Root cause of the "zebra stripe" / radial spoke artifact: disk_color_volumetric
sampled noise using the raw Cartesian pos (x,y,z). On a disk, changing the
radius r moves x/z a lot (high-frequency oscillation → stripes), while a full
angular sweep revisits correlated lattice points (weak variation), so the
stripes elongate along radius into spokes. Verified numerically: the radial
sample line oscillated filament in [0.05, 0.77] while the angular line was
symmetric (phi=0 and phi=pi identical at 0.2475).
Fix: sample noise in polar space sp = (r/inner, phi*2.5 + rot, h/half_thickness).
This decouples radial and angular axes so turbulence flows tangentially — the
physically correct pattern for a rotating fluid. Post-fix radial/angular
variance ratio is 0.50 (radial smoother than angular), eliminating the spoke
bias.
Two secondary fixes in the same function:
- Density: replaced the hard smoothstep(0.3,0.7) cut (which made the slab
patchy and over-transparent) with a soft 0.35+0.65*noise ramp.
- Brightness: raised the floor to 0.5+filament (was filament alone) so the
disk reads as luminous plasma instead of dim brown.
- Guarded log(r) with max(r,0.1) for robustness near the inner edge.
xfy
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