def is_square(contour):
"""
Squareness checker
Square contours should:
-have 4 vertices after approximation,
-have relatively large area (to filter out noisy contours)
-be convex.
-have angles between sides close to 90deg (cos(ang) ~0 )
Note: absolute value of an area is used because area may be
positive or negative - in accordance with the contour orientation
"""
area = math.fabs( cv.ContourArea(contour) )
isconvex = cv.CheckContourConvexity(contour)
s = 0
if len(contour) == 4 and area > 1000 and isconvex:
for i in range(1, 4):
# find minimum angle between joint edges (maximum of cosine)
pt1 = contour[i]
pt2 = contour[i-1]
pt0 = contour[i-2]
t = math.fabs(angle(pt0, pt1, pt2))
if s <= t:s = t
# if cosines of all angles are small (all angles are ~90 degree)
# then its a square
if s < 0.3:return True
return False
def findSquares4(img, storage):
N = 11
sz = (img.width & -2, img.height & -2)
timg = cv.CloneImage(img); # make a copy of input image
gray = cv.CreateImage(sz, 8, 1)
pyr = cv.CreateImage((sz.width/2, sz.height/2), 8, 3)
# create empty sequence that will contain points -
# 4 points per square (the square's vertices)
squares = cv.CreateSeq(0, sizeof_CvSeq, sizeof_CvPoint, storage)
squares = CvSeq_CvPoint.cast(squares)
# select the maximum ROI in the image
# with the width and height divisible by 2
subimage = cv.GetSubRect(timg, cv.Rect(0, 0, sz.width, sz.height))
# down-scale and upscale the image to filter out the noise
cv.PyrDown(subimage, pyr, 7)
cv.PyrUp(pyr, subimage, 7)
tgray = cv.CreateImage(sz, 8, 1)
# find squares in every color plane of the image
for c in range(3):
# extract the c-th color plane
channels = [None, None, None]
channels[c] = tgray
cv.Split(subimage, channels[0], channels[1], channels[2], None)
for l in range(N):
# hack: use Canny instead of zero threshold level.
# Canny helps to catch squares with gradient shading
if(l == 0):
# apply Canny. Take the upper threshold from slider
# and set the lower to 0 (which forces edges merging)
cv.Canny(tgray, gray, 0, thresh, 5)
# dilate canny output to remove potential
# holes between edge segments
cv.Dilate(gray, gray, None, 1)
else:
# apply threshold if l!=0:
# tgray(x, y) = gray(x, y) < (l+1)*255/N ? 255 : 0
cv.Threshold(tgray, gray, (l+1)*255/N, 255, cv.CV_THRESH_BINARY)
# find contours and store them all as a list
count, contours = cv.FindContours(gray, storage, sizeof_CvContour,
cv.CV_RETR_LIST, cv. CV_CHAIN_APPROX_SIMPLE, (0, 0))
if not contours:
continue
# test each contour
for contour in contours.hrange():
# approximate contour with accuracy proportional
# to the contour perimeter
result = cv.ApproxPoly(contour, sizeof_CvContour, storage,
cv.CV_POLY_APPROX_DP, cv.ContourPerimeter(contours)*0.02, 0)
# square contours should have 4 vertices after approximation
# relatively large area (to filter out noisy contours)
# and be convex.
# Note: absolute value of an area is used because
# area may be positive or negative - in accordance with the
# contour orientation
if(result.total == 4 and
abs(cv.ContourArea(result)) > 1000 and
cv.CheckContourConvexity(result)):
s = 0
for i in range(5):
# find minimum angle between joint
# edges (maximum of cosine)
if(i >= 2):
t = abs(angle(result[i], result[i-2], result[i-1]))
if s<t:
s=t
# if cosines of all angles are small
# (all angles are ~90 degree) then write quandrange
# vertices to resultant sequence
if(s < 0.3):
for i in range(4):
squares.append(result[i])
return squares