CSC151 2007S, Class 05: Tools for Scripting the Gimp

Admin:
* Are there final questions on Assignment 2?
  * I spent too long on it!  THEN STOP!  (And tell me you spent 3 hours)
  * I've forgotten every bit of math I ever learned
    * Partner with someone who does
    * Ask a professional mathematician
    * Ask someone
    * Describe what you want to do approximately
      * Suppose f(x) is a function that determines whether x is in a
        circle. ...
* Reading for Wednesday: Turtle Graphics.
* No office hours this morning.  Sorry.  I will be around this afternoon.
* A few printed labs available, for those who like dead trees.
* Quiz: What stairway will they close next?

Overview:
* A model of images, extended.
* Coding algorithms for drawing.
* Lab

Computer Science is ... the study of algorithms AND data structures
* There are many ways to represent the same kind of information
* ANd to think about generating info
* We will play with lots of image models as a way to think about ways to
  structure and organize information
  * And a way to describe algorithms

[Sam draws a really bad looking face on the whiteboard and asks how
we might describe it]
* (1) As a lot of dots, most of which are white
* (2) As four shapes
  * A circle
  * Two big dots
  * A slightly-tilted half-circle
* (3) In terms of the GIMP commands someone might use to create it
  * Choose the ellipse tool
  * Click at (0,0) and drag to (100,100)
  * Select "Stroke Selection" from the "Edit" menu 
  * ...
* (4) In terms of the turtle commands someone might use to create it
* And in many other ways.

Let's focus on GIMP commands
* Focus is on building the image, not on what the result is
  * Imperative - Sequence of orders to do something

These commands are simple enough that we can represent them as
procedures that tell the computer to do the same thing

* CHoose the ellipse tool and then select the region from
  0,0 to 50,80
  + (image-select-ellipse! ... 0 0 50 80)
* Stroke that selection
  + (image-stroke! ...)

An example (done live)

canvas
> (image-show canvas)
(1)
> (image-select-ellipse! canvas selection.replace 150 150 200 200)
Error: eval: unbound variable: selection.replace 

> (image-select-ellipse! canvas selection-replace 150 150 200 200)
1
> (image-stroke! canvas)
(#t)
> (context-set-fgcolor! "red")
#t
> (image-fill! canvas)
1
> (context-set-fgcolor! "blue")
#t
> (context-set-brush! "Circle Fuzzy (09)")
"Circle Fuzzy (09)"
> (image-stroke! canvas)
(#t)
> (context-update-displays!)
(#t)

== Some Notes ==

* It's okay to make mistakes.  (Note that Sam made mistakes in the
  code above.)
* You often won't see the results of your commands until either (a)
  you click in the drawing window or (b) you type the wonderful
  (context-update-displays!)
* No one (except Sam) knows everything.  Observe how often Max says
  "I'm not sure".
* Whoops ... I guess Sam doesn't know everything
  (image-select-ellipse! image selection left top WIDTH HEIGHT)
* If DrFu starts saying "Success" in response to every command, it is
  very confused.  Except when you load a file, "Success" means
  "I'm broken".  SAVE, quit, and restart.
* Sometimes it's useful to sketch things on paper
* Why are images numbers?  GIMP stores them in a table