Held: Friday, 4 September 2015

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

• No attendance today (the quizzes suffice).
• Notes from the Thursday 10am review sessions get posted to the eboards, provided someone shows up for the review sessions.
• Note: If I haven't responded to an email within 24 hours, I've probably lost it in my inbox. Try resending (I will not take offense).

• Before today's lab (and perhaps before every lab you do for the next week or so), please type the following command in the terminal window.

  /home/rebelsky/bin/csc151-update
  

• Friday PSA.

Upcoming Work

• One reading for Monday: Transforming RGB Colors
• Assignment 2 due Tuesday. Your assignment partner is the person (or people) at the same computer as you.
  • I will take questions on the assignment at the start of class today and Monday.
• Lab writeup (due before class Tuesday)
  • Email to grader-151-01@cs.grinnell.edu (not to me).
  • Subject: CSC 151 Lab Writeup Class 5 (Your Names)
  • Problems:

Extra Credit Opportunities

Academic

• Tuesday, noon, CS Table, JRC 224C. Topic TBD.

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 1:30 and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker

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