Author:

• Name: Mauro Persano
  Location: BR - Federative Republic of Brazil (Brazil)

To build:

    make

There is an alternate version of this entry which should work for Windows by setting binary mode on stdout. See Alternate code below.

To use:

    ./persano p q [num frames]

Try:

    ./try.sh

Open the GIF files with a viewer capable of displaying animated GIFs and watch closely at the bottom of the picture (you may need to use gifsicle - a great tool to manipulate animated GIFs - to slow down the animation and to compress the GIFs generated by the entry).

Alternate code:

This (untested as we have no Windows system) version uses the Windows API functions _setmode() and _fileno(3) to set binary mode on stdout which should let it work with Windows.

Alternate build:

    make alt

Alternate use:

Use persano.alt as you would persano above.

Alternate try:

    ./persano.alt 3 16 > knot3-16.gif

Follow the same instructions, using different tools if necessary, as the above try section.

Judges’ remarks:

If your system is one of those that distinguish between text and binary streams, you must know what to do to switch the standard output into binary mode (e.g., adding a line to the source code or specifying a linker option).

What needs to be done to fix the problem?

Solving the author’s exercises below may or may not help.

At the time of judging, the approximation of π used in the program could not be found on https://mathworld.wolfram.com.

Author’s remarks:

knot writes animated GIF images of rendered torus knots to standard output.

SYNOPSIS

    ./persano p q [num frames]

DESCRIPTION

A (p,q)-torus knot is a closed, non-intersecting curve on the surface of a torus in R3 that winds p times around one cycle of the torus and q times around the other. p and q should be coprime; otherwise, the resulting curve will not be a knot (that is, imagining the resulting curve as a loop of rubber, there will be a way to bend, twist and stretch it, without cutting it ¹, until it ends up looking just like a circle embedded in R3 ²).

Sample usage:

    ./persano 3 2 1 > foo.gif

writes a non-animated GIF (a single frame) stream of a rendered trefoil knot to the file foo.gif.

    ./persano 5 2

writes a GIF stream with a rendering of Solomon’s Seal knot to standard output; since the number of frames is not specified, an animation with 40 frames will be written.

Please view the code with 8-character tabs!

OUTPUT FORMAT

The program writes fully valid GIF 89 streams.

It is suggested that the output be viewed with a program capable of displaying animated GIFs, such as almost any graphical HTTP browser.

Due to code size constraints, full LZW compression was not implemented ³. In fact, the output images are even somewhat expanded, thanks to GIF overhead.

The GIF format supports only a limited number of bits per pixel, so the program uses ordered dithering to emulate full RGB output.

PORTABILITY

The program was tested on Linux, with gcc 3.1; Cygwin, with gcc 3.3; and Solaris/SPARC, with Sun’s C compiler.

The only major portability issue should be reliance on ASCII.

It should also be mentioned that the program won’t work on systems that distinguish between text and binary streams; however, since hardly anyone uses such systems in this day and age, this is a minor issue. ⁴

EXERCISES FOR THE READER

1. (1 point) The surface is composed of many small polygons. Try to make it smoother by increasing the number of polygons.

2. (2 points) Figure out how to change the radius of the tube cross section.

3. (5 points) Locate the famous transcendental constant π in the code.

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Footnotes

1. You can call such a transformation an “ambient isotopy” if you want to impress people at parties.↩︎

2. Also known as the “unknot”, which sounds slightly less impressive than “ambient isotopy”.↩︎

3. Unfortunately (for the author), the convenient excuse that LZW compression was not implemented in order to avoid the Unisys patent can no longer be used.↩︎

4. ;)↩︎

Primary files

• persano.c - entry source code
• Makefile - entry Makefile
• persano.alt.c - alternate source code
• persano.orig.c - original source code
• try.sh - script to try entry

Secondary files

• 2005_persano.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

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