Held: Tuesday, 2 February 2016

Back to Outline 05 - RGB Colors. On to Outline 07 - Transforming Images.

Summary

Today we explore common ways in which to transform colors.

Related Pages

• Reading: Transforming RGB Colors
• Lab: Transforming RGB Colors
• EBoard 6 (Source 6)

Overview

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

Administrivia

• Continue partners!
• Quiz 1 returned.
• Yes, I really do try to respond to questions via email and text. (At least I think I've done so so far.)

Reminders

• Office hours: MTWF 10-11, Tu 1-2.
  • Sign up at http://rebelsky.youcanbook.me.
  • Also feel free to stop by when my door is open.
  • Or to email me for an appointment.
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Weekly review sessions:
  • Wednesday at 8pm in the CS Commons with Zacharcy
  • Thursday at 10am in this room with SamR
  • Thursday at 8pm in the CS Commons with Kumar

Upcoming Work:

• Readings for Wednesday:
  • Transforming Images
• Assignment 2 due TONIGHT! at 10:30 p.m.
• Lab Writeup: Exercise _
  • Send email titled CSC 151 Lab Writeup 6 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Friday.

Extra Credit

If you know of other events, please suggest them! (I might not always agree, but it never hurts to ask.)

Academic

• Community Conversations on Race, 7:30 p.m., TONIGHT Lazier 3rd Floor Lounge
• Next week's Rosenfield Symposium on Campaign Finance Reform.

Peer

• Track and Field Grinnell Invite February 6. (10 a.m.)
• Swimming February 6 vs. UN and IAState. (1 p.m.)

Questions from Sam

What does round do?

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.)
• MediaScript comes with a variety of built-in color transformations.

Building New Transformations

• How do we build our own transformation?
• We'll learn a variety of techniques over the next few classes.
• Right now, our first technique is composition.

An Old Example

This example comes from when we covered procedure definitions before covering colors.

• 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.