skills/shader-dev/reference/camera-effects.md

Camera Effects Detailed Reference

Prerequisites

• Ray marching fundamentals (ray origin, ray direction)
• Multipass buffers (for accumulation-based DoF)
• Hash functions for stochastic sampling

Thin Lens Model Derivation

A real camera lens focuses light from a focal plane onto the sensor. Points not on the focal plane project to a circle of confusion (CoC) on the sensor.

Circle of Confusion Formula

CoC = |S2 - S1| × A × f / (S1 × (S2 - f))

Where:

• S1 = focal distance (distance to in-focus plane)
• S2 = object distance
• A = aperture diameter
• f = focal length

Simplified for Shaders

CoC ≈ apertureSize × |depth - focalDistance| / depth

Ray-Based Implementation

Instead of computing CoC per pixel, we model the physical process:

1. Choose a random point on the aperture disk → new ray origin
2. The focal point (where the original ray hits the focal plane) stays fixed
3. New ray direction = normalize(focalPoint - newOrigin)
4. Average many such samples → natural bokeh with correct occlusion

Aperture Shape

• Circular: vec2 p = sqrt(r) * vec2(cos(a), sin(a)) — uniform disk
• Polygonal: reject samples outside polygon for hexagonal/octagonal bokeh
• The sqrt(r) is critical for uniform distribution (area-preserving)

Poisson Disk Sampling

Pre-computed 16-point Poisson disk for blur kernels:

const vec2 poissonDisk[16] = vec2[](
    vec2(-0.94201624, -0.39906216), vec2(0.94558609, -0.76890725),
    vec2(-0.09418410, -0.92938870), vec2(0.34495938,  0.29387760),
    vec2(-0.91588581,  0.45771432), vec2(-0.81544232, -0.87912464),
    vec2(-0.38277543,  0.27676845), vec2(0.97484398,  0.75648379),
    vec2(0.44323325, -0.97511554),  vec2(0.53742981, -0.47373420),
    vec2(-0.26496911, -0.41893023), vec2(0.79197514,  0.19090188),
    vec2(-0.24188840,  0.99706507), vec2(-0.81409955,  0.91437590),
    vec2(0.19984126,  0.78641367),  vec2(0.14383161, -0.14100790)
);

Advantages over regular grid: no structured aliasing patterns, better coverage per sample count.

Motion Blur Approaches

Stochastic Time Sampling (Ray Marching)

For each pixel, pick a random time within the shutter interval:

t_sample = iTime + (rand - 0.5) * shutterDuration

Use t_sample for all scene animation. Accumulate multiple frames for convergence.

Velocity Buffer (Post-Process)

1. Render scene + store per-pixel velocity vectors
2. For each pixel, sample along the velocity direction
3. Weight samples by distance from center (triangle filter)

Hybrid

Use temporal accumulation (TAA-style) with per-frame time jitter — converges over frames with no per-frame cost increase.

Film Grain Characteristics

Real film grain properties:

• Luminance-dependent: More visible in shadows, less in highlights
• Temporally varying: Different pattern each frame (use fract(iTime) in hash seed)
• Spatially uncorrelated: Use pixel coordinates in hash, not UV (grain should be screen-resolution)
• Intensity: 0.02-0.05 for subtle, 0.1+ for stylized/vintage look