Pure Rain Smooth – Rain Filter on Meshes

Abstract
Pure Rain Smooth is a stochastic mesh–smoothing add‑on for Blender. It applies noise droplets always pushing inward—never zero, never outward—exactly as prescribed by the Rain Filter. Curvature‐conditioned magnitude and antifase direction yield an emergent "low‑pass" - like effect without explicit convolutions.

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Introduction

The Rain Filter proposes a smoothing approach in which random noise acts as an emergent filter. Unlike deterministic methods—e.g. classical Laplacian—this model drops stochastic perturbations onto the mesh and applies them in antifase with the vertex normal. The final result is global smoothing, yet the magic lies in the intermediate steps and their controlled randomness.

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Core Doctrine

Principle	Implementation in v1.5.1
Antifase push	Displacement direction is always inverse to the phase (in meshes it is asume as unipolar signal)
Droplets never zero	Every selected vertex has a non‑zero chance (density) each sub‑pass; magnitude random(0,1) ensures some push.
Curvature‑weighted magnitude	logCoeff = log(1+steepness·κ)/log(1+steepness) (κ ≥ 0). Peaks ≈ 1, planes ≈ ½, valleys > 0.
No deterministic kernels	Smoothing emerges from repeated noise impacts—no Laplacian matrix, no explicit diffusion.

Optional

Extra	Purpose
Tangential Passes	Hits along two orthogonal directions for isotropic feel.
Interleaved Tangents	Shuffle order Normal → T1 → T2 within each iteration (acts like a "detangler").
Burst Rain Mode	Timer‑driven loop; click Play for continuous drizzle, Pause to stop.

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Algorithm

1. Local relief  κ = max((v − centroid) · n, 0)
2. Log gain  logCoeff = log(1 + steepness · κ) / log(1 + steepness)
3. Drop magnitude  |Δ| = intensity · rand(0,1) · logCoeff
4. Direction  –n or tangents (optional).
5. Clamp  |Δ| ≤ maxDisp
6. Iterate  Fixed Iterations per click, or endless timer in Burst mode.

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Parameters

UI Control	Range	Core / Extra	Notes
Intensity	0 – 5	Core	Global strength multiplier.
Density	0 – 1	Core	Probability each vertex is hit per sub‑pass.
Iterations	1 – 500	Core	Hidden in Burst mode.
Steepness	0.1 – 10	Core	0.1 ≈ linear, 10 very logarithmic.
Max Disp	0.001 – 1 BU	Core	Safety clamp per droplet.
Tangent Rain	bool	Extra	Add two orthogonals.
Interleaved Tangents	bool	Extra	Cycle N→T1→T2 every loop.
Interval (s)	0.01 – 5	Extra	Timer period in Burst mode.

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Usage

1. Edit Mode → select the vertices you want to tame.
2. Set sliders (Intensity, Density, etc.).
3. • One‑shot: press Rain.
   • Continuous: open Burst Rain Mode, set Interval, press Play (stop with Pause).
4. Optionally enable Tangential Passes or Interleaved for a rounder wash.

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Roadmap

• Weight‑map modulation (paint where it rains harder).
• Audio port for the same concept in DSP.

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Credits

Algorithm & concept — David Rodríguez
El Mar, que fue mi inspiracion y refencia.