Held: Monday, 10 February 2014

Back to Outline 12 - Raster Graphics and RGB Colors. On to Outline 14 - Transforming Images.

Summary

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

Related Pages

• Reading: Transforming RGB Colors
• Lab: Transforming RGB Colors
• EBoard (Source) (HTML) (PDF)

Overview

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

Administrivia

• Tuesday's reading: Transforming Images
• Lab writeup: Exercises 5ace and 6ad
  • Subject: CSC 151.02 Writeup 9: Transforming RGB Colors (YOUR NAME)
• Warning! I may not be able to hold a review session this week. I'll keep you posted.
• Both the graders and I are a bit behind. I expect that I'll get you grades before tomorrow's class.
• We should probably talk for a few minutes about Friday's quiz.
• Extra credit:
  • Thursday extra, Thursday @ 4:30 p.m., Sci. 3821, Red/Black trees.
  • CS Table Friday @ noon: Law, Order, and Computers

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.