CSC151 2007S, Class 05: RGB Colors

Admin:
* My laptop died, so I am even further behind on everything.  I apologize.
* Fruit
* Remember: Everything I hand out in class (more or less) is available online through the course Web site.
* Make sure that it's clear when you're asking a question and when you're submitting a solution.  "HW3" can mean either.
  * HELP NEEDED on HW3
  * QUESTION on HW3
* We'll spend a few minutes discussing what I intend for assignment 3.

Overview:
* Problem: representing colors.
* The RGB representation.
* Complementary colors.

Some quick notes:*
* Problems in CS: Algorithms and Data
  For algs, need: Basic operations (learning()
        Naming
	Choices (coming late next week)
	Repeat operations (coming tomorrow and beyond)
	Name collections of operations
* How about data
  * In the end, it's all 0's and 1's
  * And we know how to translate those to numbers (and vice versa)
  * We will therefore often think aqbout representing data as numbers,
    even when the data seems non-numeric
* Also care about
  * Is it easy to compute with
  * Is it easy to use

* RGB is the simplest technique for representing colors
  * By combining red, green, and blue we can get almost any color
  * Matches physical device
* How do we phrase the amount of red, green, and blue?
  * Some do percentages
  * We'll use 0..255

Updates/Clarifications
* You can AND SHOULD write defintions in the interactions
* When you call rgb->cname, YOU HAVE TO CREATE THE RGB FIRST
  E.g.,
  (rbg->cname (rbg.new 255 0 0))
  NOT
  (rbg->cname 255 0 0)
* (rgb->cname redcomp greencomp bluecomp)

Notices
* Lab hours
  * Should be TC present 6-midnight Mo-Th; also 5-midnight on Sunday
  * TAs will be available Sun-Thu 9-10ish

Lessons
* Nesting is natural and appropriate
* Some of this stuff is "learning the vocabulary"
  * You need to know what operations are available to you and what they do
* There is some confusion about naming 
  * Names we define ourself don't have quotes and are defined with define
    (define pretty (rgb->new 123 32 97))
  * Color names do have quotes and are built-in
    "magenta"
  * Things we define ourselves we can use when the appropriate type is
    expected
    * Since pretty is an rgb, we can write
      (image.set-pixel! canvas 10 10 pretty)
    * Since "magenta" is an rgb, we cannot write
      (image.set-pixel! canvas 10 10 "magenta")
    * Since canvas is NOT an rgb, we should not be able to write   
      (image.set-pixel! canvas 10 10 canvs)
    * Since we haven't defined magenta, we cannot write
      (image.set-pixel! canvas 10 10 magenta)
    * But we could define magenta 
      (define magenta (cname->rgb "magenta"))