GAN Watercolor (Helena Sarin — Neural Bricolage / #latentDoodles)

Math essence

Stroke is a densely-sampled curved path through 3-5 control points; rasterised as a Gaussian-thickness mask onto a float canvas; composited with alpha α. Vertical bleed = 1-D Gaussian convolution along y (kernel σ). Global fade: canvas ← canvas · γ + (1-γ)·paper. Palette is a K-row RGB table (3 fixed Sarin palettes or a low-frequency HSV random walk).

Parameters

Reference

First 60 lines of reference.py. Full source: library_external/ml_artists/SarinWatercolor/reference.py

"""SarinWatercolor — Helena Sarin "GAN + watercolor" kernel.

Helena Sarin works at the seam of GANs and analog watercolor: she
trains generative models on her own watercolor paintings and sketches
("#latentDoodles", *An Eternal Tangle of What Ifs*, *GAN Weaver*),
then curates the output back into physical paintings.  Her aesthetic
is a *low-frequency palette with watercolor bleed*: a small handful
of colours per piece, layered translucently, with the soft wet-blur
edges that come from pigment diffusion in paper.

We replicate the algorithmic kernel:
  1. Maintain a small fixed palette of K colours (Sarin-style: 5-7).
  2. At each step, pick a palette colour and stroke shape (a
     parametric cubic spline or short curve segment with a few
     control points).
  3. Paint the stroke into a float canvas with low alpha so layers
     accumulate translucently.
  4. Apply a vertical "wet bleed" — a 1D Gaussian blur along the
     y-axis — *after* each stroke, mimicking watercolor diffusion
     through paper grain.
  5. Slowly fade the whole canvas so newer strokes dominate while
     older ones remain as ghostly underlayers.

References:
- https://www.artnome.com/news/2018/11/14/helena-sarin-why-bigger-isnt-always-better-with-gans-and-ai-art
- https://www.nvidia.com/en-us/research/ai-art-gallery/artists/helena-sarin/
- https://aiartists.org/helena-sarin
"""
from __future__ import annotations

from typing import ClassVar

import numpy as np


class SarinWatercolor:
    """Layered watercolor strokes sampled from a small Sarin-style palette.

    Parameters
    ----------
    dim : int
        Square canvas side length (px).
    n_palette : int
        Number of palette colours K (Sarin typically uses 5-7).
    palette_seed : int
        Seed used to draw the synthetic palette from a low-frequency
        random walk.
    strokes_per_step : int
        Strokes drawn per ``step`` call.
    alpha : float
        Per-stroke opacity (kept low for true watercolor layering).
    bleed_sigma : float
        Vertical bleed Gaussian sigma (px).  0 disables bleed.
    fade : float
        Per-step multiplicative canvas fade (close to 1.0 keeps old
        strokes; lower lets newer ones dominate).
    palette_name : str
        ``"sarin_warm"``, ``"sarin_cool"``, ``"sarin_folk"`` or
        ``"random"`` for a generated palette.
    """