line detection

Hi there,
I'm experimenting with image recognition in Nodebox 1.9.5.
This code detects lines in small images up to 200x200px, but doesn't work with big or edge-detected images:

def houghTransform(im):
    width, height = im.size
    size(width, height)
    rmax = int(round(math.sqrt(height**2 + width**2)))
    print rmax
    acc = zeros((rmax, 180))
    for x,y in [(x,y) for x in range(1, width) for y in range(1, height)]:
        if im.getpixel((x,y)) == 255:
            for m in range(1, 180):
                arg = (m*math.pi) / 180
                r = int(round( (x * math.cos(arg)) + (y * math.sin(arg)) ))
                if 0 < r < rmax:
                    acc[r][m] += 1
    max = 0
    for x,y in [(x,y) for x in range(rmax) for y in range(180)]:
        if acc[x][y] > max:
            max = acc[x][y];
            r, theta = (x,y)
   
    piTheta180 = (math.pi * theta)/180
    cosTheta = math.cos(piTheta180)
    sinTheta = math.sin(piTheta180)
 
    if (r/sinTheta) > height:
        m = -1 * math.tan(piTheta180)
        c = int(round(r/cosTheta))
        upper = (c,0)
        lower = (int(round(abs((m*512)+c))),512)
    else:
        m = (-1.0) / math.tan(piTheta180)
        c = int(round(r/sinTheta))
        c2 = int(round(r/cosTheta))
        upper = (c2,0)
        lower = (0,c)
    return upper, lower, r*cosTheta, r*sinTheta
 
import math
from Numeric import zeros    
 
img = "image.jpg"
 
from PIL import Image
im = Image.open(img)
im.show()
points = houghTransform(im)
 
upper = points[0]
ux = upper[0]
uy = upper[1]
lower = points[1]
lx = lower[0]
ly = lower[1]
 
print points, upper, ux, uy, lx, ly
stroke(0)
line(ux,uy,lx,ly)
Needs PIL.
Can you help fide the bug?

Oh, it simply works only with one-channel grayscale images.