Held: Friday, 5 September 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 shapes.

Related Pages

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

Overview

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

Administrivia

• New lab partners.
  • Take a playing card.
  • Sit at the computer on the playing card.
  • Introduce yourself to your partner when he/she/zi sits down.
• No attendance today.
• You can view notes from yesterday's review session
• Our final is scheduled for Friday of finals week. It is optional. I am happy to work out other times with you individually.
• I have flash drives with virtual Linux computers that you can run on your Mac, MS Windows, or Linux computer, along with instructions. Feel free to take a flash drive, but please bring them back after you get it installed.
• Since it's a weekend, a few reminders.
  • Decide what you want and what you feel comfortable with.
  • And please, if you drink, please drink responsibly.

Upcoming Work

• Lab writeup for today (due Monday): Exercise 4, parts a, c, and e.
  See link on schedule or list of labs for link to submit it.
• HW 2 is due Tuesday.
• Readings for Monday:
  • Writing Your Own Procedures
  • How Scheme Evaluates Expressions (take 2)

Extra Credit Opportunities

• Note: Please send a short (one paragraph) reflection on any extra credit activity you attend. And reflect, don't summarize.

Academic

• Awesome show at the Faulconer gallery.
• Convo next Wednesday at noon. "Limiting Armed Drone Proliferation" by Micah Zenko, the Douglas Dillon fellow in the Center for Preventive Action at the Council on Foreign Relations and vice chair of the World Economic Forum Global Agenda Council on Terrorism.
  • Sam's comments on the value of convo
• Any other event in the Rosenfield Drones program.
• CS Extra, Thursday, September 11: Ajuna Kyaruzi '17 on being a SysAdmin

Peer Support

• Football Games (???)
• Women's Volleyball home matches (???)
• Men's Tennis home matches (???)
• Women's Tennis (at Luther, noon and 3:30 p.m. Saturday)
• Anna Christie, Oct. 9-12 (SB plays Marthy)

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 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.