7edf5bc4d0
Major updates: - Added 35+ new skills from awesome-opencode-skills and antigravity repos - Merged SEO skills into seo-master - Merged architecture skills into architecture - Merged security skills into security-auditor and security-coder - Merged testing skills into testing-master and testing-patterns - Merged pentesting skills into pentesting - Renamed website-creator to thai-frontend-dev - Replaced skill-creator with github version - Removed Chutes references (use MiniMax API instead) - Added install-openclaw-skills.sh for cross-platform installation - Updated .env.example with MiniMax API credentials
72 lines
2.9 KiB
Markdown
72 lines
2.9 KiB
Markdown
# Anti-Aliasing Detailed Reference
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## Prerequisites
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- Understanding of screen-space derivatives (`dFdx`, `dFdy`, `fwidth`)
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- Multipass buffer setup (for TAA)
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- Basic signal processing concepts
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## Sampling Theory (Nyquist)
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The **Nyquist-Shannon theorem** states: to accurately represent a signal, sampling rate must be ≥ 2× the highest frequency present. In shader terms:
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- Pixel grid = sampling rate
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- Procedural detail / edge sharpness = signal frequency
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- When detail frequency > pixel frequency → aliasing (moiré, crawling edges)
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**Solutions**: either increase sampling rate (SSAA) or reduce signal frequency (analytical AA, filtering).
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## SSAA Implementation Details
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### Jitter Patterns
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- **Grid**: `offset = vec2(m, n) / AA - 0.5` — simple, uniform coverage
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- **Rotated grid (RGSS)**: 4 samples at rotated positions — better edge coverage for near-horizontal/vertical lines
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- **Halton sequence**: quasi-random low-discrepancy — best coverage for high sample counts
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### Performance
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AA=2 (4 samples) is the practical limit for real-time SDF scenes. AA=3 (9 samples) for offline/screenshot quality only.
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## SDF Analytical AA Deep Dive
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### Why `fwidth` Works
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`fwidth(d) = abs(dFdx(d)) + abs(dFdy(d))` approximates how much the SDF value changes across one pixel. Using this as the smoothstep width:
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- Edge transition spans exactly ~1 pixel regardless of zoom level
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- No texture sampling needed — purely analytical
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- Works for any SDF shape
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### Signed Distance to Coverage
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For a 2D SDF with value `d` at a pixel center:
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```
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coverage ≈ clamp(0.5 - d / fwidth(d), 0.0, 1.0)
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```
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This maps the signed distance to an approximate pixel coverage, equivalent to a box filter over the pixel footprint.
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## TAA with Neighborhood Clamping
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Full TAA pipeline:
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1. **Jitter**: offset pixel center by Halton(2,3) sequence each frame
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2. **Render**: full scene at jittered position → Buffer A
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3. **Reproject**: use motion vectors to find previous frame's pixel for current position
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4. **Clamp**: restrict history color to the min/max of current frame's 3×3 neighborhood (prevents ghosting)
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5. **Blend**: `output = mix(current, clampedHistory, 0.9)`
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### Neighborhood Clamping
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```glsl
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vec3 minCol = vec3(1e10), maxCol = vec3(-1e10);
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for (int x = -1; x <= 1; x++)
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for (int y = -1; y <= 1; y++) {
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vec3 s = texelFetch(currentBuffer, ivec2(fragCoord) + ivec2(x,y), 0).rgb;
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minCol = min(minCol, s);
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maxCol = max(maxCol, s);
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}
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vec3 clampedHistory = clamp(history, minCol, maxCol);
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```
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## FXAA Algorithm Walkthrough
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1. **Luma computation**: Convert 5 samples (center + NSEW) to luminance
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2. **Edge detection**: `lumaRange = lumaMax - lumaMin` — skip if below threshold
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3. **Edge orientation**: Compare horizontal vs vertical luma gradients to determine edge direction
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4. **Sub-pixel blending**: Sample along the edge direction at 1/3 and 2/3 offsets
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5. **Quality**: The simplified version uses 2 taps; full FXAA 3.11 uses up to 12 taps along the edge for better endpoint detection
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