This programs draws a perspective view of an object that has been defined as a wire frame and analyzes the image to remove the hidden lines.
Flexibility in choosing how to display computer-generated three-dimensional drawings has become increasingly important in recent years. A major consideration is the enhancement of the realism and aesthetics of the presentation. A polygonal representation of objects, even with hidden lines removed, is not always desirable. A more pleasing pictorial representation often can be achieved by removing some of the remaining visible lines, thus creating silhouettes (or outlines) of selected surfaces of the object. Additionally, it should be noted that this silhouette feature allows warped polygons. This means that any polygon can be decomposed into constituent triangles. Considering these triangles as members of the same family will present a polygon with no interior lines, and thus removes the restriction of flat polygons.
SILHOUETTE is a program for calligraphic drawings that can render any subset of polygons as a silhouette with respect to itself. The program is flexible enough to be applicable to every class of object. SILHOUETTE offers all possible combinations of silhouette and nonsilhouette specifications for an arbitrary solid. Thus, it is possible to enhance the clarity of any three-dimensional scene presented in two dimensions. Input to the program can be line segments or polygons. Polygons designated with the same number will be drawn as a silhouette of those polygons.
This program was released by NASA through COSMIC as ARC-12721. The italicized text above is from the official NASA release.
The program SKETCH (later superceeded by SILH), written by David Hedgley of the NASA Dryden Flight Research Center has long been famous as the most widely distributed code by the NASA service known as COSMIC. All over the world, people have obtained this program and adapted it to the task of displaying three-dimensional objects as wire-frame pictures in perspective with hidden lines removed. Today, the leading-edge computer graphics wizards are making color pictures with continuous shading that are remarkable in their realism. But, for those of us with black and white printers, wireframe is still the best way to show an object like an airplane so that it can be visualized.
I remember jumping at the chance to use this program in 1982, when I was looking for a way to check the input for an aerodynamic panel code known as PANAIR. I composed a simple program that read a PANAIR input file and called SKETCH to produce the scene. That program is the basis of the program in this collection.
Since that time, NASA has settled on a single consistent format for describing wireframe objects which is referred to as the Langley Wire Frame Geometry Standard (LaWGS). This program HLP (for hidden line program) is an update of the old program (called PANSKETCH) that I wrote, now updated to read LaWGS files and use the newer SILH (for silhouette) program from David Hedgley. The output is a plot file, encoded for gnuplot, of the object in question.
A supplementary program called PrintHLP is also provided that will convert this gnuplot file into appropriate format for printing on a PostScript printer or on a Hewlett-Packard compatible printer.