CSC151.02 2015S, Class 16: Transforming Images

Overview

Preliminaries

Do we have to document every procedure?

6P's required for those that say "write and document"; just a sentence for every other procedure.

What does purity have to do with procedures? Are unicorn horns involved?

A "pure" procedure does not modify its parameters or the environment. Things like drawing-hshift and irgb-redder, but not image-set-pixel!.

We can write new Boolean procedures.
Computers are built from basic Boolean procedures (well, hardware that implements them)
Boolean values and binary go hand in hand.
It's useful to make a chart of expected outputs.
- Finite for Booleans
- Perhaps too large for other inputs
- Once you've "sampled" the chart, you can write test cases

We can compose procedures: Make a new procedure that applies multiple procedures. Why do we care?
- Something in Scheme that's easier to read - fewer parentheses, shorter, more concise, less repetitious, less repetitious (image-variant kitten (o rgb-redder rgb-bluer))
- vs. (image-variant (image-variant kitten rgb-bluer) rgb-redder)
- Potentially more efficient, particularly when we think about making images, in which setting and getting pixels is expensive.
image-variant takes a procedure as a parameter and makes a variant of a picture by applying that procedure to every pixel
- A form of repetition
- Procedures can be parameters
- image-variant is pure, does not modify the image, creates a new one
- image-transform is impure, it modifies the image