Lasers through Portals?
Does anyone know if it is possible to transmit a laser through a portal? I have a puzzle idea that requires this feature. I have tried using env_beam and env_laser, but both seem to require a starting entity and an ending entity.
Next, I tried using an npc_sniper, but it's beam will not pass through a portal.
I also tried a workaround using the portal turrets, but the rocket turret doesn't seem to be affected by the AI relationship entity, so it kept trying to shoot me, instead of aiming where it was told to.
The closest I got was using a floor turret, but the problem is, i really need the beam to point downwards from it's starting point, which the turret doesn't do, as it thinks it has been knocked over.
I'm probably missing something really simple, but I have been experimenting for a few hours with this, and come up with nothing. 
I'm not really sure what it is you're trying to setup, but... You're correct, env_beam and env_laser both require a starting and endpoint. I don't think the actual beam will "travel" through a portal, since all it does is just render a line from A to B - but the good news is that it will render through a wall, the end points don't need to have a line of sight.
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Basically, I have a large room with a toxic water floor, and moving platforms with buttons, travelling around on a semi-random path. On one side of the room, I have an entity spawner, dropping the object that is needed to trigger the buttons.
The reason i want the laser is to help with targetting, as at the moment, it is very frustrating to fire a portal at the ceiling and judge where the object will hit on the floor, as the room is quite tall.
I figured that a laser, pointing down to the portalable floor where the object starts from would make life easier for players, as when they fire the portal into the ceiling over the slime, they have a bit of feedback on where their portal is, in relation to the floor.
Sorry if this doesn't make much sense. It works better as a puzzle than I have made it sound here, as it s hard to describe. (plus some details are secret) 
Solution: make any ceiling over water non-portable
The ceiling needs to be portalable, as the platforms are moving over the water, and the objects needs to be dropped down from a ceiling portal, onto the moving platforms.
I could completely redesign the puzzle, but i'd rather not at this stage. I just hoped there was some kind of entity that could direct a laser through a portal, as the turrets in portal can do this with no problems.
New solution: Place a spotilight on your platforms facing upwards and parent them to the platforms. Make sure you set the radiuses to very small or else you might get a compile error relating to too many dynamic lights on a brush.
Are you referring to a light_spot, or an npc_spotlight. I thought light_spots were static, but I may be wrong.
As for npc_spotlights, i only just found out they even existed lol. They look more promising. Is there a limit to how many you can safely use without a performance hit? Because I need quite a few to use the method you described, due to the amount of platforms i am using in this puzzle.
Also, i just found a workaround for the method I originally wanted to use, but it seems like overkill. Basically it involves a friendly turret, and a set of prop_portals to direct it's laser downwards to my entity_spawner. Again, this will cause a performance hit by having 2 sets of portals open, but it seems to work as a crude method of doing what I wanted.
I will experiment with both methods anyway, and figure out which suits my purposes best. Thanks for the ideas.
If anyone has any other solutions, I would be GLaD to hear them. 
Well, depending on the effect you want, you can either use light_dynamic (bad idea) or point_spotlight (probably a better idea) to achieve the effect.
A light_dynamic is just that, a dynamic light source. However, keep in mind that Source handles these HORRIBLY, and if you have more than 2 or 3 things that will need light_dynamics parented to them, you probably don't want to go this route. Also keep in mind that the size of the light_dynamic's field will determine how much lag it causes - a larger one will be harder to draw than a smaller one.
A point_spotlight doesn't emit any light (by default anyway), and just creates the visual spotlight effect. There is a flag to have it generate dynamic light, but again, you probably don't want to do this.
EDIT light_spot's are infact static. I have no clue what npc_spotlight does.
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npc_spotlight is just a spotlight that looks at enemies. I am pretty sure they are used in the Nova Prospekt levels.
You could always make the floor have the texture with a couple different color tiles (usually used to indicate flinging but useful for directing where to put a portal. Or you could also put an npc_bullseye which will cause the player to "auto-aim" at that spot.
That's what I figured, but I've never looked closely at that VMF so I wasn't sure.
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