Q.1
How do we iterate over just the pixels on the left in this example?
By only going through half the pixels in the picture
We don't. We go through the pixels on the right and compute the pixels on the left.
We don't. We go through all pixels, but we have a conditional that decides if X coordinate of the pixel is on the left. If it is, we compute the pixel on the right.
By ignoring all pixels on the right.
Q.2
To mirror from right to left, we would change
Swap the target pixel and the source pixel
Everything -- it's too hard to do with this example
Our reference frame