Using the Functional Graphics Library with Gambit

Operating principle

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.

Extensions

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
  (resize-image
   (filled-triangle 0 1 0 -1 1 -1)
   25))

(define test-bb
 (filled-triangle 0 1 0 -1 1 -1 25))

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