Non local maxima suppression in python
Solution 1
This may not be entirely correct, but it works better on a small test case
def nonMaximalSupress1(image,NHoodSize):
#
dX, dY = NHoodSize
M, N = image.shape
for x in range(0,M-dX+1):
for y in range(0,N-dY+1):
window = image[x:x+dX, y:y+dY]
if np.sum(window)==0:
localMax=0
else:
localMax = np.amax(window)
maxCoord=np.unravel_index(np.argmax(window), window.shape) + np.array((x,y))
#suppress everything
image[x:x+dX, y:y+dY]=0
#reset only the max
if localMax > 0:
print localMax
print "max coord is ", maxCoord
image[tuple(maxCoord)] = localMax
return image
I've used local variables to make things easier to read, and tweaked the loop ranges. But the big change is in how I index image
. Especially when indexing with slices, you must use one set of brackets.
image[x:x+dX, y:y+dY]
is the correct way to select a window, not image[x:x+dX][y:y+dY]
.
It can be cleaned up a bit more by modifying the window
. Since it is a view
, changing it changes image
.
def nonMaximalSupress2(image,NHoodSize):
#
dX, dY = NHoodSize
M, N = image.shape
for x in range(0,M-dX+1):
for y in range(0,N-dY+1):
window = image[x:x+dX, y:y+dY]
if np.sum(window)==0:
localMax=0
else:
localMax = np.amax(window)
maxCoord = np.argmax(window)
# zero all but the localMax in the window
window[:] = 0
window.flat[maxCoord] = localMax
return image
Solution 2
How about something like this:
# Use the max filter to make a mask
roi = 3
size = 2 * roi + 1
image_max = ndimage.maximum_filter(image, size=size, mode='constant')
mask = (image == image_max)
image *= mask
# Remove the image borders
image[:roi] = 0
image[-roi:] = 0
image[:, :roi] = 0
image[:, -roi:] = 0
# Optionally find peaks above some threshold
image_t = (image > peak_threshold) * 1
# get coordinates of peaks
f = np.transpose(image_t.nonzero())
jfalkson
Updated on June 04, 2022Comments
-
jfalkson almost 2 years
Goal: To input an image (2d numpy array) and a window size, and output the same array with the local maxima remaining, but 0 elsewhere.
What I am struggling with: I think I made a stupid mistake in my code, maybe a few typos in my loop but I am not sure (the local maxima are only on the left side of the image, which is not true). As I note below I would also welcome any easy tricks with OpenCV or numpy to make this solution shorter.
similar questions: Finding Local Maxima in an Image
and Find local maxima in grayscale image using OpenCV
Mine is different because: I am looking to surpress all but local maxima. I don't have to use my code below, I haven't been able to find a built in function of opencv or numpy to do what I need to (harris corners in cv will implicitly do this as one of the steps, but I need something to perform the sole operation I need). I read a little bit on dilate and wasn't sure if that would be useful here as well.
What I have tried so far.
def nonMaximalSupress(image,NHoodSize): #For for x in range(0,image.shape[0]-1): if x+NHoodSize[0]<image.shape[0]: #while we can still take a square #print "AHH ", image.shape startWindow=0 for y in range(startWindow,image.shape[1]-NHoodSize[1]): #try: if np.sum(image[x:x+NHoodSize[0]][y:y+NHoodSize[1]])==0: localMax=0 else: localMax = np.amax(image[x:x+NHoodSize[0]][y:y+NHoodSize[1]]) #except ValueError: #localMax=0 #print "local max is ", localMax maxCoord=np.unravel_index(np.argmax((image[x:x+NHoodSize[0],y:y+NHoodSize[1]])), image.shape)+np.array((x,y)) #print "X is %r, Y is %r, max coord is %r \n y+nhood is %r" %(x,y,maxCoord,y+NHoodSize[1]) #suppress everything image[x:x+NHoodSize[0]][y:y+NHoodSize[1]]=0 #reset only the max #print maxCoord if localMax > 0: print localMax print "max coord is ", maxCoord[0], maxCoord[1] image[maxCoord[0]][maxCoord[1]]=localMax #increment y x+=NHoodSize[0] return image