determine if Line-of-Sight exists between two objects?

I need to determine whether an uninterrupted line of sight exists between two objects. I was thinking I would just fire an invisible object from one to the other and see if it collides with anything on the way, but is there a more efficient way?

Crank your draw distance. Anchor your camera on object A, pivot your camera to object B? (^_^)

There's no way that doesn't involve one of the following: a sensor or sensor array and perhaps some calculation per each object, flinging things as mentioned, or a client modification to do a raycast on command (which isn't guaranteed to work anyway as not all objects may be in view).

and then what, write some machine vision artificial intelligence in Linden Script?

sorry, that reply was @sideways.

@Tyken, that's what I'm asking - can this be done with sensors alone?

Sensor, narrow range (like a beam), just as long as it is within 96 meters should work.
EDIT: No it wouldn't because of a sensor caveat(especially when you figure in mega prims):

"sensors detect objects only if their center falls in the detection cone (see below). This makes it harder to use sensors for pathfinding, especially avoiding big objects like walls."

So it sounds like your original idea is the best one.

Does object A already know the position vector of object B? If so, I'm thinking object A could run a sensor with a very small angle pointed directly at object B, then do some math on llDetectedPos (knowing the positions of both A and B) on any other objects detected to determine if they are closer or further away than B. Anything closer impedes line of sight.

since this sensor geometry is a cone and not a cylinder, it doesn't matter how narrow I make the range, over a large enough distance it's going to detect things that are not directly in the line of sight. also, as far as I can tell, at least in the case of llSensor(), you can only detect objects and avatars, not land. so their might not be any avatars in the way, but there could still be a great big mountain.

so I started working on having my object rez an invisible "bullet" to fling at the other thing. i'll put collision detectors on the bullet eventually to see what it comes across on the way. BUT it appears you cannot set the velocity of an object that isn't physical. but then on the other hand, if I make the thing physical, it's going to travel on a parabola, not a line, due to gravity. aaargh....

Try llSetBuoyancy(1);
http://wiki.secondlife.com/wiki/LlSetBuoyancy

Good thought about land, I was thinking within the 96 meter sensor limit that a usably narrow cone would be possible. And Jesse found the real clinker in her edit. Good luck!

Unfortunately I believe sensors also only detect objects whose CENTERS are within the sensor cone, so it wouldn't quite be like a full ray-casting solution. The projectile collision detection might be the best option available unfortunately.

ooh, buoyancy will help a lot, but then there's this problem: "Wind can cause the prim to drift." if you just use llSetPos(), will the object get collision events on the way? will it follow a straight line?

Note that your bullet won't detect phantom objects, even if they're opaque. That may be fine, though. Peraps it can check the collided objects for phantom -- I think thats one of the attributes you can get.

I wouldn't worry about wind, you'll never notice it.

current nearly-working solution is to rez the "bullet" and in on_rez call llSensor() to search the thing the bullet is supposed to aim at. then in the sensor() callback, I call llMoveToTarget(). (this is with a physical bullet now.) this works and the collision detection works on the way (land_collision works too.) now I just have to work around the fact that llMoveToTarget() is limited to 60m. cuz it wouldn't be Second Life if things actually did what I need them to and in a simple way.

Sounds like the right (only) approach to me.

There's a user function at http://lslwiki.net/lslwiki/wakka.php?wakka=llMoveToTarget that might help.

Just rez a bullet towards target at a constant velocity, there is just absolutely no need to do llMoveToTarget..

In fact, I did the exact same thing with tracing a ray via a invisible bullet. Just sensor object B, rez bullet towards it at a constant velocity, when it hits something, make it tell key of what it hit, so object A can compare it against object B's key.

the suggested user funciton on LSL wiki doesn't seem to work. it just sends the bullet drifting in a random direction.

@Nexii - If you use that technique, how do you get the bullet to move on a straight line? the velocity parameter in llRezObject is only used if the object is physical, but if the object is physical and all you do is give it an initial velocity, the wind is going to blow it around and gravity is going to pull it down.

Wind has no effect on objects in Second Life (exceot for flexi-prim effects). Gravity does have an effect, but llSetBuoyancy() or llSetForce() can be used to counteract it.