Created
June 29, 2018 12:03
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Two methods sampling on a sphere:
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#!/usr/bin/python3 | |
# -*- coding: utf-8 -*- | |
# | |
# http://marc-b-reynolds.github.io/math/2018/06/21/SFPoints4ET.html | |
import numpy as np | |
""" | |
// constant turning rate: | |
// TX = cos(2pi K) | |
// TY = sin(2pi K) | |
// K = frac(phi) = 1/phi = (sqrt(5)-1)/2 | |
const double TX = -0.73736887807831985597317725478205829858779907226562; | |
const double TY = -0.67549029426152362720614519275841303169727325439453; | |
typedef struct { | |
double x,y; // incrementally computed point on circle | |
double z,dz; // incrementally computed height on cap | |
} sf_walk_t; | |
// n = number of points to generate | |
// h = height of cap (ex: half-sphere=1, full-sphere=2) | |
void sf_walk_init(sf_walk_t* w, uint32_t n, float h) | |
{ | |
w->x = 1.0; | |
w->y = 0.0; | |
w->z = 1.0; | |
w->dz = h/n; | |
} | |
void sf_walk_next(sf_walk_t* w, float* v) | |
{ | |
double x=w->x, y=w->y; | |
double ct,st; | |
// current disc to cap mapping values | |
ct = w->z; | |
st = sqrt(1-ct*ct); | |
// output current point on cap | |
v[0] = (float)(st*x); | |
v[1] = (float)(st*y); | |
v[2] = (float)(ct); | |
// update point on circle: turn by 2pi*K | |
w->x = TX*x-TY*y; | |
w->y = TY*x+TX*y; | |
// update height in cap position | |
w->z -= w->dz; | |
} | |
""" | |
def Fib(n, r, h=2.0): | |
""" | |
n = number of points to generate | |
h = height of cap (ex: half-sphere=1, full-sphere=2) | |
""" | |
TX = -0.73736887807831985597317725478205829858779907226562 | |
TY = -0.67549029426152362720614519275841303169727325439453 | |
wx, wy, wz, dz = 1.0, 0.0, 1.0, h/n | |
pts = [] | |
for i in range(n): | |
x = wx | |
y = wy | |
ct = wz | |
st = np.sqrt(1 - ct*ct) | |
nx = st * x | |
ny = st * y | |
nz = ct | |
wx = TX*x - TY*y | |
wy = TY*x + TX*y | |
wz -= dz | |
pts.append((nx, ny, nz)) | |
return pts | |
# https://www.cmu.edu/biolphys/deserno/pdf/sphere_equi.pdf | |
def Uniform(N, r = 1.0): | |
points = [] | |
cnt = 0 | |
a = 4 * np.pi / N | |
d = np.sqrt(a) | |
M = int(np.round(np.pi / d)) | |
d_theta = np.pi / M | |
d_phi = a / d | |
for m in range(M): | |
theta = np.pi * (m + 0.5) / M | |
M_phi = int(round(2 * np.pi * np.sin(theta) / d_phi)) | |
for n in range(M_phi): | |
phi = 2 * np.pi * n / M_phi | |
x = r*np.sin(theta)*np.cos(phi) | |
y = r*np.sin(theta)*np.sin(phi) | |
z = r*np.cos(theta) | |
points.append((x,y,z)) | |
return points | |
def Test_Uniform(): | |
import numpy as np | |
import matplotlib.pyplot as plt | |
from mpl_toolkits import mplot3d | |
pts = np.array(Uniform(100)) | |
ax = plt.axes(projection='3d') | |
ax.scatter3D(pts[:, 0], pts[:, 1], pts[:, 2], cmap='Greens') | |
plt.show() | |
def Test_Fib(): | |
import numpy as np | |
import matplotlib.pyplot as plt | |
from mpl_toolkits import mplot3d | |
pts = np.array(Fib(n = 1000, r = 10, h = 0.1)) | |
ax = plt.axes(projection='3d') | |
ax.scatter3D(pts[:, 0], pts[:, 1], pts[:, 2], cmap='Greens') | |
plt.show() | |
if __name__ == '__main__': | |
Test_Uniform() | |
Test_Fib() |
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