Class 10: Raster Graphics and RGB Colors

Held: Tuesday, February 9, 2010

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:

• EBoard.
• Lab: Raster Graphics and RGB Colors.
• Reading: Raster Graphics: Images from Pixels and Colors and RGB Colors.

Notes:

• Reading for tomorrow: Transforming RGB Colors.
• Reminder: If you're going to miss class (or if you missed class), please send me a note at the earliest possible time.
• Due to popular request, I will be shifting the time of quizzes. How does Mondays sound?
• Due to the weather, our schedule has shifted a bit. We hope to have Prof. Kluber visit on Friday.
• Are there questions on Assignment ?
• EC for Thurday's convocation.
• EC for today's Health Fair.
• It's new partner day!

Overview:

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

Raster

• 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 giong on?
• Well, most computers have different techniques for representing really small integers (particularly integers in the range [0..255]) 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 that representation can't read it.