Initial commit: add all skills files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shader-dev/reference/sdf-tricks.md

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+# SDF Tricks Detailed Reference
+
+## Prerequisites
+- Understanding of signed distance fields and ray marching
+- Basic SDF primitives and boolean operations
+- FBM / procedural noise fundamentals
+
+## Lipschitz Condition and FBM Detail
+
+An SDF must satisfy the **Lipschitz condition**: `|f(a) - f(b)| ≤ |a - b|` (gradient magnitude ≤ 1). This guarantees that stepping by the SDF value is always safe — no surface exists within that radius.
+
+When adding FBM noise to an SDF, the noise derivatives can violate Lipschitz:
+- Raw noise amplitude of 0.1 with frequency 20 has gradient ~2.0, breaking the condition
+- This causes ray marching to overshoot, creating holes and artifacts
+
+**Solutions**:
+1. **Amplitude limiting**: Keep `amplitude × frequency < 1.0` across all octaves
+2. **Distance fade**: `d += amp * fbm(p * freq) * smoothstep(fadeStart, 0.0, d)` — detail only appears near the surface where overshoot distance is small
+3. **Step size reduction**: Multiply ray step by 0.5-0.7, trading speed for stability
+
+## Bounding Volume Strategies
+
+### Hierarchical Bounding
+For scenes with N objects, test bounding volumes in order of increasing cost:
+```
+Level 1: Scene bounding sphere (1 evaluation)
+Level 2: Object group bounds (few evaluations)
+Level 3: Individual object SDF (full cost)
+```
+
+### Spatial Partitioning
+For repeating structures, combine domain repetition with bounds:
+```glsl
+float map(vec3 p) {
+    vec3 q = mod(p + 2.0, 4.0) - 2.0;  // repeat every 4 units
+    // Only evaluate detail if within local bounding sphere
+    float bound = length(q) - 1.5;
+    if (bound > 0.2) return bound;
+    return detailedSDF(q);
+}
+```
+
+## Binary Search Convergence
+
+After N iterations of binary search, the position error is `initialStep / 2^N`:
+- 4 iterations: 1/16 of initial step size
+- 6 iterations: 1/64 of initial step size (sub-pixel at typical resolutions)
+- 8 iterations: 1/256 (overkill for most uses)
+
+6 iterations is the practical sweet spot — gives sub-pixel precision without wasting GPU cycles.
+
+## XOR Operation Mathematics
+
+`opXor(a, b) = max(min(a, b), -max(a, b))`
+
+This is equivalent to: `union(a, b) AND NOT intersection(a, b)` — the symmetric difference. Geometry exists where exactly one shape is present but not both. Useful for creating lattice structures and interlocking patterns.
+
+## Interior SDF Pattern Techniques
+
+When the camera is inside an SDF (d < 0), the negative distance still gives useful information:
+- `abs(d)` gives distance to nearest surface from inside
+- Combine with repeating patterns using `fract()` to create infinite interior structures
+- Use `max(outerSDF, innerSDF)` to confine interior patterns within the outer shell