Detect squares (paintings) in images and draw contour around them using python

Question

I'm trying to detect and draw a rectangular contour on every painting on for example this image:

I followed some guides and did the following:

1. Grayscale conversion
2. Applied median blur
3. Sharpen image
4. Applied adaptive Threshold
5. Applied Morphological Gradient
6. Find contours
7. Draw contours

And got the following result:

I know it's messy but is there a way to somehow detect and draw a contour around the paintings better?

Here is the code I used:

path = '<PATH TO THE PICTURE>'

#reading in and showing original image
image = cv2.imread(path)
image = cv2.resize(image,(880,600)) # resize was nessecary because of the large images
cv2.imshow("original", image)
cv2.waitKey(0)
cv2.destroyAllWindows() 

# grayscale conversion
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
cv2.imshow("painting_gray", gray)
cv2.waitKey(0)
cv2.destroyAllWindows() 

# we need to find a way to detect the edges better so we implement a couple of things
# A little help was found on stackoverflow: https://stackoverflow.com/questions/55169645/square-detection-in-image 
median = cv2.medianBlur(gray,5)
cv2.imshow("painting_median_blur", median) #we use median blur to smooth the image
cv2.waitKey(0)
cv2.destroyAllWindows() 

# now we sharpen the image with help of following URL: https://www.analyticsvidhya.com/blog/2021/08/sharpening-an-image-using-opencv-library-in-python/ 
kernel = np.array([[0, -1, 0],
                [-1, 5,-1],
                [0, -1, 0]])
image_sharp = cv2.filter2D(src=median, ddepth=-1, kernel=kernel)
cv2.imshow('painting_sharpend', image_sharp)
cv2.waitKey(0)
cv2.destroyAllWindows()

# now we apply adapptive thresholding
# thresholding: https://opencv24-python-tutorials.readthedocs.io/en/latest/py_tutorials/py_imgproc/py_thresholding/py_thresholding.html#adaptive-thresholding 
thresh = cv2.adaptiveThreshold(src=image_sharp,maxValue=255,adaptiveMethod=cv2.ADAPTIVE_THRESH_GAUSSIAN_C,
                            thresholdType=cv2.THRESH_BINARY,blockSize=61,C=20)   
cv2.imshow('thresholded image', thresh)
cv2.waitKey(0)
cv2.destroyAllWindows()

# lets apply a morphological transformation
kernel = np.ones((7,7),np.uint8)
gradient = cv2.morphologyEx(thresh, cv2.MORPH_GRADIENT, kernel)
cv2.imshow('dilated image', gradient)
cv2.waitKey(0)
cv2.destroyAllWindows()



# # lets now find the contours of the image
# # find contours: https://docs.opencv.org/4.x/dd/d49/tutorial_py_contour_features.html
contours, hierarchy = cv2.findContours(gradient, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
print("contours: ", len(contours))
print("hierachy: ", len(hierarchy))
print(hierarchy)


cv2.drawContours(image, contours, -1, (0,255,0), 3)
cv2.imshow("contour image", image)
cv2.waitKey(0)
cv2.destroyAllWindows() 

Tips, help or code is appreciated!

Answer 1

Here's a simple approach:

1. Obtain binary image. We load the image, grayscale, Gaussian blur, then Otsu's threshold to obtain a binary image.

2. Two pass dilation to merge contours. At this point, we have a binary image but individual separated contours. Since we can assume that a painting is a single large square contour, we can merge small individual adjacent contours together to form a single contour. To do this, we create a vertical and horizontal kernel using cv2.getStructuringElement then dilate to merge them together. Depending on the image, you may need to adjust the kernel sizes or number of dilation iterations.

3. Detect paintings. Now we find contours and filter using contour area using a minimum threshold area to filter out small contours. Finally we obtain the bounding rectangle coordinates and draw the rectangle with cv2.rectangle.

---

Code

import cv2

# Load image, grayscale, Gaussian blur, Otsu's threshold
image = cv2.imread('1.jpeg')
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(gray, (13,13), 0)
thresh = cv2.threshold(blur, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)[1]

# Two pass dilate with horizontal and vertical kernel
horizontal_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (9,5))
dilate = cv2.dilate(thresh, horizontal_kernel, iterations=2)
vertical_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (5,9))
dilate = cv2.dilate(dilate, vertical_kernel, iterations=2)

# Find contours, filter using contour threshold area, and draw rectangle
cnts = cv2.findContours(dilate, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]
for c in cnts:
    area = cv2.contourArea(c)
    if area > 20000:
        x,y,w,h = cv2.boundingRect(c)
        cv2.rectangle(image, (x, y), (x + w, y + h), (36, 255, 12), 3)

cv2.imshow('thresh', thresh)
cv2.imshow('dilate', dilate)
cv2.imshow('image', image)
cv2.waitKey()