Minor issues in the Computer Graphics chapter

[caitlind]

I have some suggestions for few minor changes I think could be made to the graphics chapter to make it more understandable:

• In section 12.2 Before the block interactives it would be good to explicitly say what directions negative and positive numbers will move the box in.

• The first interactive is quite confusing when you start using it, because of how the camera moves and how it is difficult to understand whether you are moving or if the block is moving.
  I'm not sure how or if this actually needs to be changed in any way (since it already explains in the description that the camera follows the box) but if anyone has any ideas it could be worth exploring them! Maybe adding visible axes?

• In section 12.2 starting from the "Swapping coordinates" interactive the idea of how the image is being rotated around the x and y axes needs to be explained more clearly. When I first read it I thought the word "rotated" had been used incorrectly and it should have been "mirrored".
  I think some of this part needs re-writing and maybe should be divided into "rotation" and then "swapping coordinates. The interactive chould have 3 phases - rotating around x axis only, rotating around y axis only, and then both.
  

• In section 12.2.3 3D Transforms the word "vector" is used without being defined or explained. In this context it is about using vectors for translation (not vector graphics). The concept of a vector should probably be explained here.

[eAlasdair]

In section 12.2 starting from the "Swapping coordinates" interactive the idea of how the image is being rotated around the x and y axes needs to be explained more clearly. When I first read it I thought the word "rotated" had been used incorrectly and it should have been "mirrored".
I think some of this part needs re-writing and maybe should be divided into "rotation" and then "swapping coordinates. The interactive chould have 3 phases - rotating around x axis only, rotating around y axis only, and then both.

This was probably a misinterpretation by the original author:

The shape in the interactive used to be an arrow - which was symmetrical about the y axis.
When you swap x and y of a shape you flip it about the x=y axis, which – if it's symmetrical – happens to look exactly the same as a rotation.

Now that the shape is asymmetrical it's obvious that a rotation isn't what the text describes. We need to find a new way of telling the user to change the coordinates so that the shape rotates and doesn't flip

[eAlasdair]

In section 12.2.3 3D Transforms the word "vector" is used without being defined or explained. In this context it is about using vectors for translation (not vector graphics). The concept of a vector should probably be explained here.

Vectors are first used early on in 12.2.1 but are only named in teacher notes. Perhaps instead of a pseudo-explanation of a matrix within the content we should have proper jargon busters for Matrix and Vector

[eAlasdair]

Another issue:

• The content mentions point (3, 1) as the rightmost corner of the shape. This refers to the original arrow. The rightmost points of the current shape are (2, 3) and (2, 1)

[caitlind]

Vectors are first used early on in 12.2.1 but are only named in teacher notes. Perhaps instead of a pseudo-explanation of a matrix within the content we should have proper jargon busters for Matrix and Vector

A jargon buster would be great for this

[eAlasdair]

Now that the shape is asymmetrical it's obvious that a rotation isn't what the text describes. We need to find a new way of telling the user to change the coordinates so that the shape rotates and doesn't flip

Further to this: A clockwise rotation of 90 degrees that the text intends can be obtained by multiplying the points by the matrix:

| 0 1|
|-1 0|

(x, y) = (y, -x)

wikipedia

i.e. Swap the coordinates but multiply the new y coordinate by -1