CSC151 2007S, Class 37: Color Palettes

* Normal start-of-class time for discussion of campus issues.

Admin:
* EC for
  + Attending Wednesday evening Con Brio concert (DEFINITELY 9:30 in the pub).
  + Attending Thursday noon applied studio recital in Sebring-Lewis.
  + Attending Thursday 4:15 talk on Calculus for Dogs.
  + Attending Friday evening talk by Patch Adams.
  + Attending Saturday evening 10 p.m. RHPS (Jillian IS Columbia)
* Given the slew of email I received last night, I expect that we should 
  talk about Assignment 8.
* Reading for tomorrow: Vectors.

Overview:
* Saving space by approximating colors.
* One strategy: Web-safe colors.
* Another strategy: Color palettes.
* Choosing palette colors.
* Using different palettes.
* Lab!

==Assignment 8==

Approaching the problem:

* Kernel has *four* parameters
  * Current column
  * Current row
  * Color most recently seen
  * Count of times we've seen that color
  (let kernel ((col _)
               (row _)
               (color _)
               (count _))
    ...
* Four possibilities
  * Reached end of row
    Move on to beginning of next row
  * Current row and column suggest that you've reached the end of the  
    image, so ... 
  * Color at current (col,row) matches current color.  
    Recurse on the  next position, incrementing the count and color
    (kernel ...
            (+ count 1))
  * Color at current (col,row) is a new color, so ...
    Write the previous information (color and count), APPROXIMATELY
    (write-rgb color)
    (write-int count)
    Recurse with: Next column, same row, color just seen, count of 1
    (kernel (+ 1 col)
            row
            (image-get-pixel image col row)
            1)
==Context: Representing Colors with Fewer Characters==

* Storing images in files
* Attempting to make more concise
* To make files more concise, SACRIFICE
  * Computation time
  * Human readability
* Today, Sacrifice accuracy for even smaller files
* Goal: represent each color with ONE number between 0 and 255.
* Strategy: Approximate colors

==Strategy: Web-Safe Colors==

* Component to level Turn 0..255 into 0..5
  * Multilply by 5/255 and then round
* Turn three levels into an integer between 0 and 255
  * Treat as three digits in a base 6 number
     rgb
     36*r + 6*g + 1*b

* Now, given one of these numbers, how do we go in the reverse direction?
  * Extract levels from the integer
    We have 36*r + 6*g + 1*b
    * Red: Throw away the 6*g + 1*b and then divide by 36
      * AKA divide by 36, rounding down: quotient
    * Green: Throw away 36*r and 1*b and then divide by 6
      * AKA modula by 36 (or subtract red-just-computed * 36)
        quotient by 6
    * Blue: Throw away 36*r + 6*g
  * Turn levels (0..5) back into components (0..255)
    (let ((red-compononent (* red-level (/ 255 5))))
  * Turn components back into colors
    (rgb-new red-component green-component blue-component)

==Strategy: Color Palettes==

* Instead of starting with the computed set of colors above, choose the
colors based on the image
* List all the colors in your chosen set
* The list gives an encoding: To encode the nth color, use the value n
* To decode, use list-ref
* A LOT LIKE CHARACTERS

* What if you want to encode a color not in the list?
  Use the code of the closest color in the list
* HOw do we find that closest color?
  * Generalize problem from last exam

==Lab Notes==

* HA HA HA.  We had no time left for lab.