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

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

• 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

• 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

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

Lab