## Friday, May 27, 2011

### Ellipse detection in image by using Hough transform

How we can detect ellipses in images ? One way is to use Hough transform. I will use Hough transform algorithm variant created by Yonghong Xie and Qiang Ji. That algorithm pseudo-code:

1. Store all edge pixels in a one dimensional array.
2. Clear the accumulator array.
3. For each pixel (x1, y1), carry out the following steps from (4) to (14).
4. For each other pixel (x2, y2), if the distance between
(x1, y1) and (x2, y2) is greater than the required least
distance for a pair of pixels to be considered then
carry out the following steps from (5) to (14).
5. From the pair of pixels (x1, y1) and (x2, y2) calculate the center,
orientation and length of major axis for the assumed ellipse.
6. For each third pixel (x, y), if the distance between
(x, y) and (x0, y0) is greater than the required least
distance for a pair of pixels to be considered then
carry out the following steps from (7) to (9).
7. Calculate the length of minor axis.
8. Increment the accumulator for this length of minor axis by 1.
9. Loop until all pixels are computed for this pair of pixels.
10. Find the maximum element in accumulator array. The
related length is the possible length of minor axis
for assumed ellipse. If the vote is greater than the
required least number for assumed ellipse, one
ellipse is detected.
11. Output ellipse parameters.
12. Remove the pixels on the detected ellipse from edge pixel array.
13. Clear accumulator array.
14. Loop until all pairs of pixels are computed.


Proof-of-concept algorithm implementation in Python:
import sys
from PIL import Image,ImageFilter, ImageDraw
from math import *

# some global constants
EL_COVERAGE_RATIO = 0.9
EL_VERIFICATION_DISTANCE = 1.
EL_PATH_POINTS = 51
MIN_MINOR_FREQUENCY = 30
MIN_HALF_MAJOR = 8
MIN_HALF_MINOR = 6

def distance(p1,p2):
x1,y1 = p1
x2,y2 = p2
return sqrt((x1-x2)**2 + (y1-y2)**2)

def nonnegative(v):
return v if v >= 0 else 0

def parametricEllipse(center, a, b, angle):
xc,yc = center
elx = lambda t: xc + a * cos(t) * cos(angle) - b * sin(t) * sin(angle)
ely = lambda t: yc + a * cos(t) * sin(angle) + b * sin(t) * cos(angle)
return [(int(elx(2.*pi*x/float(EL_PATH_POINTS-1))),int(ely(2.*pi*x/float(EL_PATH_POINTS-1)))) for x in range(EL_PATH_POINTS)]

assert len(sys.argv)!=2, "missing input and/or output file !"

im = Image.open(sys.argv[1])
width, height = im.size
io = Image.new('RGB',(width, height),(255,255,255))
draw = ImageDraw.Draw(io)

# converting image to grayscale
im = im.convert('L')

# detecting edge pixels
im = im.filter(ImageFilter.FIND_EDGES)

# converting to binary image
im = im.convert('1')
pxy = []

# extracting binary pixels coordinates
for x in range(width):
for y in range(height):
if pixels[x,y]==255:
pxy.append((x,y))

# applying Hough transform for ellipses detection.
# algorithm is taken from this paper:
cIx = -1
colors = [(255,0,0),(0,200,0),(0,0,255)]
for x1,y1 in pxy:
for x2,y2 in pxy:
bbins = {}
dist = distance((x1,y1),(x2,y2))
if dist >= 2*MIN_HALF_MAJOR:
cent = ((x1+x2)/2.,(y1+y2)/2.)
a = dist/2. # semi-length of major axis
alfa = atan2((y2 - y1),(x2 - x1))
for rx,ry in pxy:
d = distance((rx,ry),cent)
if d >= MIN_HALF_MINOR:
f = distance((rx,ry),(x2,y2))
cost = (a**2. + d**2. - f**2.)/(0.00001+2.*a*d)
b = sqrt(nonnegative((a**2. * d**2. * (1.-cost**2.))/(0.00001 + a**2. - d**2. * cost**2.)))  # semi-length of minor axis
b = int(b)
if bbins.has_key(b):
bbins[b]+=1
elif b > 0:
bbins[b]=1
bbins_rev = dict([(v,k) for k,v in bbins.iteritems()])
max_freq = max(bbins_rev.keys())
bmax = bbins_rev[max_freq]
# Did we found probable ellipse ?
if max_freq >= MIN_MINOR_FREQUENCY and alfa >=0.0 and bmax >= MIN_HALF_MINOR:
elData = parametricEllipse(cent, a, bmax, alfa)
supported = []
supportRatio = 0.0
# counting how much pixels lies on ellipse path
for i in range(EL_PATH_POINTS):
elx,ely = elData[i]
for x,y in pxy:
if distance((elx,ely),(x,y)) <= EL_VERIFICATION_DISTANCE:
supported.append((x,y))
supportRatio += 1./float(EL_PATH_POINTS)
supported = list(set(supported))
# if number of pixels on ellipse path is big enough
if supportRatio >= EL_COVERAGE_RATIO:
cIx = (cIx+1)%3
print "coverage %.2f" % supportRatio,"frequency ", max_freq, "center ", cent, "angle %.2f" % alfa, "axes (%.2f,%.2f)" % (a, bmax)
# removing founded ellipse pixels from further analysis
for p in supported:
pxy.remove(p)
# drawing founded ellipse
for i in range(EL_PATH_POINTS):
elx,ely = elData[i]
if i < EL_PATH_POINTS-1:
draw.line(elData[i] + elData[i+1], fill=colors[cIx])
io.save(sys.argv[2])
print "***************************************************************"
print "************************** DONE *******************************"
print "***************************************************************"


(Prototype algorithm is slow, tested only on 50x50 images). So, by running this algo on this image:
we will get such algorithm output:

Have fun in computer vision !

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