user2955708 - 1 year ago 274
Python Question

# Maximum likelihood pixel classification in python opencv

I would like to perform pixel classification on RGB images based on input training samples of given number of classes. So I have e.g. 4 classes containing pixels (r,g,b) thus the goal is to segment the image into four phases.

I found that python opencv2 has the Expectation maximization algorithm which could do the job. But unfortunately I did not find any tutorial or material which can explain me (since I am beginner) how to work with the algorithm.

Could you please propose any kind of tutorial which can be used as starting point?

Update...another approach for the code below:

``````  **def getsamples(img):
x, y, z = img.shape
samples = np.empty([x * y, z])
index = 0
for i in range(x):
for j in range(y):
samples[index] = img[i, j]
index += 1
return samples
def EMSegmentation(img, no_of_clusters=2):
output = img.copy()
colors = np.array([[0, 11, 111], [22, 22, 22]])
samples = getsamples(img)
#em = cv2.ml.EM_create()
em = cv2.EM(no_of_clusters)
#em.setClustersNumber(no_of_clusters)
#em.trainEM(samples)
em.train(samples)
x, y, z = img.shape
index = 0
for i in range(x):
for j in range(y):

result = em.predict(samples[index])[0][1]
#print(result)
output[i][j] = colors[result]
index = index + 1
return output
smallImg = small = cv2.resize(img, (0,0), fx=0.5, fy=0.5)
output = EMSegmentation(img)
smallOutput = cv2.resize(output, (0,0), fx=0.5, fy=0.5)
cv2.imshow('image', smallImg)
cv2.imshow('EM', smallOutput)
cv2.waitKey(0)
cv2.destroyAllWindows()**
``````

convert C++ to python source

``````import cv2
import numpy as np

def getsamples(img):
x, y, z = img.shape
samples = np.empty([x * y, z])
index = 0
for i in range(x):
for j in range(y):
samples[index] = img[i, j]
index += 1
return samples

def EMSegmentation(img, no_of_clusters=2):
output = img.copy()
colors = np.array([[0, 11, 111], [22, 22, 22]])
samples = getsamples(img)
em = cv2.ml.EM_create()
em.setClustersNumber(no_of_clusters)
em.trainEM(samples)
means = em.getMeans()
covs = em.getCovs()  # Known bug: https://github.com/opencv/opencv/pull/4232
x, y, z = img.shape
distance = [0] * no_of_clusters
for i in range(x):
for j in range(y):
for k in range(no_of_clusters):
diff = img[i, j] - means[k]
distance[k] = abs(np.dot(np.dot(diff, covs[k]), diff.T))
output[i][j] = colors[distance.index(max(distance))]
return output