Outline 06: Transforming Colors

Held: Monday, 7 September 2015

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

Overview

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

Administrivia

New partners! Please grab a card to identify your computer.
Happy Labor Day!
Quiz 1 returned. If you had trouble on part 1, please try to speak with me.
I've added additional office hours at 8:30-9:00 M-F to give you more opportunities to speak with me. Still bookable at https://rebelsky.youcanbook.me.
Before today's lab (and perhaps before every lab you do for the next week or so), please type the following command in the terminal window.
```
/home/rebelsky/bin/csc151-update
```

Upcoming Work

One reading for Tuesday: Transforming Images
Assignment 2 due Tuesday.
- I will take questions on the assignment at the start of class today and Tuesday.
Lab writeup (due before class Wednesday)
- Email to grader-151-01@cs.grinnell.edu (not to me; don't forget the cs).
- Subject: CSC 151 Lab Writeup Class 6 (Your Names)
- Problems:

Extra Credit Opportunities

Academic

Tuesday, noon, CS Table, JRC 224C. Mobile networks in developing countries.
Convocation, Thursday. (Sam's convocation spiel.)

Peer Support

Pals of PALS, pals@grinnell.edu, normally Saturday at 1:30 and Mondays at 4:45. Requires sign up in advance.
Pun Club, Saturdays, 4pm, Way over Younker

Notes on Quiz 1

Quizzes are ...
- An opportunity for me to discover what you do and don't understand.
- An opportunity for you to discover what you do and don't understand.
- An incentive for you to review materials.
- Intended to be relatively "low stakes".
Note 1: Every real number in DrRacket is also rational.
Note 2: ceiling gives an inexact output when given an inexact input.
Note 3: Although I think of the card as the input to the algorithm in problem 2, I accepted anything that was argued well.

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.