Watershed Segmentation excluding alone object?

Question

Problem

Using this answer to create a segmentation program, it is counting the objects incorrectly. I noticed that alone objects are being ignored or poor imaging acquisition.

I counted 123 objects and the program returns 117, as can be seen, bellow. The objects circled in red seem to be missing:

Using the following image from a 720p webcam:

Code

import cv2
import numpy as np
import matplotlib.pyplot as plt
from scipy.ndimage import label
import urllib.request


# https://stackoverflow.com/a/14617359/7690982
def segment_on_dt(a, img):
    border = cv2.dilate(img, None, iterations=5)
    border = border - cv2.erode(border, None)

    dt = cv2.distanceTransform(img, cv2.DIST_L2, 3)
    plt.imshow(dt)
    plt.show()
    dt = ((dt - dt.min()) / (dt.max() - dt.min()) * 255).astype(np.uint8)
    _, dt = cv2.threshold(dt, 140, 255, cv2.THRESH_BINARY)
    lbl, ncc = label(dt)
    lbl = lbl * (255 / (ncc + 1))
    # Completing the markers now.
    lbl[border == 255] = 255

    lbl = lbl.astype(np.int32)
    cv2.watershed(a, lbl)
    print("[INFO] {} unique segments found".format(len(np.unique(lbl)) - 1))
    lbl[lbl == -1] = 0
    lbl = lbl.astype(np.uint8)
    return 255 - lbl


# Open Image
resp = urllib.request.urlopen("https://i.sstatic.net/YUgob.jpg")
img = np.asarray(bytearray(resp.read()), dtype="uint8")
img = cv2.imdecode(img, cv2.IMREAD_COLOR)

## Yellow slicer
mask = cv2.inRange(img, (0, 0, 0), (55, 255, 255))
imask = mask > 0
slicer = np.zeros_like(img, np.uint8)
slicer[imask] = img[imask]

# Image Binarization
img_gray = cv2.cvtColor(slicer, cv2.COLOR_BGR2GRAY)
_, img_bin = cv2.threshold(img_gray, 140, 255,
             cv2.THRESH_BINARY)

# Morphological Gradient
img_bin = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN,
        np.ones((3, 3), dtype=int))

# Segmentation
result = segment_on_dt(img, img_bin)
plt.imshow(np.hstack([result, img_gray]), cmap='Set3')
plt.show()

# Final Picture
result[result != 255] = 0
result = cv2.dilate(result, None)
img[result == 255] = (0, 0, 255)
plt.imshow(result)
plt.show()

Question

How to count the missing objects?

Answer 1

Answering your main question, watershed does not remove single objects. Watershed was functioning fine in your algorithm. It receives the predefined labels and perform segmentation accordingly.

The problem was the threshold you set for the distance transform was too high and it removed the weak signal from the single objects, thus preventing the objects from being labeled and sent to the watershed algorithm.

The reason for the weak distance transform signal was due to the improper segmentation during the color segmentation stage and the difficulty of setting a single threshold to remove noise and extract signal.

To remedy this, we need to perform proper color segmentation and use adaptive threshold instead of the single threshold when segmenting the distance transform signal.

Here is the code i modified. I have incorporated color segmentation method by @user1269942 in the code. Extra explanation is in the code.

import cv2
import numpy as np
import matplotlib.pyplot as plt
from scipy.ndimage import label
import urllib.request


# https://stackoverflow.com/a/14617359/7690982


def segment_on_dt(a, img, img_gray):

    # Added several elliptical structuring element for better morphology process
    struct_big = cv2.getStructuringElement(cv2.MORPH_ELLIPSE,(5,5))
    struct_small = cv2.getStructuringElement(cv2.MORPH_ELLIPSE,(3,3))

    # increase border size
    border = cv2.dilate(img, struct_big, iterations=5)
    border = border - cv2.erode(img, struct_small)




    dt = cv2.distanceTransform(img, cv2.DIST_L2, 3)
    dt = ((dt - dt.min()) / (dt.max() - dt.min()) * 255).astype(np.uint8)

    # blur the signal lighty to remove noise
    dt = cv2.GaussianBlur(dt,(7,7),-1)

    # Adaptive threshold to extract local maxima of distance trasnform signal
    dt = cv2.adaptiveThreshold(dt, 255, cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY, 21, -9)
    #_ , dt = cv2.threshold(dt, 2, 255, cv2.THRESH_BINARY)


    # Morphology operation to clean the thresholded signal
    dt = cv2.erode(dt,struct_small,iterations = 1)
    dt = cv2.dilate(dt,struct_big,iterations = 10)

    plt.imshow(dt)
    plt.show()

    # Labeling
    lbl, ncc = label(dt)
    lbl = lbl * (255 / (ncc + 1))
    # Completing the markers now.
    lbl[border == 255] = 255

    plt.imshow(lbl)
    plt.show()

    lbl = lbl.astype(np.int32)
    cv2.watershed(a, lbl)
    print("[INFO] {} unique segments found".format(len(np.unique(lbl)) - 1))
    lbl[lbl == -1] = 0
    lbl = lbl.astype(np.uint8)
    return 255 - lbl

# Open Image
resp = urllib.request.urlopen("https://i.sstatic.net/YUgob.jpg")
img = np.asarray(bytearray(resp.read()), dtype="uint8")
img = cv2.imdecode(img, cv2.IMREAD_COLOR)


## Yellow slicer
# blur to remove noise
img = cv2.blur(img, (9,9))

# proper color segmentation
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)  
mask = cv2.inRange(hsv, (0, 140, 160), (35, 255, 255)) 
#mask = cv2.inRange(img, (0, 0, 0), (55, 255, 255))

imask = mask > 0
slicer = np.zeros_like(img, np.uint8)
slicer[imask] = img[imask]



# Image Binarization
img_gray = cv2.cvtColor(slicer, cv2.COLOR_BGR2GRAY)

_, img_bin = cv2.threshold(img_gray, 140, 255,
             cv2.THRESH_BINARY)


plt.imshow(img_bin)
plt.show()
# Morphological Gradient
# added
cv2.morphologyEx(img_bin, cv2.MORPH_OPEN,cv2.getStructuringElement(cv2.MORPH_ELLIPSE,(3,3)),img_bin,(-1,-1),10)
cv2.morphologyEx(img_bin, cv2.MORPH_ERODE,cv2.getStructuringElement(cv2.MORPH_ELLIPSE,(3,3)),img_bin,(-1,-1),3)

plt.imshow(img_bin)
plt.show()

# Segmentation
result = segment_on_dt(img, img_bin, img_gray)
plt.imshow(np.hstack([result, img_gray]), cmap='Set3')
plt.show()

# Final Picture
result[result != 255] = 0
result = cv2.dilate(result, None)
img[result == 255] = (0, 0, 255)
plt.imshow(result)
plt.show()

Final results : 124 Unique items found. An extra item was found because one of the object was divided to 2. With proper parameter tuning, you might get the exact number you are looking. But i would suggest getting a better camera.

Answer 2

Improving Accuracy

Detecting missing objects

im_1, im_2, im_3

I've count 12 missing objects: 2, 7, 8, 11, 65, 77, 78, 84, 92, 95, 96. edit: 85 too

117 found, 12 missing, 6 wrong

---

1° Attempt: Decrease Mask Sensibility

#mask = cv2.inRange(img, (0, 0, 0), (55, 255, 255))  #Current
mask = cv2.inRange(img, (0, 0, 0), (80, 255, 255))   #1' Attempt

inRange documentaion

im_4, im_5, im_6, im_7

[INFO] 120 unique segments found

120 found, 9 missing, 6 wrong

Answer 3

Looking at your code, it is completely reasonable so I'm just going to make one small suggestion and that is to do your "inRange" using HSV color space.

opencv docs on color spaces:

https://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_imgproc/py_colorspaces/py_colorspaces.html

another SO example using inRange with HSV:

How to detect two different colors using `cv2.inRange` in Python-OpenCV?

and a small code edits for you:

img = cv2.blur(img, (5,5))  #new addition just before "##yellow slicer"

## Yellow slicer
#mask = cv2.inRange(img, (0, 0, 0), (55, 255, 255))   #your line: comment out.
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)  #new addition...convert to hsv
mask = cv2.inRange(hsv, (0, 120, 120), (35, 255, 255))  #new addition use hsv for inRange and an adjustment to the values.