CSC151 2007S, Class 04: Raster Graphics

Admin:
* Assignments
  * I expect to read over HW1 tonight and HW2 tomorrow afternoon.
  * Assignment 3 is now available.
  * When working in groups, you need only submit one assignment per group.
  * Short reading on RGB colors for Thurday.
* Extra Credit
  * Three extra credit talks on Thursday.
  * Cross-country's one Home meet on Saturday.

Overview:
* Representing images, revisited.
* Pixels and colors: The basics.
* The graphics coordinate system.
* Key functions.
* Lab a

Working with this representation
* Create a new image of specified width and height
  image.new
* Open an existing image
  image.load
* Get information about that image
  image.width
  image.height
* Get and set pixels
  image.get-pixel
  image.set-pixel!

* Observation: All of these operations have the same prefix "image."
  * Not required by the language
  * But imposed by programmer

Some hints and corrections
* To define the landscape image
  (define landscape (image.new 9 5))
* To set the background color
  (envt.set-bgcolor! (cname->rgb "yellow"))

Expected lessons
* How to use the basic operations
* In an image of width w (9) and height h (5), the last column is w-1 (8)
  and the last row is h-1 (4)
* If the last column is c and c is even, the center is c/2
* Naming is your friend
  (define color (cname->rgb "red"))
* Sometimes you need to do a sequence of operations 
  (define color (cname->rgb "red"))
  (image.set-pixel! landscape 0 0 color)
* You can also using nesting to sequence
  (image.set-pixel! landscape 0 0 (cname->rgb "red"))

Unexpected lessons
* DrFu is not so reliable
* Sam and Emily and Dr Davis don't always have the answers
* It may be better to do everything in the interactions window (or in the
  definitions window, but not both)