The source code for the program to generate the pages is written in the Adobe PostScript language. This is because a jpeg or tiff containing the images at 1200 dpi would be tens to hundreds of megabytes in size and therefore unwieldy and non portable. (Jpegs are bitmaps and generally lossy compression.) Whereas PostScript and PDF are vector representations so the image size in this case is only 10K bytes or so. Very small and portable to a wide range of printers because of the free Acrobat Reader.
The link above is to a wikipedia article with external links to the PostScript Language Reference Manual, tutorials, etc. PostScript is a simple byte code interpreted language (much like Forth) written in postfix notation.
Many higher end printers will accept PostScript files directly, in which case you can bypass the pdf step while debugging, by just queuing the program file to the printer. For wider distribution and to cover all the non-PostScript printers, pdf is used.
Postscript is converted to pdf by Adobe Distiller, part of the full Adobe Acrobat package. (You understand that Adobe Acrobat and Adobe (Acrobat) Reader are separate, with Adobe Acrobat being a boxed product.) You do not need the latest versions of Acrobat (8 at this writing) however. Acrobat 5.0.5 will do nicely, and is frequently available on eBay for $40 or less.
An alternative to buying Acrobat is to use the new Create Adobe PDF Online website. This has a free trial for the first 5 conversions, then you can signup to pay either $10/month or $99/year to do Distiller runs through the website. Still, it seems like the $40 Acrobat 5.0.5 versions floating around may be a better deal.
There is a free version of PostScript related software called GhostScript. It is included with all Linux operating systems and is available also for Windows and Mac. However, the Distiller features of GhostScript appear to be broken for the PostScript language features (Tiled Patterns) used in this image. (The pdf generated is multimegabytes!) So don't attempt to convert this ps to pdf using GhostScript.
There is another free package called PStill from Germany. It looks promising but I havent tried it. It might do the ps to pdf conversion ok. Let me know if you test it.
This is the PostScript source code. The file name is "pni.ps", standing for PostScript (Tiling Pattern) Network of Indra. "ps" is the extension used for PostScript files. Ultimately I think we will setup a wiki with the various versions of the file, if enough people are making modifications to it. But for now it is here. The file below is in plain text format (.txt) so you can see what it looks like without downloading and opening a text editor. After right-clicking and saving it on your desktop, just change the extension to .ps.
After opening the file you will see that it has many options for changing the image characteristics. These are typically set with booleans "true" or "false". The top section of the file is for Distiller setup, then is the section with most of the casual user adjustable features such as tpi (triangles per inch), image type, paper type, etc.
The primary determinant of field strength seems to be the number of triangles on the page. Not their sizes. (This is considering the FOL figure as consisting of rounded edge subtriangles). So if an excellent and sharp quality image exists with no noise or digital artifacts, "high density pages" (20 triangle bases or glyphs per horizontal inch) are about 5 times more field strength than the normal or default "low density" pages (10 glyphs/inch or 'gpi').
Our initial experiments were with the triangle (flat sided) figures, not FOL. With flat sides, 10 and even 20 gpi were possible because of the simplicity of the line segments and how they render into dots. However with FOL, our default density isonly 5 glyphs per inch, to get decently smooth curves at 1200 dpi.
We have found the default images to be plenty strong enough for our purposes, especially when laid out in a 4 column by 8 row matrix under the bed. Even a single layer of these is quite strong. You could double up or more if you wanted to add more pages. But jumping to a five times intensity increase -- that sounds like a rather abrupt increment.
However some people will obviously want to experiment with other densities. Just be careful. At 20 gpi (flat sided triangles), the 1200 and 2400 laser printers we had access to (e.g. Xerox 4110 with a Fiery RIP controller at FedExKinkos) -- had quite noticible noise under a magnifier. This could be termed bit jitter or raster jitter.
The same raster is there at 10 gpi as at 20 gpi, but at 10 gpi the lines and heart image are large enough that the overall percentage of image jitter is negligable. It is just barely noticeable around the edges. But at 20 tpi, the triangle size, line widths and heart image are half the size and then the bit jitter starts to be more serious. If you are exposed to a large stack of jittery images, especially at 20 tpi -- take care. (I've done it.) It is a harsh experience.
Photo printers capable of 2400 or 4800 would likely be able to handle high density 20 gpi. Another type of printer I investigated is the Kodak 5634 DI direct digital offset press (from Presstek originally). These take a pdf file and directly expose an offset plate via laser cutting on the press. The RIP is at 2540 dpi (the metric resolution family.) Since it is offset, liquid ink is used instead of toner particles, so the 2540 resolution should be capable of 20 gpi. The local printer I talked to here quoted me at about $80 for 1000 copies. Not bad! But since I've barely scratched the surface of what 10 gpi (flat triangles) or 5 gpi (FOL) can do energetically, I passed for the moment.
As you can see the intention is rather general, and by following the code it is possible for you to invent your own node symbols and experiment. Or place features elsewhere in the geometry.
The introduction explaining how these work is here. Usage here.