StackOverflow Problems

Real-world problems to test your skills on!

In [4]:
%matplotlib inline
In [5]:
import matplotlib.pyplot as plt
import numpy as np
from skimage import (filters, io, color, exposure, feature,
                     segmentation, morphology, img_as_float)

Parameters of a pill

(Based on StackOverflow

Consider a pill from the NLM Pill Image Recognition Pilot (../images/round_pill.jpg). Fit a circle to the pill outline and compute its area.


  1. Equalize (exposure.equalize_*)
  2. Detect edges (filter.canny or feature.canny--depending on your version)
  3. Fit the CircleModel using measure.ransac.

Alternative: morphological snakes

NOTE: this is expensive to compute, so may take a while to execute

In [ ]:
# Initial level set
pill = color.rgb2gray(image)
pill = restoration.denoise_nl_means(pill, multichannel=False)

level_set = segmentation.circle_level_set(pill.shape, radius=200)
ls = segmentation.morphological_chan_vese(pill, 80, init_level_set=level_set, smoothing=3)

fig, ax = plt.subplots(1, 1, figsize=(8, 8))

ax.imshow(pill, cmap="gray")
ax.contour(ls, [0.5], colors='r');

Counting coins

Based on StackOverflow

Consider the coins image from the scikit-image example dataset, shown below. Write a function to count the number of coins.

The procedure outlined here is a bit simpler than in the notebook lecture (and works just fine!)


  1. Equalize
  2. Threshold (filters.threshold_otsu)
  3. Remove objects touching boundary (segmentation.clear_border)
  4. Apply morphological closing (morphology.closing)
  5. Remove small objects (measure.regionprops)
  6. Visualize (potentially using color.label2rgb)
In [3]:
from skimage import data
fig, ax = plt.subplots()
ax.imshow(data.coins(), cmap='gray');


Based on

Consider the zig-zaggy snakes on the left (../images/snakes.png).
Write some code to find the begin- and end-points of each.


  1. Threshold the image to turn it into "black and white"
  2. Not all lines are a single pixel thick. Use skeletonization to thin them out (morphology.skeletonize)
  3. Locate all snake endpoints (I used a combination of scipy.signal.convolve2d [find all points with only one neighbor], and np.logical_and [which of those points lie on the snake?] to do that, but there are many other ways).


How many blue M&Ms are there in this image (../images/mm.jpg)?


  1. Denoise the image (using, e.g., restoration.denoise_nl_means)
  2. Calculate how far each pixel is away from pure blue
  3. Segment this distance map to give a "pill mask"
  4. Fill in any holes in that mask, using scipy.ndimage.binary_fill_holes
  5. Use watershed segmentation to split apart any M&Ms that were joined, as described in

Alternative approach:

Viscous fingers

Based on StackOverflow:

Consider the fluid experiment on the right (../images/fingers.png). Determine any kind of meaningful boundary in this noisy image.


  1. Convert to grayscale
  2. Try edge detection (feature.canny)
  3. If edge detection fails, denoising is needed (try restoration.denoise_tv_bregman)
  4. Try edge detection (feature.canny)