# Class 10: Raster Graphics and RGB Colors

Back to Laboratory: Writing Your Own Procedures. On to Transforming Colors.

This outline is also available in PDF.

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:

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.

Back to Laboratory: Writing Your Own Procedures. On to Transforming Colors.

Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

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Samuel A. Rebelsky, rebelsky@grinnell.edu