Using the Functional Graphics Library with EdScheme for the Macintosh

The fungraph.scm library provides the procedures for producing graphical images described in Concrete Abstractions. Because these procedures operate like mathematical functions, we refer to them as functional graphics, hence the name fungraph.

Making images appear

Contrary to the book, images are not automatically displayed. For example, you can not simply evaluate (line -1 -1 1 1) to see a diagonal line appear on your screen. Instead, you have to explicitly apply the draw procedure to the image, as in (draw (line -1 -1 1 1)). Note that this wrapping of draw around images should only be done for top-level evaluations typed in by the user, not internally to procedures that operate on images. For example, in Chapter 1's application section, you might do (draw (pinwheel rcross-bb)) to see what a pinwheeled rcross-bb looks like, but you wouldn't use draw anywhere in pinwheel. Pinwheel still constructs an image just as in the book; the only difference is that you now need to make that resulting image appear on the screen by using draw.


In addition to the procedures described in the textbook, there are three additional ones:

(draw image)
This is described above.
(mirror-image image)
Like quarter-turn-right or invert, this takes an image and makes another, related image. In the case of mirror-image, the new image is the same size as the original, and is formed by flipping the original image around a vertical axis, as though it were viewed in a mirror.
(resize-image image width height)
This makes a new image similar to the original, but stretched or shrunk as necessary to have the specified width and height. This may also imply a change in the image's proportions. The width and height must both be exact, positive integers. The unit of measure is the "turtle step" defined by EdScheme.
(resize-image image size)
This is a shorthand, equivalent to (resize-image image size size), i.e., the specified size is used as both the width and the height of the new image.
(resize-image image)
This is a further shorthand, equivalent to (resize-image image default-image-size default-image-size), i.e., the width and height are both taken as the value of default-image-size. As described below, this is the same name that controls the size of the images created by line and filled-triangle.

The images produced by line and filled-triangle are of a size determined by the value of the name default-image-size. Initially this name is defined as 60. This is a compromise size, rather small for the fractals in chapter 4 and a bit large for each basic block of the quilts in chapters 1-3. You can redefine it to another value (which must be an exact, positive integer) to suit your own needs. Or, you can use the resize-image procedure described above. Or, as a final option, you can specify a size as an additional argument to the basic image construction procedures, as in the following example:

(filled-triangle 0 1 0 -1 1 -1 25)
You can specify a size in this way with line as well as filled-triangle, and can also specify two values, for width and height, as with resize-image.

The overlay and stack procedures are not restricted to two arguments, but rather can take one or more.

For more information, see the parent web page, or contact Max Hailperin:
Mathematics and Computer Science Department
Gustavus Adolphus College
800 W. College Avenue
St. Peter, MN 56082