CSC151 2007S, Class 09: Writing Your Own Color Transformations

Admin:
* No Rice Krispy (R) treats today.  (Pop Tarts (R) as an alternative)
* Notes on HW4 will be available soon
* Are there questions on Homework 5?
* In case you can't tell, I like to teach using a hybrid prospective-experiential-retrospective technique.  I consider it particular important to reflect on what you might have learned after you attempt the learning.
* Reading for Friday: Numeric Values in Scheme.
* Dr. Davis and I are talking about DrFu this afternoon at 4:15 in 3821 (I think).  The talk is meant primarily for CS folks, but if you want to come and listen, I will give you extra credit.

Overview:
* What is a procedure?
* Describing procedures.
* Syntax: Writing procedures.
* Use in filters

To write algorithms, you may need 
* The ability to name things: define
* Basic instructions: rgb.new, rgb.red, rgb.redder, image.new, ...
* The ability to sequence computations
  * Write them one after another in the definitions pane
  * Write them one after another in the interactions pane
  * (compose function1 function2) gives you a function that first applies
    function2 and then function1
  * Nesting: Write one instruction within another; the inner one is computed
    first
* Create new functions
  * Convenience: Encapsulate (and may name) computations
  * TODAY'S TOPIC
* Loop - Do things multiple times, to different values
  * Do something to each pixel - image.map, image.map!
    * image.map creates a new image, does not change the existing image
    * image.map! doesn't create a new image, instead changes the existing image
* Conditionals
  * Haven't learned yet, but coming next week

Goals:
* Minimize errors

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Encapsulating instructions in Scheme
* Idea: You have a computation, and would like to do in more general contexts
  * Repeatedly
  * Use *here* and *here* and *here*
* Traditionally think about these in terms of
  "Given *these values*, we compute *this value*"
* Example from colors
  * "Given a color, which we call c, compute a new color ..."
  * "Given grades on homeworks 1 2 and 3, compute the homework average"
* In Scheme
  (lambda (param1 param2 param3) expression-to-compute-result)
* More typically, we name these things
(define proc
  (lambda (param1 param2 param3)
    expression))
(define draw-blended-pixel
  (lambda (position)
    (image.set-pixel! blendy position 0 (rgb.new (+ initial-red  (* red-diff (/ position 10))) ...))))
(draw-blended-pixel 0)
(draw-blended-pixel 1)
(draw-blended-pixel 2)
(draw-blended-pixel 3)
(draw-blended-pixel 4)
(draw-blended-pixel 5)
(draw-blended-pixel 6)

Today: Image map and color computation
(image.map (lambda (color) (rgb.new (+ 32 (rgb.red color)) (rgb.blue color) (rgb.green color))) some-image)