Apply a mask

Although masks are binary, they can be applied to images to filter out pixels where the mask is False.

NumPy's where() function is a flexible way of applying masks. It takes three arguments:

np.where(condition, x, y)

condition, x and y can be either arrays or single values. This allows you to pass through original image values while setting masked values to 0.

Let's practice applying masks by selecting the bone-like pixels from the hand x-ray (im).

Create a Boolean bone mask by selecting pixels greater than or equal to 145.
Apply the mask to your image using np.where(). Values not in the mask should be set to 0.
Create a histogram of the masked image. Use the following arguments to select only non-zero pixels: min=1, max=255, bins=255.
Plot the masked image and the histogram. This has been done for you.

Exercise

Apply a mask

Instructions