Neon Trail Rendering¶

The neon trail system renders camera-facing ribbon trails behind each N-body sphere with a physically-inspired neon tube glow profile. Trails emit HDR light that interacts with the bloom post-process to create wide, colorful neon halos.

Visual Profile¶

The fragment shader (shaders/trail.frag) simulates a neon tube with three concentric glow layers:

Layer	Exponent	Weight	Visual Role
Core	`u_core_exp` (default 12.0)	45%	Tight white-hot filament
Inner	`core_exp × 0.25`	35%	Medium saturated color glow
Outer	0.8 (fixed)	20%	Soft diffuse halo for bloom

White-Hot Center¶

Real neon tubes appear near-white at their brightest point. The shader desaturates the center by mixing towards vec3(peak) (luminance-preserving white) proportionally to the core intensity:

vec3 neon_color = mix(vColor, hot_white, core * 0.7);
neon_color *= (1.0 + core * 2.0);  // HDR boost for bloom

HDR / Bloom Interaction¶

The CPU-side build_ribbon() function pre-multiplies colors by the HDR intensity parameter. Combined with the shader's core boost, the center of the ribbon reaches values well above the bloom threshold (default 1.0), triggering a wide bloom halo that forms the characteristic neon glow.

Runtime Controls¶

All three neon parameters are adjustable at runtime via keyboard:

Key	Action
`I`	Cycle active parameter: Intensity → Core → Width
`Shift+I`	Increase the selected parameter
`Ctrl+I`	Decrease the selected parameter

Parameters¶

Parameter	Default	Step	Min	Description
Intensity	5.0	±0.5	0.5	HDR intensity multiplier — controls overall brightness and bloom response
Core	12.0	±2.0	2.0	Core tightness exponent — higher = thinner bright filament, lower = diffuse glow
Width	0.24	±0.02	0.04	Ribbon half-width in world units — physical thickness of the trail

Each change displays an on-screen notification with the current value.

Architecture¶

graph TD
    A[TrailRenderer::neon] -->|intensity, width| B[build_ribbon CPU]
    A -->|core_exp| C[trail.frag GPU]
    B -->|HDR vertices| D[VBO Upload]
    D --> E[glMultiDrawArrays]
    C -->|Neon profile| E
    E -->|HDR output| F[Bloom Post-Process]
    F -->|Wide halo| G[Final Composite]

Key Files¶

File	Role
`shaders/trail.frag`	Neon glow profile (3-layer Gaussian, white-hot core)
`shaders/trail.vert`	Camera-facing ribbon vertex transform
`include/trail_renderer.h`	`TrailNeonParams` struct + defaults
`src/trail_renderer.c`	Ribbon geometry builder + uniform upload
`src/app_input.c`	`I` / `Shift+I` / `Ctrl+I` keyboard handlers
`src/app_binding.c`	F2 help overlay registration

Tuning Guide¶

More glow, less sharp center: Decrease Core (e.g., 4.0–6.0)
Laser-thin bright line: Increase Core (e.g., 20.0+)
Stronger bloom halo: Increase Intensity (e.g., 8.0–10.0)
Subtle trails: Decrease Intensity (e.g., 1.0–2.0) and Width (0.08)
Fat neon tubes: Increase Width (e.g., 0.40+)