Anti-Aliasing

Anti-Aliasing

Aliasing occurs when continuous signals are sampled at discrete intervals, causing jagged edges (jaggies), moiré patterns, and temporal flickering. Anti-aliasing techniques reduce these artifacts.

Supersampling (SSAA)

Renders the scene at a higher resolution then downsamples. The simplest but most expensive anti-aliasing method.

Supersampling 4x:

  Render at 4x resolution:
  For each pixel, compute 4 sub-pixels

  +---+---+
  | 0 | 1 |   4 samples per pixel
  +---+---+
  | 2 | 3 |
  +---+---+

  pixel_color = (c0 + c1 + c2 + c3) / 4

  4x SSAA: 4x rendering cost
  16x SSAA: 16x rendering cost

Multisampling (MSAA)

Like supersampling but shares computation within each triangle. Only samples that fall inside the triangle contribute. More efficient than full SSAA.

MSAA with 4 samples:

  Sample positions within pixel:
  +-------+-------+
  |   s0  |  s1   |
  +-------+-------+
  |   s2  |  s3   |
  +-------+-------+

  For each triangle covering the pixel:
    Count how many samples fall inside
    Shade only once per triangle
    Blend based on coverage

  Coverage = samples_inside / total_samples
  pixel = coverage * triangle_color + (1-coverage) * background

Post-Process Anti-Aliasing

Techniques like FXAA and SMAA work on the final rendered image. They are fast but may blur textures or miss thin features.

FXAA (Fast Approximate AA):
  1. Detect edge pixels based on luminance
  2. Determine edge direction
  3. Blend pixel with neighbors along edge
  
  Very fast, works on any hardware
  May blur fine details

SMAA (Subpixel Morphological AA):
  1. Edge detection
  2. Pattern recognition
  3. Blending
  
  Better quality than FXAA, moderate cost

Temporal Anti-Aliasing (TAA)

Uses information from previous frames to increase effective sample count. Each frame uses a different sample offset, and results are accumulated over time.

TAA Process:
  1. Jitter sample position each frame
     (using Halton sequence or similar)
  2. Render with single sample at jittered position
  3. Combine current frame with accumulated history
  4. Apply motion blur correction

  Over N frames: effectively N samples per pixel
  Cost: ~1x + history buffer + blending

  Pros: High quality, low per-frame cost
  Cons: Ghosting artifacts on fast motion