Held: Wednesday, 17 September 2014

Back to Outline 11 - A Design Perspective. On to Outline 13 - Testing Your Procedures, Revisited.

Summary

We consider the basics of raster graphics, a common technique for describing and representing images. We also consider RGB colors, one of the most common mechanisms for representing colors digitally.

Related Pages

• Reading: Raster Graphics
• Reading: RGB Colors
• Lab: Raster Graphics and RGB Colors
• EBoard (Source) (HTML)

Overview

• Representing images, revisited.
• Pixels and colors: The basics.
• RGB colors.
• Those weird numbers.

Administrivia

• New lab partners today.
• HW 2 returned soon.
• Review sessions tonight, Thursday at 10, and Thursday night.
• I've rearranged the schedule of topics slightly to give us time to talk about testing on Friday.
  • The two sections will be out of synch for the next week or two.

Notes from Epilogues on HW 3

• Not all of you filled in the epilogues. Please take the time to do so. Evidence suggests that you learn better if you bookend your assignments: Think about the work in advance, do the work, reflect on it.
  • This approach can also help with more general skills, such as reading tasks and assessing workload.
• There was a huge range of time spent, from 3 to 9 hours.
  • The 9-hour group noted that they needed to ask for help earlier.
  • Other long groups noted that they needed to ask for help earlier.
  • Still others just noted that there were a lot of hard problems on the assignment. We thought of 2 as hard, but mostly variants of the same hard problem.
• A variety of notes about group work
  • Working with others makes it more efficient.
  • Scheduling is hard.
  • What should I do when my partner seems to understand it better than I do?
  • What should I do when I have an idea, but I can't convey it to my partner?
  • What should we do when we each want to try a different approach to a problem?
• Other important lessons
  • DrRacket likes to crash. Save early and often.
  • It can be difficult to communicate an approach to your colleagues.

Upcoming Work

• Lab Writeup: Exercise 10 <151-2014F-w12>, due Friday.
• Exam 1 distributed. Due next Tuesday evening.
  • We will go over the policies today
  • We will go over the problems on Friday (and Thursday, if you'd like).
• Study for Friday's quiz.
  • Friday's quiz: Drawings, Side effects, Documentation, Procedures
• No readings for Friday. (You may want to revisit the reading on testing.)

Campus Events of Interest

• Rosenfield seminar on Scotland, Thursday afternoon/evening.
• Rebecca Hamlin book talk today at 4:15 pm.
• Internship talks Wednesday and Thursday afternoons.

Extra Credit Opportunities

Academic

• Convocation TODAY: President Kington
• CS Table Friday: Browser Tracking
• Campus Town Hall September 30

Peer Support

• Football Games (???)
• Men's Tennis (???)
• Women's Tennis (???)
• Anna Christie, Oct. 9-12 (SB plays Marthy)

Miscellaneous

• Friends of Drake Library needs help setting up for their annual booksale on Thursday, October 2 (conveniently, the same night as Grinnell High School's homecoming parade). You can help a good cause and probably get a few free books, too.

Raster Graphics

• Another model of images.
• This one corresponds closely to how computers actually represent images.
• The image is a grid of colors.
• You already know about the numbering of the grid points:
  
  • Rows and columns numbered starting at 0
  • Rows numbered from top to bottom
  • Columns number from left to right
• It's painful to have to set individual pixels, but it can be useful.
  • At some point "under the hood", most programs eventually deal with individual pixels.

Colors

• Many ways to represent colors, too.
• Goals: Unambiguous, fast to process, compact
• The color names we've been using are
  • Ambiguous
  • Slow to process
  • Long
• Whoops!

RGB Colors

• The most common internal representation of colors on computers.
• We think of a color as the combination of three primaries: red, green, and blue.
  • These are the primaries for the so-called additive colors
  • You are probably used to the primaries being red, yellow, and blue, but those are the subtractive colors
• On computers, we represent each component as a number between 0 and 255, inclusive.
• It turns out that you can shove four numbers, each between 0 and 255, into the internal representation of an integer.
  • So, the red, green, and blue components are three of those numbers.
  • What's the forth? In multi-layer images, it's the alpha channel.
• We use rgb-new to create these colors.
• We use rgb-red, rgb-green, and rgb-blue to extract the corresponding components.

Those Weird Color Numbers

• We're representing colors as three integers, each in the range [0..255].
• Yet when we ask MediaScript for a color, we tend to get a single integer which is rarely in that range?
• What's going on?
• Well, most computers have different techniques for representing really small integers (particularly integers in the range 0 to 255, inclusive) and standard-length integers. We're using the smaller representation, but shoving three of those into a standard-length integer.
  • We use that representation because it makes everything faster.
• Even those of us who designed and implemented that representation can't read the values presented in that representation.
• The formula for computing those numbers:
  • 256256red + 256*green + blue