Using the Functional Graphics Library with Gambit

The unix-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. This version of the library only works on unix-like systems (such as Linux), hence the other part of the name.

Operating principle

This library displays an image by writing it out as an Encapsulated PostScript (EPS) file and then invoking a seperate program to display the file. Normally this is unobtrusive, since the displaying is automatically done whenever the read-eval-print loop encounters a value that is an image, and because the file is deleted after it has been displayed. However, you need to be aware of what is going on behind the scenes in order to tailor the library to the requirements of particular installations, and in order to help troubleshoot any problems that arise. Each time a value resulting from a top-level evaluation is an image, the following steps occur:
  1. A filename is created of the form imageN.eps, where N is replaced by the number of of the image (first 1, then 2, etc.). The file is in the current directory.
  2. The image is written out to that file in EPS format, using the save-image-as-epsf procedure described later.
  3. The ghostview program, a publically available PostScript previewer, is invoked to display the file. Gambit continues without waiting for ghostview, but arranges that when the user has quit out of ghostview, the rm program is used to delete the file. It is easy to change the commands used in the library, for example to specify a different previewer.
  4. Gambit displays something like #[image 1 100x100] as a textual version of the value. The number 1 in this example corresponds to the filename image1.eps, and can be used to tell which ghostview window is displaying which image, if you have several up. The 100x100 in the example is the size of the image.


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

(save-image-as-epsf image filename)
This procedure writes the image out to an Encapsulated PostScript file with the given filename.
(ps-image PostScript-string)
This is an advanced feature that allows you to make an image with any arbitrary string of PostScript code used to specify the contents.
(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 point, i.e., 1/72 of an inch.
(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, filled-triangle, and ps-image.

The images produced by line, filled-triangle, and ps-image are of a size determined by the value of the name default-image-size. Initially this name is defined as 100. This is a compromise size, a bit small for the fractals in chapter 4 and rather 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. For example, when defining the basic blocks for quilting, it may make sense to do so as in the following example:

(define test-bb
   (filled-triangle 0 1 0 -1 1 -1)
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:
(define test-bb
 (filled-triangle 0 1 0 -1 1 -1 25))
You can specify a size in this way with line and ps-image 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