Class 5: RGB Colors

Held: Monday, 30 January 2017

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

Preliminaries

Overview

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

Related Pages

• Eboard 5 (Source)

• Reading: Design and Color

• Reading: RGB Colors

• Lab: RGB Colors

Updates

News / Etc.

• Quiz 1 returned (by mentors).
• HW 1 returned (via email).
• New partners! Grab a card, find a computer, introduce yourself.
• I apologize for the confusion on the self-checks for colors.

Reminders

• The CS department maintains a mailing list of events and opportunities (and other things of potential interest).
• Starting this week, we will have mentor sessions on Wednesday and Thursday evenings from 8:00-9:00 p.m. Wednesdays will be more Q&A, Thursdays will include sample quizzes.
• We have tutors available Sunday through Thursday evening from 7-10 p.m. in Science 3813/15.

Upcoming Work

• Assignment 2. Due Tuesday!
• Reading: Transforming RGB Colors.
• Lab writeup: Exercise 7. Due Wednesday before class.

Extra credit (Academic/Artistic)

• CS Table, Tuesday, noon, middle PDR: Planning for the semester.
• Thursday extras, Thursday, 4:15 p.m., Science 3821: LaTeX

Extra credit (Peer)

• Swim meet this coming weekend.
• Ritalin Test Squad Improv Troupe is looking for new members and will be holding try outs this weekend. Auditions will be on Saturday Feb 4th, 1-3 pm in The Wall (Bucksbaum 152).

Good things to do

• Care for the people around you.

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