Class 35: Complete Scripts

Held: Wednesday, 2 April 2003

Summary: Today we consider how to write procedures for Script-Fu.

Related Pages:

• EBoard.
• First Reading on Script-Fu.
• First Script-Fu Lab.

Notes:

• Are there questions on Exam 2?
• No lab today. We'll work in small groups and then as a class.
• No reading tonight. Spend your time on the exam!

Overview:

• Writing Script-Fu Procedures.
• An example: Drawing a circle.
• Project: Drawing a Random Circle.
• Project: Creating a New Image.
• Project: Drawing a Star.
• Project: Tracing a Picture.
• Project: Choose Your Own.

Procedures in Script-Fu

• As you should know from your long experience with Scheme, it is relatively easy to group a number of individual commands together into a procedure.
• Hence, rather than typing instructions one-at-a-time into the Gimp window, we can
  1. group the commands within a Scheme procedure;
  2. put the procedure definition in a file;
  3. load the file in the Script-Fu console; and
  4. execute the procedure.
• Unfortunately, the Gimp does not provide an editor for Script-Fu. Hence, you'll need to write them in DrScheme.
  • You can't run them in DrScheme, though.
• We'll do some examples together.

An Example: Drawing A Circle

• Some folks don't like describing circles with left/right/width/height.
• They'd rather describe them with center/radius.
• Let's write a procedure to draw a circle.

  ;;; Procedure:
  ;;;   circle
  ;;; Parameters:
  ;;;   image, the id of an image
  ;;;   layer, the id of a layer on that image
  ;;;   x, an integer
  ;;;   y, an integer
  ;;;   r, an integer
  ;;; Purpose:
  ;;;   Draws a circle centered at (x,y) with radius r using the
  ;;;   current foreground color and brush.
  ;;; Produces:
  ;;;   Nothing, or so I hope.
  ;;; Preconditions:
  ;;;   image and layer define an existing image and layer
  ;;;   x, y, and r are all non-negative
  ;;; Postconditions:
  ;;;   The specified layer now contains a circle with the specified
  ;;;   dimensions.
  (define circle
    (lambda (image layer x y r)
      (gimp-ellipse-select image (- x r) (- y r) (+ r r) (+ r r) REPLACE 0 0 0)
      (gimp-edit-stroke layer)
      (gimp-selection-none image)))
  

Some Projects

• Working in small groups (to be assigned), work on the following questions. You don't need to write working code; simply think about the structure.
  • We'll stop ever five minutes or so to discuss the next project in the sequence. If you finish early, move on to the next one.
• Unpredictable Circles: Express our dada-ist tendencies by drawing a circle in a random color at a random place with a random width using a random brush.
• Image Creation: Because it's a pain to type the dozen or so commands that we use to create and display a new image, we might want to create a procedure, (create-image width height), which creates a new image of the specified width and height.
  • Return a list that consists of the image-id and the layer-id of the new image.
• Star Drawing: Figure out how to draw a five-pointed star given a center and a radius (distance from center to point).
  • You do not need to express this in Script-Fu, but you should think about how to compute the location of the points of the star.
• Tracing Existing Images: Draw a line in the current foreground color around anything in the current image.
• Choose Your Own: Think of some interesting task like those we've just discussed.