Working with skimage
- Be aware, that learners might get surprising results in the Keeping only low intensity pixels exercise, if
plt.imshowis called without thevmaxparameter. A detailed explanation is given in the Plotting single channel images (cmap, vmin, vmax) callout box.
Blurring
- Take care to avoid mixing up the term “edge” to describe the edges of objects within an image and the outer boundaries of the images themselves. Lack of a clear distinction here may be confusing for learners.
Questions from Learners
Q: Where would I find out that coordinates are x,y not r,c?
A: In an image viewer, hover your cursor over top-left (origin) the move down and see which number increases.
Q: Why does saving the image take such a long time? (skimage-images/saving images PNG example)
A: It is a large image.
Q: Are the coordinates represented x,y or r,c in the code (e.g. in array.shape)?
A: Always r,c with numpy arrays, unless clearly specified otherwise - only represented x,y when image is displayed by a viewer.
Take home is don’t rely on it - always check!
Q: What if I want to increase size? How does skimage upsample? (image resizing)
A: When resizing or rescaling an image, skimage performs interpolation to up-size or down-size the image. Technically, this is done by fitting a spline function to the image data. The spline function is based on the intensity values in the original image and can be used to approximate the intensity at any given coordinate in the resized/rescaled image. Note that the intensity values in the new image are an approximation of the original values but should not be treated as the actual, observed data. skimage.transform.resize has a number of optional parameters that allow the user to control, e.g., the order of the spline interpolation. The scikit-image documentation provides additional information on other parameters.
Q: Why are some lines missing from the sudoku image when it is displayed inline in a Jupyter Notebook? (skimage-images/low intensity pixels exercise)
A: They are actually present in image but not shown due to interpolation.
Q: Does blurring take values of pixels already blurred, or is blurring done on original pixel values only?
A: Blurring is done on original pixel values only.
Q: Can you blur while retaining edges?
A: Yes, many different filters/kernels exist, some of which are designed to be edge-preserving.
Troubleshooting
Learners reported a problem on some operating systems, that Shift+Enter is prevented from running a cell in Jupyter when the caps lock key is active.