Introduction to Morphological Operators

In this tutorial you are gonna explore kornia.morphology, that’s Kornia’s module for differentiable Morphological Operators.

By the end, you will be able to use morphological operations as easy as:

new_image = morph.operation(original_image, structuring_element)

But first things first, let’s prepare the environment.

Download Kornia

If you don’t have Kornia installed, you can download it using pip.

%%capture
!pip install git+https://github.com/kornia/kornia

Prepare the image

With kornia.morphology, you can apply morphological operators in 3 channel color images. Besides, all operators are differentiable. Let’s download and read an image.

%%capture
!wget 'https://image.shutterstock.com/image-photo/portrait-surprised-cat-scottish-straight-260nw-499196506.jpg' -O img.jpg

We can use can use OpenCV to load the image.

import cv2
 
img = cv2.imread("img.jpg") # Download Image
img = cv2.cvtColor(img, cv2.COLOR_BGR2RGB) # Define RGB

Structuring element

We have the original image ready to go, now we need the second part in the operation, the structuring element (aka Kernel).

The kernel must be a 2-dim tensor with odd sides, i.e. 3x3.

import torch
import kornia as K

device = 'cpu' # 'cuda:0' for GPU
kernel = torch.tensor([[0, 1, 0],[1, 1, 1],[0, 1, 0]]).to(device)

# to torch.tensor
img_t = K.image_to_tensor(img, keepdim=False)
img_t = img_t.float() / 255.

/home/docs/checkouts/readthedocs.org/user_builds/kornia-tutorials/envs/latest/lib/python3.7/site-packages/tqdm/auto.py:22: TqdmWarning: IProgress not found. Please update jupyter and ipywidgets. See https://ipywidgets.readthedocs.io/en/stable/user_install.html
  from .autonotebook import tqdm as notebook_tqdm

Making plots!

In this tutorial we are gonna compare the images before and after transforming them.

It make sense to create a function to plot and see the changes!

import matplotlib.pyplot as plt
from matplotlib import rcParams

def plot_morph_image(tensor):

  # kornia.tensor_to_image
  image = K.tensor_to_image(tensor.squeeze(0)) # Tensor to image

  # Plot before-after
  rcParams['figure.figsize'] = 20 ,20
  fig, ax = plt.subplots(1,2)
  ax[0].axis('off')
  ax[0].imshow(img)
  ax[1].axis('off')
  ax[1].imshow(image)

Morphology

The main goal of kornia.morphology is that you could easily implement several morphological operator as follows:

new_image = morph.operation(original_image, structuring_element)

Let’s check them all!

Dilation

from kornia import morphology as morph

dilated_image = morph.dilation(img_t, kernel) # Dilation
plot_morph_image(dilated_image) # Plot

Erosion

eroded_image = morph.erosion(img_t, kernel) # Erosion
plot_morph_image(eroded_image) # Plot

Open

opened_image = morph.opening(img_t, kernel) # Open
plot_morph_image(opened_image)

Close

closed_image = morph.closing(img_t, kernel) # Close
plot_morph_image(closed_image) # Plot

Morphological Gradient

graded_image = morph.gradient(img_t, kernel) # Morphological gradient
plot_morph_image(1. - graded_image)

Bottom Hat

bottom_image = morph.bottom_hat(img_t, kernel) # Black Hat
plot_morph_image(1. - bottom_image)

Top Hat

toph_image = morph.top_hat(img_t, kernel) # Top Hat
plot_morph_image(1. - toph_image)

Conclusion

And that’s it!

Now you know how to use Kornia to apply differentiable morphological operations in your PyTorch pipeline.

Many thanks for using Kornia, and have fun!