Held: Monday, 27 January 2014

Back to Outline 04 - Computing with Symbols and Numbers. On to Outline 06 - Writing Your Own Procedures.

Summary

We explore a special kind of values, values that represent images. We call this the "drawings as values" model, and describe images by transforming and composing very simple pictures.

Related Pages

• Reading: Drawings as Values
• Lab: Drawings as Values
• EBoard (Source) (HTML) (PDF)

Overview

• Representing images.
• Thinking about drawings through composition/decomposition.
• The underlying representation.
• Pure approaches vs impure approaches.

Administrivia

• Lab partners assigned for the week. (This is a "same sex" week as part of our broader partner preference experiment.)
• We had a family emergency this weekend, so it will take me a few more days to respond to your introductory surveys. (Yours is my largest class, so you folks are last.)
• Study break in the CS commons at 8pm tonight. Home-made (well, probably apartment-made) cookies.
• The TC Corps is hiring! Contact [tranchri] or [cohnhann] for details.
  • Probably other people, too.
• I've graded your first quiz. I'll return it during class, possibly while you are doing lab.
  • No other attendance.
• I've changed my homework grading strategy a bit, to "Excellent, Very Good, Good, Fair, Poor" (it's more or less the same as the old check system, but with new words). Updates to the handouts will follow.
• Note that if you use the "Source" version of an EBoard, you should see the stuff within a minute or two of me updating it.
• I think I've responded to all of the lab writeups I've received.
  • Please send lab writeups to me. Your graders don't need to see them.
• We will continue today's lab tomorrow.
  
  • Please read ahead: Procedures as values
• Today's lab writeup (due Friday): Exercise 4, parts a, c, d, f, and g
  • Email subject: CSC 151 Writeup 3: Drawings as Values (YOUR NAME(s) HERE)
  • You can, but need not, write up the lab with your partner.
• Extra credit:
  • Thursday at 4:15: Spencer Liberto, Lea Marolt Sonnenschein, and Daniel Torres on Ushahidi and more.
  • CS Table Friday at Noon: The ACM Code of Ethics.
  • Convo Feb. 5. (I'll give my "Why go to convo" lecture closer to the date.)
  • Others?

Representing Images

• One of the themes of this course is that there is more than one way to represent an image.
• In this course, we will also consider multiple computational ways to think about images.
• Today we explore our first one, based on simple shapes. We call this model drawings as values.

Drawings

• Your questions about a new type should be

  • What kinds of values do I have?
  • How do I represent those values?
  • What operations do I have?

• We start with a few basic values (the unit square and the unit circle).

  • We represent these as drawing-unit-square and drawing-unit-circle
  • These are values, not procedures.
• We add a few operations (procedures)
  • Scaling
    • If d is a drawing, (scale-drawing amt d) is a drawing
  • Shifting
    • If d is a drawing, (hshift-drawing amt d) is a drawing
  • Recoloring
    • If d is a drawing, (recolor-drawing color d) is a drawing
  • Grouping
    • If d1 and d2 are drawings, so is (drawing-group d1 d2)
• Once we've described a drawing, we can view it with (image-show (drawing->image d width height))
• We will explore what benefits (and obstacles) this new approach gives us.
• This model of images models one of the ways we sometimes think about images: Every image can be thought of as a composition of simpler components.
  • We just take that idea to the extreme.

Representing Drawings

• There is an underlying representation.
• You shouldn't need to know it.
• But it can be helpful to look.

Lab

• Do The lab.
• Be prepared to reflect.

Purity (Optional)

• Note that the definition of drawings uses induction, in much the same way that the definition of whole numbers uses induction.
• For whole numbers:
  • 0 is a whole number
  • If w is a whole number, 1+w is a whole number
• For drawings
  • The unit circle is a drawing.
  • The unit square is a drawing.
  • If d is a drawing, then a shifted version of d is a drawing.
  • If d is a drawing, then a scaled version of d is a drawing.
  • If d1 and d2 are drawings, the grouping of d1 and d2 is a drawing.
• When we build a new whole number from a previous whole number (by adding 1), we don't affect the previous whole number.
• Similarly, when we build a new drawing from a previous drawing, we don't affect the previous drawing.
• Contrast this with the GIMP tools, in which each command changes (sometimes irrevocably) the underlying image.
• When operations do not affect the underlying data structure, we call them pure operations.