Bramble Circle in Grease Pencil

circ_grease_pencil.py

import bpy
import math
from mathutils import Vector

grease_data = bpy.data.grease_pencils.new("Stroke")
grease_layers = grease_data.layers
active_layer = grease_layers.new("Layer")
grease_frames = active_layer.frames
active_frame = grease_frames.new(
    bpy.context.scene.frame_current,
    active=True)
frame_strokes = active_frame.strokes

arc_count = 24
point_count = 64

i_range = range(0, arc_count, 1)
i_to_theta = math.tau / arc_count

j_range = range(0, point_count, 1)
j_to_theta = math.tau / point_count
j_to_fac = 1.0 / (point_count - 1.0)

theta_offset = math.tau / 3.0
orbit_radius = 1.0
minor_radius = 0.5

min_pressure = 0.5
max_pressure = 5.0

for i in i_range:
    i_theta = i * i_to_theta

    active_stroke = frame_strokes.new()
    active_stroke.line_width = 12.0
    active_stroke.hardness = 1.0
    active_stroke.start_cap_mode = 'FLAT'
    active_stroke.end_cap_mode = 'FLAT'

    stroke_points = active_stroke.points
    stroke_points.add(point_count)

    orbit = orbit_radius
    if i % 2 != 0:
        orbit = orbit_radius * 0.875

    origin = Vector(
        (orbit * math.cos(i_theta),
         orbit * math.sin(i_theta),
         0.0))

    j_theta_origin = math.pi + (i_theta - theta_offset)
    j_theta_dest = math.pi + (i_theta + theta_offset)

    for j in j_range:
        j_fac = j * j_to_fac
        j_theta = (1.0 - j_fac) * j_theta_origin \
            + j_fac * j_theta_dest

        point = stroke_points[j]

        osc = 0.5 + 0.5 * math.cos(math.tau * j_fac - math.pi)
        point.pressure = (1.0 - osc) * min_pressure \
            + osc * max_pressure

        point_local = Vector(
            (minor_radius * math.cos(j_theta),
             minor_radius * math.sin(j_theta),
             0.0))
        point.co = origin + point_local


grease_obj = bpy.data.objects.new(grease_data.name, grease_data)
bpy.context.collection.objects.link(grease_obj)