Uniformly distribute x points inside a circle

Question

I would like to uniformly distribute a predetermined set of points within a circle. By uniform distribution, I mean they should all be equally distanced from each other (hence a random approach won't work). I tried a hexagonal approach, but I had problems consistently reaching the outermost radius.

My current approach is a nested for loop where each outer iteration reduces the radius & number of points, and each inner loop evenly drops points on the new radius. Essentially, it's a bunch of nested circles. Unfortunately, it's far from even. Any tips on how to do this correctly?

Answer 1

The goals of having a uniform distribution within the area and a uniform distribution on the boundary conflict; any solution will be a compromise between the two. I augmented the sunflower seed arrangement with an additional parameter alpha that indicates how much one cares about the evenness of boundary.

alpha=0 gives the typical sunflower arrangement, with jagged boundary:

With alpha=2 the boundary is smoother:

(Increasing alpha further is problematic: Too many points end up on the boundary).

The algorithm places n points, of which the kth point is put at distance sqrt(k-1/2) from the boundary (index begins with k=1), and with polar angle 2*pi*k/phi^2 where phi is the golden ratio. Exception: the last alpha*sqrt(n) points are placed on the outer boundary of the circle, and the polar radius of other points is scaled to account for that. This computation of the polar radius is done in the function radius.

It is coded in MATLAB.

function sunflower(n, alpha)   %  example: n=500, alpha=2
    clf
    hold on
    b = round(alpha*sqrt(n));      % number of boundary points
    phi = (sqrt(5)+1)/2;           % golden ratio
    for k=1:n
        r = radius(k,n,b);
        theta = 2*pi*k/phi^2;
        plot(r*cos(theta), r*sin(theta), 'r*');
    end
end

function r = radius(k,n,b)
    if k>n-b
        r = 1;            % put on the boundary
    else
        r = sqrt(k-1/2)/sqrt(n-(b+1)/2);     % apply square root
    end
end

Answer 2

Stumbled across this question and the answer above (so all cred to user3717023 & Matt).
Just adding my translation into R here, in case someone else needed that :)

library(tibble)
library(dplyr)
library(ggplot2)

sunflower <- function(n, alpha = 2, geometry = c('planar','geodesic')) {
  b <- round(alpha*sqrt(n))  # number of boundary points
  phi <- (sqrt(5)+1)/2  # golden ratio

  r <- radius(1:n,n,b)
  theta <- 1:n * ifelse(geometry[1] == 'geodesic', 360*phi, 2*pi/phi^2)

  tibble(
    x = r*cos(theta),
    y = r*sin(theta)
  )
}

radius <- function(k,n,b) {
  ifelse(
    k > n-b,
    1,
    sqrt(k-1/2)/sqrt(n-(b+1)/2)
  )
}

# example:
sunflower(500, 2, 'planar') %>%
    ggplot(aes(x,y)) +
    geom_point()