Held: Tuesday, 22 September 2015

Back to Outline 14 - Boolean Values and Predicate Procedures. On to Outline 16 - 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 15 (Source 15)

Overview

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

Administrivia

• Continue partners!
• Review sessions:
  • Wednesday, 8pm (Zachary)
  • Thursday, 10am (Sam)
  • Thursday, 8pm (Caleb)
• Lab and reading in the back of the room. I don't know why the reading printed so small.
• I don't know why it's so hard to print from DrRacket. I'm looking into the issue.
• I hear that some of you are starting to get the "cs rush" - the joy of realizing that you've solved a hard problem.

Upcoming Work

• Reading for Wednesday Homogeneous Lists
• Exam 1
  • Electronic due TONIGHT: http://bit.ly/151-2015F-exam1
  • Printed version due TOMORROW
  • Epilogue due TOMORROW NIGHThttp://bit.ly/151-2015F-exam1epi
• Today's Lab writeup (due before class Friday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 15 (Your Names)
  • Exercise:

Extra Credit Opportunities

Academic

• Any visit to the current show in the Faulconer gallery
• No CS Table this week - It's advising week!
• Wednesday CS Extra this week, 4:15 in 3821: Ursula Woltz on Building Coding Communities
• Thursday CS Extra this Week, 4:15 in 3821: SamR on The Architecture of Mediascript

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included), Sunday at 4:45, and Mondays at 4:45. Requires sign up in advance. Fundraising activity forthcoming, too!
• Pun Club, Saturdays, 4pm, Way over Younker.
• Singers concert on Family weekend! Sept. 26, 2pm., Sebring-Lewis
• Orchestra concert on Family weekend! Sept. 26, 2pm., Sebring-Lewis

Non Extra-Credit Activities

• EE's show in Smith
• Grinnell Homecoming Parade Thursday at 5:30 p.m. - Small town homecoming is an interesting experience that everyone should see once

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.