Held: Monday, 6 October 2014

Back to Outline 21 - Naming Local Values. On to Outline 23 - Revisiting Lists.

Summary

We consider a simple approach to working with images - looping over the positions in an image and computing a new color based on positional information. We then explore some applications of this approach.

Related Pages

• Reading: Building Images by Iterating Over Positions
• Lab: Building Images by Iterating Over Positions
• EBoard 22 (Source 22) (HTML 22)

Overview

• Models of images.
• Iterating over positions: Images as functions from position to color.
• Blends and other positionally-computed images.
• Drawing simple shapes.
• Some strange computations.

Administrivia

• Maintain partners!
• On Wednesday, I spent about twenty minutes going over the material. On Friday, I spent no time. What's the right amount of time for you? (I will close my eyes while you answer.)
  • 5 minutes, or less
  • Around 10 minutes.
  • Around 20 minutes.
  • Whatever.
• If you'd like to see a very different approach to today's material (not in Scheme, but some similar ideas), head over to the beta of my summer students' software at http://glimmer.grinnell.edu
• Office hours
  • Normal (MThF 1:45-3:15; walking 1:15-1:45)
  • In the Grill 8:00-9:30 Wed and Fri
  • In my office 8:00-9:30 Tue and Thu

Upcoming Work

• Writeup (due Wed): Problem 3, http://bit.ly/151-2014F-w22
• HW 5 due Tuesday.
• Exam 2 will be distributed on Wednesday.
• Reading for Tuesday:
  • Heterogeneous Lists

Cool Things Coming to Campus

• Two versions of Anna Christie!
• Live Streaming of Verdi's Macbeth, Saturday at 11:55 a.m.

Extra Credit Opportunities

Academic

• Grinnell Prize events this week (particularly events related to the prize winners whose project involves Web tools)

Peer Support

• Football (Saturday at 1pm)
• Men's Tennis (???)
• Women's Tennis (???)
• Swimming (???)
• Karan's radio show 11pm Thursday nights on KDIC
• Evan's radio show 5pm Friday nights on KDIC
• Donna's radio show Sunday midnight on KDIC
• The College's version of Anna Christie, Oct. 9-12 (SB plays Marthy)

Models of images

• We've seen a variety of models of images.
  • Images that we describe by composing basic shapes.
  • Images that we create by transforming existing images.
  • Images that we create by using the Gimp tools.
  • Images that we create by a combination of techniques.
• But there are certainly other models of image making. We'll learn at least two more this semester.
• Why are we learning so many models of image making? Because the basic operations you have affect the way you think about problem solving.

The image-compute model

• In the image-variant model, we built images with a starting image and a function from colors to colors.
• We will consider a variant of this model - images as functions from positions to colors.
• That is, given an (x,y) pair, use a function to determine a color for the pixel at (x,y).
• Syntax: (image-compute *function* *width* *height*)
  • No exclamation point because we are not changing anything that alread exists; we are simply computing a new image.

Examples

• Color blends are the most natural thing to make.
• We can also make shapes by using conditionals.
• Given that colors are just numbers, we can also just do numeric computations and see what they give us.