rLIC

Line Integral Convolution for Python, written in Rust

rLIC (pronounced 'relic') is a highly optimized, minimal implementation of the Line Integral Convolution algorithm for in-memory numpy arrays, written in Rust.

Development status and scope

rLIC is currently in beta. As of version 0.5.0, the only public API, rlic.convolve, is considered feature complete and stable. However, minor behavior changes may still happen, particularly where performance can be improved as a result.

Post-processing is out-of-scope for this library. For histogram equalization, an operation very commonly applied to LIC images, please see rLIC's sister project ahe, which follows the same minimalist philosophy and originated within rLIC's code base.

Free-threading support

rlic.convolve is trivially thread-safe, because it does not mutate any external data. As of version 0.5.1, Wheels are not yet distributed for free-threaded CPython, but this build target is still supported and tested.

Installation

python -m pip install rLIC

Examples

rLIC consists in a single Python function, rlic.convolve, that convolves a texture image (usually noise) with a 2D vector field described by its components u and v, via a 1D kernel array. The result is an image where pixel intensity is strongly correlated along field lines.

Let's see an example. We'll use matplotlib to visualize inputs and outputs.

import matplotlib.pyplot as plt
import numpy as np

import rlic

SHAPE = NX, NY = (256, 256)
prng = np.random.default_rng(0)

texture = prng.random(SHAPE)
x = np.linspace(0, np.pi, NY)
U = np.broadcast_to(np.cos(2 * x), SHAPE)
V = np.broadcast_to(np.sin(x).T, SHAPE)

fig, axs = plt.subplots(ncols=2, sharex=True, sharey=True, figsize=(10, 5))
for ax in axs:
    ax.set(aspect="equal", xticks=[], yticks=[])

ax = axs[0]
ax.set_title("Input texture (noise)")
ax.imshow(texture)

ax = axs[1]
ax.set_title("Input vector field")
Y, X = np.mgrid[0:NY, 0:NX]
ax.streamplot(X, Y, U, V)

Now let's compute some convolutions, varying the number of iterations

kernel = 1 - np.abs(np.linspace(-1, 1, 65))

fig_out, axs_out = plt.subplots(ncols=3, figsize=(15, 5))
for ax in axs_out:
    ax.set(aspect="equal", xticks=[], yticks=[])
for n, ax in zip((1, 5, 100), axs_out, strict=True):
    image = rlic.convolve(
        texture,
        U,
        V,
        kernel=kernel,
        boundaries="periodic",
        iterations=n,
    )
    ax.set_title(f"Convolution result ({n} iteration(s))")
    ax.imshow(image)

Polarization mode

By default, the direction of the vector field affects the result. That is, the sign of each component matters. Such a vector field is analogous to a velocity field. However, the sign of u or v may sometimes be irrelevant, and only their absolute directions should be taken into account. Such a vector field is analogous to a polarization field. rLIC supports this use case via an additional keyword argument, uv_mode, which can be either 'velocity' (default), or 'polarization'. In practice, the difference between these two modes in only visible around sharps changes in sign in either u or v, and with certain kernels. Let's illustrate one such case

import matplotlib.pyplot as plt
import numpy as np

import rlic

SHAPE = NX, NY = (256, 256)
prng = np.random.default_rng(0)

texture = prng.random(SHAPE)
kernel = 1 - np.abs(np.linspace(-1, 1, 65, dtype="float64"))

U0 = np.ones(SHAPE)
ii = np.broadcast_to(np.arange(NX), SHAPE)
U = np.where(ii < NX / 2, -U0, U0)
V = np.zeros((NX, NX))

fig, axs = plt.subplots(ncols=3, sharex=True, sharey=True, figsize=(15, 5))
for ax in axs:
    ax.set(aspect="equal", xticks=[], yticks=[])

ax = axs[0]
ax.set_title("Input vector field")
Y, X = np.mgrid[0:NY, 0:NX]
ax.streamplot(X, Y, U, V)

for uv_mode, ax in zip(("velocity", "polarization"), axs[1:], strict=True):
    image = rlic.convolve(
        texture,
        U,
        V,
        kernel=kernel,
        uv_mode=uv_mode,
        boundaries={"x": "periodic", "y": "closed"},
    )
    ax.set_title(f"{uv_mode=!r}")
    ax.imshow(image)

Memory Usage

rLIC.convolve allocates exactly two buffers with the same size as texture, u and v, regardless of the number of iterations performed, one of which is discarded when the function returns. This means that peak usage is about 5/3 of the amount needed to hold input data in memory, and usage drops to 4/3 on return.