Held: Monday, 1 February 2016

Back to Outline 04 - Computing with Symbols and Numbers. On to Outline 06 - Transforming Colors.

Summary

We consider the basic use of colors in image making and explore one of the common digital representations of colors.

Related Pages

• Reading: Design and Color
• Reading: RGB Colors
• Lab: RGB Colors
• EBoard 5 (Source 5)

Overview

• Color palettes.
• RGB colors.
• Representing RGB colors as integers.

Administrivia

• New Partners! (Take a card to figure out where you are and who your partner will be.)
• Quiz 1 to be returned tomorrow.
• A lot of snow is scheduled within the next day. Please be careful.
• The Registrar wants me to remind you that Add/Drop deadline is Friday.
• Enjoy caucusing (or watching the Caucuses) tonight!
• Email policies
  • When you send me email asking questions, it helps if you title it something like "HELP" or "QUESTION". (Yes, all caps is okay.)
  • I tend to respond to email promptly, but I get a lot of email. If I haven't responded to email within 24 hours, send me a reminder.

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 Zacharcy
  • Thursday at 10am in this room with SamR
  • Thursday at 8pm in the CS Commons with Kumar

Questions from Sam

What does round do?

Upcoming Work:

• Readings for Tuesday:
• Assignment 2 due TOMORROW! at 10:30 p.m.
• Lab Writeup: Problem _
  • Send email titled CSC 151 Lab Writeup 5 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Wednesday.

Extra Credit

Academic

• CS Table, Tuesday, Noon, White PDR: Facebook's Walled Internet. (Readings to be distributed via email.)
• Community Conversations on Race, 7:30 p.m., Tuesday, February 2, Lazier 3rd Floor Lounge

Peer

• Track and Field Grinnell Invite February 6.
• Swimming February 6 vs. UN and IAState.

Miscellaneous

• Caucus TONIGHT. (Or observe a Caucus.)

Colors

• Many ways to represent colors.
• Goals: Unambiguous, fast to process, compact
• Color names 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