CSC151.02 2014S, Class 22: Images as Functions from Position to Colors
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_Overview_

* Preliminaries.
    * Upcoming work.
    * Admin.
    * Questions.
* Topics.
    * Models of images.
    * Iterating over positions: Images as functions from position to color.
    * Examples.
        * Blends and other positionally-computed images.
        * Drawing simple shapes.
        * Some strange computations.
* Questions.
* Lab.

Preliminaries
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### Upcoming Work

* Work on homework 5.
* Reading for Wednesday: Building Data Structures with Heterogeneous Lists
* Writeup: Exercise 4
    * Subject: CSC 151.02 Writeup 14: Iterating Positions (OPTIONAL NAMES)

### Admin

* Newish lab partners.  (I didn't pay much attention to the computer's
  assignment, so you may find out that you have familiar partners.)
* I've forgotten the students who asked for individual or small group
  tutors.  Please send me an email if you're interested.
* Our computer system is (still) acting up.  I have not received any
  clear indications as to when it will be healthy again.
* Congraulations to Ajuna for her 120 in bowling yesterday.  (It beats
  my average.)
* I've finished getting all of yesterday's eboard into place.
* A few of you are frustrated that your indentation changes when you
  cut and paste into Google docs.  If you use Chromium for accessing
  Google docs, cut and paste *should* work.
* Extra credit: 
    * Tuesday Town Hall, noon in JRC 101, 7:30 p.m. in Harris.
    * Tuesday Wellness event, 5:30-8:00 pm.
    * Wednesday Royce Wolf concert.
    * Thursday CS Extras at 4:30 p.m. in 3821: New CS Curriculum.
* Other good things to do
    * Go see Martin Estrada's show in Smith
    * Go to tonight's Faulconer talk - Works with cars and video

### Questions on the Homework

_What should our solution to problem 3 look like?_

    ;;; Procedure:
    ;;;   dwelling
    ;;; Parameters:
    ;;;   width, a positive integer
    ;;;   height, a positive integer
    ;;; Purpose:
    ;;;   ...
    (define dwelling
      (lambda (width height)
        (let ([scene (image-new width height)])
          ; Standard instructions
          (context-set-fgcolor (irgb 128 64 196))
          (image-select ellipse! scene REPLACE 10 10 (- width 20) 20)
          (image-fill! scene)
          ...
          ; The required two-way decision
          (if (< width 100)
              ...
              ...)
          ; The required three-way decision
          (cond
            ([(< width height)
              ...]
             [(> width height)
              ...]
             [else
              ...]))
          ; The required one-way decision
          (when (= width 42)
            (image-show-text! scene "You got the answer!"))

          scene))))

_What do you want for the three examples?_

> Code.  We'll run the code.

Models of images
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* Course model
    * Problem solving in a formal environment
    * Idea: Different basic operations lead to different solutions
* How might you make a 100x100 image with a 50x50 black outlined circle.
    * Approach one: Use drawings as values
        * Scale the drawing unit circle by 50
        * Shift it horizontally by 50 and vertically by 50
        * Scale the drawing unit circle by 48
        * Shift it horizontally by 50 and vertically by 50
        * Recolor it white
        * Group the two circles
        * Render it with something like (image-show (drawing->image asdfasd 100 100))
    * Approach two: Use the Gimp tools
        * (context-set-bgcolor "white")
        * (define image (image-new 100 100))
        * (image-select-ellipse image REPLACE 25 25 50 50)
        * (context-set-fgcolor! "black")
        * (context-set-brush! "2. Hardness 100" 3)
        * (image-stroke! image)
        * (image-select-nothing! image)
        * (image-show image)

Iterating over positions: Images as functions from position to color
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* Images are just a bunch of pixels, each with a column and a row.
* We can write a function that assigns a color bsaed on the column and the row
* (image-compute (lambda (x y) INSTRUCTIONS-TO-GIVE-COLOR)) WIDTH HEIGHT)

Examples
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A simple color blend

    > (image-show (image-compute (lambda (x y)
                                   (irgb x 0 (- 256 x)))
                                 256 256))


Something a bit more geometric

    > (image-show (image-compute (lambda (x y)
                                   (if (< x y)
                                       (irgb 255 0 0)
                                       (irgb 0 0 0)))
                                 256 256))

Something very strange.  Yay irgb!

    > (image-show (image-compute (lambda (x y)
                                   (* 100 x y))
                                 256 256))

Something equally strange.

    > (image-show (image-compute (lambda (x y)
                                   (* 2 x y))
                                 256 256))

Questions
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Lab
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