D20 Mechanic: Infravision
We can probably make infravision functional, even usable, so long as it doesn't come up all the time.
This version of infravision is written with Reynolds' rant regarding infravision in mind.
Infravision allows creatures to see via heat, rather than light.
Only living creatures have heat signatures.
A creature's heat signature extends in a radius around it equal to its size (so a Large creature, which fills a 10-ft cube, would have a heat signature that extends for 10 feet in all directions).
A creature with infravision can see heat signatures; however, unless the signature's source is within the creature's infravision range, the creature with infravision only knows that a heat source is near, and roughly how large it is, and not anything more specific than that.
Nonliving Creatures and Objects
An object can only be seen via infravision if the object is significantly warmer or colder than its surrounding environment.
Nonliving creatures typically match their environment's temperature, but can leave signs of their passage through friction (see below).
Movement causes heat, which is visible to creatures with infravision. Even nonliving creatures, such as undead or constructs, leave sign of their passage.
Whenever a living creature touches an object, they leave a heat trace on that object. The rate at which these heat traces vanish from an object depend on the object's material - stone retains the heat for some time, while metal quickly loses the trace.
A creature with both infravision and the Track feat can track creatures using its infravision.
Nonliving creatures only leave heat trace by virtue of friction. As such, their heat traces disappear much more rapidly.
Infravision and Various Effects
Infravision interacts with various effects significantly differently than normal vision.
[Light] and [Darkness] Effects
Effects with these descriptors do not block or otherwise interfere with infravision.
The exception to this is solar effects. However, given that base d20 doesn't actually make that distinction, we have to do it ourselves.
[Cold] and [Fire] Effects
Cold effects are to infravision as darkness effects are to normal vision - they temporarily inhibit infravision. An effect with the [Cold] descriptor