CSC151.01 2009F Functional Problem Solving : Assignments

Assignment 3: Drawing Generally and Concisely

Due: 10:00 a.m., Wednesday, 16 September 2009

Summary: In this assignment, you will experiment with drawing shadows and checkerboards. You will also write a procedure to draw an interesting picture that is scaled to a given width and height.

Purposes: To further practice with two different image models: GIMP tools and drawings-as-values. To consider concise algorithms for creating images with repetitive elements. To consider methods for scaling images to a given width and height. To gain experience writing procedures.

Expected Time: Two to three hours.

Collaboration: We encourage you to work in groups of size three. You may, however, work in a group of size two or size four. You may not work alone. You may discuss this assignment with anyone, provided you credit such discussions when you submit the assignment. If you work in a group, please collaborate on every problem - do not break up the work so that one person works on problem 1, another on problem 2, and another on problem 3.

Submitting: Email your answer to . The title of your email should have the form Assignment 3: Drawing Generally and Concisely and should contain your answers to all parts of the assignment. Scheme code should be in the body of the message.

Warning: So that this assignment is a learning experience for everyone, we may spend class time publicly critiquing your work.


You will need some of the code from the lab on procedures to test the examples given in this assignment.


Problem 1: Adding shadows

a) Define a procedure that corresponds to the following documentation. Include this documentation along with your definition of the procedure.

;;; Procedure:
;;;   shadow
;;; Parameters:
;;;   drawing, a drawing
;;; Purpose:
;;;   Create a shadow for drawing.
;;; Produces:
;;;   my-shadow, a drawing
;;; Preconditions:
;;;   No additional.
;;; Postconditions:
;;;   my-shadow is the same size and shape as drawing.
;;;   my-shadow is colored grey.
;;;   my-shadow is shifted up by 3 units and left by 6 units
;;;     relative to the position of drawing.

Here are some examples of applying this procedure.

(shadow red-eye)

(shadow (drawing-hshift blue-i 10))

b) Define a procedure (pair-with-shadow drawing) that groups a drawing together with its shadow. The shadow should appear behind. For example,

(pair-with-shadow (drawing-hshift (drawing-vshift red-eye 20) 20))

(pair-with-shadow (drawing-hshift (drawing-vshift blue-i 20) 20))

Problem 2: Drawing a checkerboard, concisely

Using the drawings-as-values model, write a program to draw a checkerboard: that is, an 8x8 grid of squares in alternating black and red colors. Your code should take advantage of the many similar elements in this figure.

Recall that the drawings-as-values model includes procedures such as drawing-scale, drawing-hshift, drawing-recolor, and drawing-group.

Problem 3: Drawing interesting pictures, generally

You have already written several programs to create interesting pictures using the GIMP tools. For example, you wrote programs to make a smiley face and a house for the lab on scripting the GIMP tools. You also wrote a program to create a picture of your own design in homework 2. Recall that the GIMP tools procedures include context-set-fgcolor!, image-select-ellipse!, image-fill-selection!, and so on.

Take one of these three programs: your program to create a smiley face, your program to create a house, or your program from homework 2. Copy and paste it into the body of a new procedure, like so:

(define your-proc-name!
  (lambda (image width height)
     ; Your code for making a picture with GIMP tools goes here

Delete the line of code that defines a new image. You should not define a new image inside the procedure, because the image is one of the parameters. You may need to change the procedure definition so that the parameter name for the image matches the name of the image in your code.

Try calling your procedure. If you originally made a 200x200 image, you would call your new procedure as follows:

> (define canvas (image-new 200 200))
> (image-show canvas)
> (your-proc-name! canvas 200 200)

You should see your picture of a smiley face, or whatever, appear in the image.

But what if you wanted to make a picture that was some other size, and not 200x200? That is where the width and height parameters come in.

Modify your code, still using the GIMP tools procedures, so that the numbers in your code are related to the width and height parameters. For example, if your original program selected a 100x50 ellipse in a 200x200 image, you would modify the call to image-select-ellipse! to instead select an ellipse with the dimensions (* 0.5 width) by (* 0.25 height).

You may alter your original instructions to make them simpler, for example, changing the size and location of elements or removing elements altogether.

Your procedure may take other parameters if you wish, but this is not required. If you add extra parameters, please provide informal documentation explaining how we should use your procedure.

For example, suppose we defined a procedure called sams-smiley! that generalizes the original smiley face from the lab on scripting the GIMP tools. The generalized version takes a width and height as input and draws a simple smiley of that width and height.

Here are some examples of using sams-smiley! to make a smiley face in a 200 x 200 image.

(define canvas (image-new 200 200))
(image-show canvas)
(sams-smiley! canvas 200 200)

(define another-canvas (image-new 200 200))
(image-show another-canvas)
(sams-smiley! another-canvas 100 150)

Note that although the second smiley is much thinner and slightly shorter, the brushes used to draw it are the same. Changing brushes based on image size is beyond the expectations of this assignment.

Important Evaluation Criteria

The first criterion we will use in evaluating your work is correctness. In particular, we will check to ensure that each program or procedure generates an appropriate image.

The second criterion we will use in evaluating your work is conciseness. That is, we will look to see whether your code is short or long for the problem. We will not differentiate short and long variable names, so please use comprehensible names.

The final criterion we will use in evaluating your work is generality. That is, for questions where you are asked to write a procedure, we will look at whether your procedure behaves correctly in a variety of situations.

Extra credit opportunity

We will award extra credit for the most concise code to draw a checkerboard.

Creative Commons License

Samuel A. Rebelsky,

Copyright (c) 2007-9 Janet Davis, Matthew Kluber, Samuel A. Rebelsky, and Jerod Weinman. (Selected materials copyright by John David Stone and Henry Walker and used by permission.)

This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.