Merge pull request #86 from HealthBioscienceIDEAS/km/update-last-image-exercise

Update exercise to mention OverflowError

Author: K-Meech

episodes/what-is-an-image.md

@@ -393,9 +393,11 @@ and _overflow_.
 value. For example, storing 1000 in a `uint8` image may result in 255 being 
 stored instead (the max value)
 
-- Overflow: For NumPy arrays, values outside the valid range are 
-'wrapped around' to give the new result. For example, storing 256 in a `uint8` 
-image (max 255) would give 0, 257 would give 1 and so on...  
+- Overflow: For NumPy arrays, values outside the valid range may be 
+'wrapped around' or create an `OverflowError`, depending on what version of 
+NumPy is installed. Older versions of NumPy may wrap the values around, so 
+storing 256 in a `uint8` image (max 255) would give 0, 257 would give 1 and so 
+on... Newer versions of NumPy will raise an `OverflowError`.
 
 Clipping and overflow result in data loss - you can't get the original values 
 back! So it's always good to keep the data type in mind when doing image 
@@ -530,19 +532,19 @@ you should see the one bright pixel you just created.
 ```python
 
 image[15, 10] = 300
-viewer.layers["human_mitosis"].refresh()
-
-print(image[15, 10])
 ```
 
-```output
+```error
 
-44
-```
+OverflowError                             Traceback (most recent call last)
+Cell In[8], line 1
+----> 1 image[15, 10] = 300
 
-The new value is not correct. This is because 300 exceeds the maximum value for 
-this 8-bit image (max 255). The value therefore overflows and 'wraps around' to 
-give 44 - an incorrect value.
+OverflowError: Python integer 300 out of bounds for uint8
+```
+An `OverflowError` occurs when you try to update this pixel value.
+This is because 300 exceeds the maximum value for 
+this 8-bit image (max 255).
 
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