circular gradients?

hi, I just started playing with nodebox and I'm kind of stumped by this issue. I'm trying to make a function to produce circles or rings with gradient fill, but the result looks really ugly, with a ton of moire lines. I'm not sure what I'm doing wrong. here's the function:

def gradcirc(x=(WIDTH/2),y=(HEIGHT/2),rad=(WIDTH/2),inner=0,c=color(0.5,0,0)):
    colormode(HSB)
    nofill()
    for i in range(rad)[int(rad*inner):rad]:
        c.a = (float(i) / rad)
        stroke(c)
        oval(x-i, y-i, i*2, i*2)

oh also the colormode(HSB) should be outside (before) the def block. I changed it and didn't realize that it was a problem until after posting this.

You could widen the gap between each circle and at the same time increase the strokewidth. Less steps soften the moire effect.

colormode(HSB)
def gradcirc(x=(WIDTH/2),y=(HEIGHT/2),rad=(WIDTH/2),
             inner=0,c=color(0.5,0,0), step=2):
    nofill()
    for i in range(rad / step)[int(rad*inner):rad]:
        c.a = (float(i) / rad)
        stroke(c)
        strokewidth(step*3)
        oval(x-i*step, y-i*step, i*2*step, i*2*step)

from math import sqrt
n = 280 # adjusts the spread
r = sqrt(WIDTH*WIDTH + HEIGHT*HEIGHT)
for i in range(n):
    dx = (WIDTH-r)/2
    dy = (HEIGHT-r)/2
    fill(0, 0, float(i)/n)
    oval(dx+i, dy+i, r-i*2, r-i*2)

size(400, 400)
 
colors = [color(0.80, 0.80, 0.44),
          color(0.29, 0.27, 0.08),
          color(0.16, 0.16, 0.05),
          color(0, 0, 0),
          ]
colors.reverse()
 
def gradient(colors, i, n):
    
    l = len(colors)-1
    a = int(1.0*i/n*l)
    a = min(a+0, l)
    b = min(a+1, l)
    base = 1.0 * n/l * a
    d = (i-base) / (n/l)
    r = colors[a].r*(1-d) + colors[b].r*d
    g = colors[a].g*(1-d) + colors[b].g*d
    b = colors[a].b*(1-d) + colors[b].b*d
    
    return color(r, g, b)
 
from math import sqrt
n = 280
r = sqrt(WIDTH*WIDTH + HEIGHT*HEIGHT)
for i in range(n):
    dx = (WIDTH-r)/2
    dy = (HEIGHT-r)/2
    fill(gradient(colors, i, n))
    oval(dx+i, dy+i, r-i*2, r-i*2)