Sunday, April 4, 2021

Thoughts on spells - Sunlight and small-d darkness effects

Over the years I've revised some GURPS Magic spells for my DF Felltower game.

The Sunlight spell is an especially weird spell, in my experience.

Part of this is because of how games treat darkness - darkness is more of a tangible thing, like smoke, just without the actual smoke. It looks black, you can't see through it, it can have a shape and limits. It's like squid ink in air, really, just not substantive. Light, though, we know diffuses out over and area, and it is brighter near the source and less out from it, and can be seen from far away.

Darkness, thus, works not at all like in reality, where it is an absence of light, but we expect light to work just like in reality. It creates a lot of confusing and conflicting expectations.

Add in, then, the sunlight spell. As written, it creates "real" sunlight, for purposes of anything that is affected by sunlight.

It also creates a spell that extends upwards as far as possible - the sky, to clouds above, to a ceiling if cast inside or underground. But does it extend down, if cast higher up? The spell doesn't say yes or no.

If yes, you get delvers casting the spell on a hole in the ground at -0 for range and getting a shaft of sunlight that extends up and down as far as it can. Got a nearly bottomless pit? Why drop down an object with Continual Light on it when you can spend 2 energy and cast Sunlight and shine a shaft of light all the way down to the bottom and harm any vampires intersected by the path as a bonus. Not sure where the roof is? Cast sunlight and it'll extend upwards, too. The latter is clearly not as big of an issue as the former can be. Especially when the former goes through a No Mana Zone - is it natural sunlight, and thus ignores it, or magical, and thus ends on it and reveals it?

If no, you get the oddness of a hard stop of sunlight on nothing. That violates the way light operates in the game otherwise, where it comes from a source and diffuses out.

There isn't an easy solution here.

- If I treat Sunlight like Darkness, it is just a small area of light with sharply defined boundaries but is totally inconsistent with everything else we do with light.

- If I allow it to extend up and down, but it's really annoying that it does. It becomes a really effective flashlight and people "explore" by shining a perfect, -0 to see light down to look around.

- I could substantially change the spell to create a sunlight source, much like Continual Light, but then people will cast it on rocks and carry sunlight around.

- I could allow it only to be cast at the terminus of light. So, no shining down by casting it up in the air to look down. This has the benefit of being as written even if not as intended. You simply couldn't cast it if you weren't on the ground or some other ground surface.

I'm not sure how I want to deal with this. I think the last one might be the only way to "easily" fix the spell without having the weirdness of effectively always being only horiontally-penalized for distance as it'll always been up and down.

I'll have to give it some thought, as it's been a minor but annoying issue for all of us in the DF Felltower campaign.


  1. Well, this is obviously a spell that can have its assumptions different depending on how you want to set up the cosmology.

    With real light, if you assume plane wave (to simplify the math), light has a direction and illuminates surfaces based on that. You have shadows, and different intensities depending.

    The light from sunlight does not seem to match any sort of realistic source, otherwise you would have shadows, and problems filling an infinite space at constant intensity with a finite source.

    So, what is doing is defining a volume, and the free space within that volume is acting as a source. A weird source. 1. It is omnidirectional like a point source. 2. But, despite being a volume, it is not 'adding up'/integrating to become a larger directional source. 3. You /can't/ look at the effects from a distance, because it doesn't propagate outside the volume.

    So the basic question is not "why doesn't it behave like real light", but the magical constraints on defining the volume. Around the caster and up can be justified in a number of thematic ways. Symbolically, the sun is up even when your cosmology says the sun is on the other side of the world. Mechanically, up is opposite gravity. If you want trouble, the caster is bouncing something that radiates off of the ground, and that invisible radiation is defining the volume. Extra trouble, the caster is also casting the reflective field over the grounds surface, the radiation reflected by the field, and the thing that turns the radiation into the volume of illumination.

    Bonus question: If you have a torch or lamp that would illuminate outside of the effect of Sunlight, does the outer edge of Sunlight stop that propagation outwards? You can answer yes or no, but the implications are different. What about propagation of light /in/?

    1. All of that seems much more complicated than what we've got now.

  2. I've always gone with the last option: the spell is cast on the patch of ground to be illuminated, and range penalties are assessed accordingly.

  3. "...allow it only to be cast at the terminus of light" seems to be exactly what the designers intended, is consistent with the wording, and restricts the abuses that could come from casting it midair and allowing it to extend down to the ground, hundreds of yards away. It does cause some issues for large/tall caverns--a one hex Sunlight spell would allow you to see all the way to the ceiling, and, presumably, four hexes away in bright sunlight, twelve hexes away with darkness penalties. Whether the spell should do that is an other issue, but that seems to be what was intended. That's less of a problem than being able to cast it midair and seeing down into a really deep pit or shaft.

    1. That's where I'm leaning because of the relative simplicity in saying it has to be some kind of natural terminus.

  4. That it creates a new, temporary star when cast at night also has world building implications if you raise your eyes to the heavens and wonder about it. So maybe don't encourage anyone to play a sage.

  5. "- If I treat Sunlight like Darkness, it is just a small area of light with sharply defined boundaries but is totally inconsistent with everything else we do with light."

    But also totally consistent with the inconsistent manner in which the Spell Darkness works.

    Another way to think of Darkness is "destroy light". It's an area that light cannot escape, thermodynamics be damned, it just is.

    But for your purposes, I say lean in on the last choice.

    For me, the choice went the same, but also differently. I removed the spell from Wizards and only allow Clerics and Druids to cast it, it's literally the gods shining light down from above. It's a miracle!

  6. I would look at it as being like a light /sunlight bright with Color (full spectrum sunlight) and base it on those limitations.


Related Posts Plugin for WordPress, Blogger...