Hacking the VLM

Hacking the VLM
by Jeff Minter

The Virtual Light Machine (VLM) is one of the highest (if not the highest) point of the Atari Jaguar CD-ROM drive. It is an interactive light synthesizer; when the JagCD is used to play audio CDs, the VLM samples the music data and creates corresponding psuedo-random graphic displays. The result? Dynamic, mesmerizing light shows, all matching the music and never quite the same way twice.

In this document, Jeff Minter, the programmer of VLM, reveals a hidden back-door to the VLM that lets ambitious Jaguar owners create their own effects. Of course, this only works if you have an Atari Jaguar and a JagCD...

A Brief Introduction

1: The good news
There is a backdoor left in the VLM which allows the user to get at the edit mode that was used to create the banks of VLM effects. You can get in there and roll-your-own FX, and it's not hard to come up with stuff that is a *lot* better than many of the default FX.

2: The bad news
There is no provision for saving your hacks. They are entirely transient, and you can wave them bye-bye as soon as you switch to another FX bank -- and unfortunately, whenever you open the lid of the Toilet to change the CD, it switches banks into a special Jaguar-logo bank, and it's bye bye hacks. Bummer, I wish it didn't do that, 'coz otherwise I'd just leave my VLM on all the time and evolve 9 banksworth of top hacks...

Also, the edit interface is buggy and pretty user-hostile; its got no error-checking and its possible to crash the VLM, or slow it down to a crawl, if you tweak certain parameters out of range. Hell, it was never intended for end-users, only for use by a certain deranged bovine.

3: Why bother then?
Because it's fun. Because if you're interested, you can find out a lot about how the VLM works. Because a lot of the default FX are fairly nonoptimal, largely 'coz at the time I was designing the banks I had pneumonia and was feeling like shit, and I always find that I make better FX when I feel good. And although you can't save the hacks, you can do what I do, and keep a tape in the VCR and lay down some vid whenever you come up with a particularly nice one.

4: How do I get there?
Easy. Select the effect you want to base your hack on. Then, in VLM-mode, hold down *,1,3,0 on the Jaggi control pad. This should bring up a menu called Spectrum and Triggers. That's not it though -- that was just a red herring to confuse people looking for the hack ;-) While on the Spectrum and Triggers screen, press up, down, up, down... 8 times (so the cursor wraps from Trigger 1 to Trigger 5 and back again 8x) and then the display will change to Edit Mode. At that point you're in.

5: What do I do when I get there then?
Okay, first it helps to have some idea of what is going on inside any particular VLM setting. Each individual setting consists of up to six effects, each of which can take a shitload of parameters, which define how that effect changes over time. Parameters can be attached to waveform generators, to user control from the joypad, or to the spectrum triggers to create an audio-responsive effect.

The most common types of effect you will see in the VLM are:

Digital Video Feedback area. DVF is what gives you all those cool swirly screen-filling patterns and persistence fields. It is, however, quite an expensive effect in terms of proc power (one is throwing rather a lot of pixels around doing scaled rotates of the entire screen)... if you can live with a smaller DVF area on some of the effects the speed and responsiveness of many of the effects can be greatly enhanced. I have a whole new class of 60Hz feedback FX that I could kick myself for not putting in the default banks...
Draw Spectrum As Intensities. This basically plots the output of the FFT through the symmetry generator. On its own, or combined with DVF, this is a good setting to use for effects that you want to be precisely audio-reactive. Put it over some nice DVF or a persistence field, llovely.
Draw a Ring of Pixels. Does just that, through the sym generator, with various parameters applied so you can change phases, number of points, pixel size, etc... a very versatile thing, simple though it is.
Draw Plasma. Draws a tile of plasma on the screen. Useful for putting in the middle of a nice DVF field as a source effect. Can be programmed to flash on trigger band events too. Try hooking oscillators up to the XY position when it's in a 60Hz DVF field -- yummy.

Empty slots cannot be edited. Time was when pushing B on an empty slot would bring up two pages' worth of FX that you could build in there, but sadly that came out. I wish I had never taken it out now. Damn, and I wish I had not removed Save Bank either...

There are a few others, starfields and the like. Play with them. Some of them are quite cool.

Fiddling Around With Stuff

When you press B to edit an effect, you are presented with three choices -- Edit source function, which allows you to see and tweak the parameters that make up the basis of the effect; Edit symmetry generator, which allows you to twiddle with the sym generator, for effects that go through that pipe (not all FX do -- DVF and the plasma don't use the sym gen at all, so changing stuff on theyr sym generator pages does nothing useful); and Edit Source Waves. Each effect has 8 programmable waveform generators which can be attached to any of the parameters defining the effect or the symmetry. Edit Source Waves is where you go to adjust the speed, waveform etc of the waveform generators.

Edit Source Function

If you enter this menu, you will be presented with a list of the variables which control how that effect is displayed. In a DVF effect's Source Function menu, for example, you will see parameters controlling the Window Size, Scale, Rotate Angle, etc. You are free to cursor around this menu, twiddle the values, and see what happens to the display. (It helps to have a CD playing while you fiddle, so that all the effects are active and you can see what you are doing).

Menu items with (X,Y) after them represent pairs of variables. When you select a variable to edit, you will be presented with a slider, a 2-way slider, a position marker, or crosshairs, depending on the effect. Just use the joypad to change the values. Anything you change will stay changed for the life of your hack (that is until you change banks or open the Toilet).

While editing a variable, you may see the message 'Press * to Attach Waveforms'. If you press *, it takes you to the Attach Waveforms screen, which has a totally crappy and confusing UI, but which is nonetheless the key to the cooler aspects of VLM usage. It works in conjunction with the Waveform Edit page, and in a sane and rational world, would have been integrated in with that page.

The wonderfully-informative Attach Waveforms screen presents two rows of the numbers 1-8. The top row represents waveform generators linked to this variable. If one or more of these numbers are highlighted, it means there are generators already linked to this variable.

You can attach or detach a generator to the variable by pressing the corresponding number key 1-8. You can attach as many generators as you like -- the resultant waveform is the sum of all the input waveforms.

You also can adjust the amplitude of the waveform here. Dismally, there is no display actually showing you the amplitude -- you just have to watch the display to see the result and kinda fish about. Also, for some inane reason, if the waveform is attached to an X-component you adjust the amplitude with left/right, and if it is a Y component you use up and down. I think I was planning to put in an actual display of the waveforms here, but I got ill and never finished it off. As it is, it's sucky and counterintuitive. Oh well...

The bottom row of 8 numbers allows you to modulate the attached waveform(s) with the output of an envelope generated by any of the five spectrum trigger generators. You press #, then the number of the trigger 1-5 (6, 7 and 8 are for the three joypad buttons) to toggle the trigger attach. And for some reason, like it was never properly debugged, when you toggle a trigger off, often the digit in the bottom row will not de-highlight. Oops.

After you have attached waveforms, you will probably want to mosey on over to the

Edit Source Waves

menu. Here, you can select a waveform generator by cursoring up and down. While the cursor is on a generator, that wave is displayed in the blue box. You can increase or decrease the frequency with A and C, and slip the phase by Left/Right. You can change the wave type by pressing a number on the numeric pad. User X and User Y translate to the 'position' controlled by the joypad in VLM Interactive mode.

Edit Symmetry Generator

is a very interesting place to be for all those particle effects and anything else that goes through the sym generator. In this menu you can twiddle, attach waveforms to and generally piss about with everything to do with symmetry.

The sym generator uses two basic kinds of symmetry, planar 8-way reflection, which is quick, and rotational, which is very nice but considerably slower. The two types can be combined. Many of the items on the Symmetry page only apply to Rotational symmetry, and will have no effect if Rotational is switched off.

You set the sym types under the Symmetry Type menu, not surprisingly. Pressing 9 on this page toggles rotational sym off and on, and due to quite possibly the same bug as on the Waveform Attach screen, often the '9' in the middle will be incorrectly highlighted. It's usually pretty obvious when rotational is on though. Keys 1-8 toggle on and off the planes of the 8-way symmetry. Set up your types, and then go and have a fiddle with the parameters as in the Edit Source Function mode.

Since you can't add or delete effects, the nature of your hacks will be in part defined by what effect types are in the bank you choose to edit, so if you want to do a lot of DVF stuff, try bank 3 or 4, and if you like particles, bank 1 or 2, and so on. You can switch between, and hack, all effects within a bank, but if you change banks they are lost. Be careful!

And, basically, there we have it (as I said to Flossie as I led her into the concealing darkness of the sheep-shed). Enough stuff to get you started playing about with VLM editing. Apologies for the crappy UI, but I never had time to really bring it up to scratch before I got that bloody pneumonia. As it is it was about a year between finishing the VLM code and the release of the CD-ROM, and I wish I had been able to spend a few more months at it. Both the FX and the edit mode could have been a lot nicer!

Oh well, next time... I've already got some awesome stuff running on [closes his muzzle and remembers the three letters N, D and A] ;-)

and oh yeah, I better mention:

CAVEATS: Here Be Bugs'n'Beasties!

The error checking on the UI is at best sucky and at worst nonexistant. It is quite possible to either choke up the VLM with some incredibly intensive sym mode that will have it doing one frame per Sunday, and you can even kill it with an honest to Ghu, thank-you-and-goodnight, little-silicon-legs-in-the-air crash. There are a few danger areas which I shall warn you of now:

Positioning a DVF window too far off the edge of the screen can kill the system.
Attaching waveforms to DVF window size and position can be fatal. Look out!
Attaching waveforms to the Rotational Symmetry Order can cause it to wrap to negative, whuch translates to something greater than 32767, which, as a sym order, will cause the system to choke most heinously. Won't kill it, but the frame rate will be measured in minutes per frame, hehe...

Don't worry, killing the system will not do any permanent damage. The default banks are tucked away snug in ROM and will be restored when you restart the system, no matter how badly you take it down.

Well, I shall go and post this now... have fun and Happy Hacking!

\ (:-) - the Beastly Boanthrope /

[ Back to the Atari Archives ]