Code Notes made by Vaibhav Rathod

PathTracer Learning - Concept - Anti-Aliasing

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Written by Vaibhav Rathod

Concept: Anti-Aliasing

What Is Aliasing?

  • Aliasing: high-frequency signal sampled at too low a rate → artifacts
  • In rendering: sharp edges appear as “jaggies” (staircase pattern)
  • Nyquist theorem: must sample at 2× the highest frequency to avoid aliasing
  • Solution: anti-aliasing — reduce high frequencies before sampling

Supersampling (SSAA)

  • Render at higher resolution, downsample to final resolution
  • 4× SSAA: render at 2× width and height, average 4 pixels → 1
  • Pros: simple, correct
  • Cons: 4× more rays — too expensive for real-time

Jittered Sampling (Stochastic AA)

  • Instead of one ray per pixel center, jitter the ray within the pixel
  • ray_uv = (pixel + vec2(random(), random())) / resolution
  • With N samples per pixel: each sample uses a different jitter
  • Converts aliasing into noise — noise is less objectionable than jaggies
  • This is what path tracing does naturally (each sample is a different ray)

Stratified Sampling

  • Divide pixel into N×N strata, sample one per stratum
  • Better distribution than pure random — avoids clustering
  • For 4 samples: 2×2 grid, one sample per cell
// 4-sample stratified
for (int sy = 0; sy < 2; sy++) {
    for (int sx = 0; sx < 2; sx++) {
        vec2 jitter = (vec2(sx, sy) + vec2(random(), random())) / 2.0;
        vec2 uv = (pixel + jitter) / resolution;
        // trace ray at uv
    }
}

Temporal Anti-Aliasing (TAA)

  • Accumulate jittered samples over multiple frames
  • Each frame: use a different sub-pixel jitter pattern (Halton sequence)
  • Blend with previous frame: output = lerp(prev, current, blend_factor)
  • Effectively: N frames × 1 spp = N spp anti-aliasing
  • Jitter patterns: Halton(2,3) sequence — good low-discrepancy distribution 
    // Halton sequence for frame N
float halton(int i, int base) {
    float f = 1.0, r = 0.0;
    while (i > 0) { f /= base; r += f * (i % base); i /= base; }
    return r;
}
vec2 jitter = vec2(halton(frame, 2), halton(frame, 3));
  • Requires motion vector reprojection (same as temporal accumulation)
  • Ghosting: same issue as temporal accumulation — need rejection

MSAA (Multisample Anti-Aliasing)

  • Hardware rasterization feature — not directly applicable to ray tracing
  • Samples geometry coverage at multiple sub-pixel positions
  • Shades each pixel once but uses coverage information
  • Not useful for path tracing (we already trace multiple rays per pixel)

DLSS / FSR (AI/Spatial Upscaling)

  • Render at lower resolution, upscale with AI or spatial algorithms
  • DLSS (NVIDIA): AI-based, uses temporal data — very high quality
  • FSR (AMD): spatial algorithm, no temporal data — lower quality but universal
  • Both provide anti-aliasing as a side effect of upscaling
  • See PathTracer Learning - DLSS and Denoising

Pixel Reconstruction Filter

  • How to combine multiple samples within a pixel
  • Box filter: simple average — blurry
  • Tent filter: linear falloff — slightly better
  • Gaussian filter: smooth falloff — good balance
  • Mitchell-Netravali: negative lobes — sharper but can ring
  • For path tracing: Gaussian or Mitchell-Netravali are common choices
  • Filter radius: typically 1-2 pixels

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