Revert "remove 3 colour image"

This reverts commit b4c5b49.

Author: Robert (Bob) Turner

episodes/02-image-basics.md

@@ -7,10 +7,8 @@ questions:
 objectives:
 - "Define the terms bit, byte, kilobyte, megabyte, etc."
 - "Explain how a digital image is composed of pixels."
-- "Explain how images are stored in NumPy arrays."
 - "Explain the left-hand coordinate system used in digital images."
 - "Explain the RGB additive colour model used in digital images."
-- "Explain the order of the three colour values in skimage images."
 - "Explain the characteristics of the BMP, JPEG, and TIFF image formats."
 - "Explain the difference between lossy and lossless compression."
 - "Explain the advantages and disadvantages of compressed image formats."
@@ -22,9 +20,6 @@ upper left corner, the x-axis running to the right, and the y-axis running
 down."
 - "Most frequently, digital images use an additive RGB model, with eight bits
 for the red, green, and blue channels."
-- "skimage images are stored as multi-dimensional NumPy arrays."
-- "In skimage images, the red channel is specified first, then the green, then
-the blue, i.e., RGB."
 - "Lossless compression retains all the details in an image, but lossy
 compression results in loss of some of the original image detail."
 - "BMP images are uncompressed, meaning they have high quality but also that

episodes/03-skimage-images.md

@@ -1,15 +1,20 @@
 ---
-title: "Working with skimage"
+title: "Image Representation in skimage"
 teaching: 70
 exercises: 50
 questions:
-- "How can the skimage Python computer vision library be used to work with images?"
+- "How are digital images stored in Python with the skimage computer vision library?"
 objectives:
+- "Explain how images are stored in NumPy arrays."
+- "Explain the order of the three colour values in skimage images."
 - "Read, display, and save images."
 - "Resize images with skimage."
 - "Perform simple image thresholding with NumPy array operations."
 - "Extract sub-images using array slicing."
 keypoints:
+- "skimage images are stored as multi-dimensional NumPy arrays."
+- "In skimage images, the red channel is specified first, then the green, then
+the blue, i.e., RGB."
 - "Images are read from disk with the `skimage.io.imread()` function."
 - "We create a window that automatically scales the displayed image
 with matplotlib and calling `show()` on the global figure object."
@@ -23,8 +28,59 @@ images, e.g., `clip = image[60:150, 135:480, :]`."
 - "Metadata is not retained when images are loaded as skimage images."
 ---
 
-We have covered much of how images are represented in computer software. In this episode we will learn some more methods
- for accessing and changing digital images.
+Now that we know a bit about computer images in general,
+let us review and expand on the concepts we just learned.
+
+## Images are represented as NumPy arrays
+
+In [the Image Basics episode]({{page.root}}{% link _episodes/02-image-basics.md %}),
+we learned that images are represented as
+rectangular arrays of individually-coloured square pixels,
+and that the colour of each pixel can be represented as an RGB triplet of numbers.
+On import, skimage stores the information for each pixel in an n-dimensional NumPy arrays.
+
+The rectangular shape of the array corresponds to the shape of the image,
+although the order of the coordinates are reversed.
+The "depth" of the array for a full-colour image in skimage image is three,
+with one layer for each of the three channels.
+The differences in the order of coordinates and the order of the channel
+layers can cause some confusion,
+so we should spend a bit more time looking at that.
+
+When we think of a pixel in an image,
+we think of its (x, y) coordinates (in a left-hand coordinate system)
+like (113, 45) and its colour,
+specified as a RGB triple like (245, 134, 29).
+In an skimage image, the same pixel would be specified with
+*(y, x)* coordinates (45, 113) and *RGB* colour (245, 134, 29).
+
+Let us take a look at this idea visually.
+Consider this image of a chair:
+
+![Chair image](../fig/chair-original.jpg)
+
+A visual representation of how this image is stored as a NumPy array is:
+
+![Chair layers](../fig/chair-layers-rgb.png)
+
+So, when we are working with skimage images,
+we specify the *y* coordinate first,
+then the *x* coordinate.
+And, the colours are stored as *RGB* values -
+red in layer 0,
+green in layer 1,
+blue in layer 2.
+
+> ## Coordinate and colour channel order
+>
+> CAUTION: it is vital to remember the order of the coordinates and
+> colour channels when dealing with images as NumPy arrays.
+> *If* we are manipulating or accessing an image array directly,
+> we specifiy the y coordinate first, then the x.
+> Further, the first channel stored is the red channel,
+> followed by the green, and then the blue.
+>
+{: .callout}
 
 ## Reading, displaying, and saving images