Radius or diameter?

[delta]

When expressing a rule for a circular area of effect (like a spell, et. al.), is it better to express the size as a radius or a diameter?

Gygax goes back and forth in several texts. In Chainmail: Catapults/fireballs are given by diameter, light by radius (effectively), protection from evil by diameter, etc. In D&D Vol-1 each of those examples toggle: Fireballs by radius, light by diameter, protection from evil area by radius, etc. In Swords & Spells: Everything by diameter. In AD&D: Everything by radius (mostly, I think).

Here are some advantages of each. (Feel free to identify more, I may append this over time.)


Radius

1. Matches the mathematical definition of a circle.
2. Easier to draw physically with a compass tool.
3. If casting specifies a target point, easier to determine if someone is near that point to be affected.

Diameter

1. More comparable scale with other shapes (e.g., 2" diameter circle and 2" side square are equally wide *)
2. Matches most software tools where all shapes (token scaling, ellipses, etc.) are sized uniformly by total width.
3. Tells you size of material you need to cut from to make a template, etc.
4. Easier for the caster to estimate how many people can be caught in the area (?)

* This is most apparent if you make a big list of various spells, as per the presentation in Swords & Spells.

[hamurai]

Clearly radius!

Usually you need that when you cast a spell and want to know, who or what is affected. It's much easier to draw a line from the center of the effect to the target(s) than use a template and try to get it matching exactly with the center of the effect.
Of course, VTTs will do that for you easily, but at the table it's quite different. And a VTT will do either, really.

[delta]

Feb 22, 2023 2:19:37 GMT -6 hamurai said:

... Of course, VTTs will do that for you easily, but at the table it's quite different. And a VTT will do either, really.

Thanks for the vote!

I will say that my VTT of choice, Roll20, won't do either, to my knowledge. Drawing a circle/ellipse is by total width (not center-to-radius), and a circular token gets scaled by total width (same as any other tokens), etc. Precisely that the other day, needing to convert from radius to diameter for everything in the VTT, is what put me over the edge to ask this question.

[waysoftheearth]

it's a bit hard to imagine one without the other

[Desparil]

I put radius because most systems now specify ranges and radii in real distances rather than scale distances, in which case I wholeheartedly prefer the radius to be specified. However, if using Gygaxian scale inches, then for typesetting and readability I would prefer diameter - better to have a description say 3" diameter than 1½" radius.

[hamurai]

Feb 22, 2023 10:36:59 GMT -6 delta said:

Feb 22, 2023 2:19:37 GMT -6 hamurai said:

... Of course, VTTs will do that for you easily, but at the table it's quite different. And a VTT will do either, really.

Thanks for the vote!

I will say that my VTT of choice, Roll20, won't do either, to my knowledge. Drawing a circle/ellipse is by total width (not center-to-radius), and a circular token gets scaled by total width (same as any other tokens), etc. Precisely that the other day, needing to convert from radius to diameter for everything in the VTT, is what put me over the edge to ask this question.

True, Roll20 doesn't do that. IIRC Foundry uses radius, but it's been a while since we played D&D there, so don't quote me on that. I still fiind it a lot easier to use the diameter template in a VTT than IRL, as you can easily see if you've got your template centre exactly on the centre of the AoE.

If you've got a fixed AoE radius and a transparent template, it's also easy at the table, but most of the time the radius is not fixed for all effects.