit b858a2faa1 into master 2026-03-06 15:48:36 -06:00
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