Held: Wednesday, 11 February 2015

Back to Outline 13 - Raster Graphics and RGB Colors. On to Outline 15 - Boolean Values and Predicate Procedures.

Summary

Today we explore common ways in which to transform colors, pixels, and images.

Related Pages

• Reading: Transforming RGB Colors
• Lab: Transforming RGB Colors
• Morning EBoard 14 (Source a.m. 14)
• Afternoon EBoard 14 (Source p.m. 14)

Overview

• Review: Color basics.
• Integer-encoded RGB colors.
• Computing new colors from old.
• An example.

Administrivia

• New partners!
• Mentor session tomorrow at 8:00 p.m.
• Review sessions tomorrow at 9:00 a.m. and 1:15 p.m.
• No quiz Friday!

Upcoming Work

• Exam 1.
  • Exam due in electronic form due last night at 10:30 p.m. (free extension until tonight at 10:30 p.m.).
  • Exam due in printed form TODAY (free extension until Thursday at review time).
  • Required epilogue due TONIGHT at 10:30 p.m. (free extension until TONIGHT at 11:00 p.m.)
• No lab writeup for today (no matter what it currently says online)
• Reading for Friday
  • Boolean Values and Conditionals
• HW4 due next Tuesday (to be distributed later tonight): Color and Image transformations
  • You will write color transformations, like you are doing in class today.
  • You will learn how to apply them to images.
  • Your partner is your lab partner from today's class

Extra Credit Opportunities

Academic

• TEDx Grinnell, February 21

Peer Support (Morning Section)

• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m. Musicals.
• Swimming, Conference Championships, February 13-15, The Osgood

Peer Support (Afternoon Section)

• Swimming, Conference Championships, February 13-15, The Osgood
• Men's Basketball vs. Knox, next Wednesday at 7:30 p.m., New Darby

Recommended, But no EC

• 2 <3 GC day next Wednesday.

Review: RGB Colors

• A standard way to represent transmissive colors.
• Think of it as a combination of three colored lights of varying intensities
  • Red
  • Green
  • Blue
• Why these colors? It just works that way.
• How do we measure intensities?
  
  • Some people like a 0-1 scale
  • Computers like a 0-255 scale (powers of two rock!)
• Important procedures:
  • (irgb-new red green blue) - create a new color
  • (image-set-pixel! image col row color) - set the color of a pixel
  • (image-get-pixel image col row) - get the color of a pixela
  • (irgb-red color) - get the red component
  • (irgb-green color) - get the green component
  • (irgb-blue color) - get the blue component
  • (color->rgb-list color) - get a list of the three components
  • (color->irgb color) - convert some representations to the RGB representation.

Transforming Colors

• Today we are exploring ways to transform colors.
• Why?
  • Deepens our understanding of colors.
  • We can apply a color transformation to a pixel in the image.
  • We can apply a color transformation to each pixel in an image. (More on that tomorrow.)

An Example

• MediaScript comes with a variety of built-in color transformations.
• However, we can also define our own (yay!).
• The normal framework for a color transformation is

(define color-transform
  (lambda (color)
    (irgb-new ___ ; computation of red component
              ___ ; computation of green component
              ___ ; computation of blue component
              )))

• Let's consider a simple one: We'll keep only the blue component of an image.
• How would we write this?

(define only-blue
  (lambda (color)
    (irgb-new ___
              ___
              ___)))

• Some tests
• While we'll spend today using this idea with pixels, for this example, we'll explore the effect on a sample image.