Held: Tuesday, 16 February 2016

Back to Outline 13 - Testing Your Procedures, Revisited. On to Outline 15 - Homogeneous Lists: Making and Manipulating Groups of Drawings .

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 14 (Source 14)

Overview

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

Administrivia

• New partners!
• New lab.

Reminders

• Office hours: MTWF 10-11, Tu 1-2.
  • Sign up at http://rebelsky.youcanbook.me.
  • Also feel free to stop by when my door is open.
  • Or to email me for an appointment.
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Weekly review sessions:
  • Wednesday at 8pm in the CS Commons with ...
  • Thursday at 10am in this room with SamR!
  • Thursday at 8pm in the CS Commons with ...

Upcoming Work:

• Reading for Wednesday:
  • Making and Manipulating Homogeneous Lists
• Exam 1
  • Exam due TONIGHT at 10:30 p.m.
  • Epilogue due Tuesday at 11:00 p.m.
  • Cover sheet due Wednesday at the start of class.
• Lab Writeup: Exercise _
  • Send email titled CSC 151 Lab Writeup 13 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Wednesday.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit".
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic

• On Being Muslim in the US, Wednesday, 11:30-1:00, JRC 101.
• Convocation: Hilary Mason '00 on Data Science and Social Good. (11 a.m., Thursday, JRC 101)
• CS Extras: Hilary Mason on Fast Forward Labs. (4:30 p.m., Thursday, Science 3821)

Peer

• Swim Meet, Friday, Saturday, and Sunday, Osgood Pool. (30 min.)

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio
• Pun club Saturdays at 4pm in Younker (max 2)
• Electronic Potpourri on KDIC Fridays at Five (TONIGHT)
• Space Odyssey KDIC Fridays at Six

No Extra Credit, But Still Good

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.