Alright, I've tried doing some searches, and I've come up pretty empty. So as far as I can see, this is not possible... but as far as I know, China doesn't exist and everyone is playing a big joke on me.
In other words, just because I don't know it, doesn't mean it's impossible.

Ehem More to the point.

Is it possible to create a brush that detects if there are no portals in it, deactivate it from start, then reactivate it through Output of another object?

See, there's an area in my level, like most Portal maps, inwhich you can throw a cube in, and get yourself stuck. Unlike the maps that come with the game though, my map is relatively larger, and I need the person to be able to carry the cube to various locations, so I can't just throw grills in to box area's off.

So what I was thinking about doing is adding in brushes to the small room and have them perform outputs when there are no Portals in there, but only enable them whenever a box is in there, and the door is shut.

Possible?

PS: Smaller lower priority question that people will most likely answer long before my initial question, but... Portal Pair ID's, what exactly are those? I have an idea, but I'd like an unbiased conformation here.

I think you can create a brush (Trigger brush would probably be best) and tie it to a func_portal_detector (Ctrl-T). That has outputs for detecting Portal 1, Portal 2, a linked portal, or both portals, but no output for detecting no portals...
Maybe you could place a few of these outside the area where the player gets stuck along with a trigger inside for detecting the player themselves...

portal pair ids allow you to make more than 2 portals

the default it 0 so i make a red, orange yellowish w/e colour portal and a blue one and they will link.

I change it to 1 and make a new red, orange yellowish w/e portal then a blue one and the others 2 portals will stay there and the first 2 i made with a linkage of 1 will link to let you go though them. i think the max is something like 3 or 4 so you can't make many of them.

So here's a question. How can you make it so that it detects 2 Portals on 2 different portal detectors, as a link? Like if you have 2 portals in a single room but on different walls and inside different detectors. I kind of thought that's what ID's were.

no its completely different, i don't understand the portal detectors, but go into a map then put the command into console and change the linkage so that you can make 4, 6, or maybe even 8 portals, i will make a demo shortly

edit: demo is uploaded .dem, the exact command is change_portalgun_linkage_id <number of the portal link here>

CassataGames wrote:
So here's a question. How can you make it so that it detects 2 Portals on 2 different portal detectors, as a link? Like if you have 2 portals in a single room but on different walls and inside different detectors.

I would do this by using math_counter. Everytime a portal is created inside the portal detector, the math_counter's value would be increased by one. But hey, it has a glitch - the portal detectors don't have output for NOT detecting portals (like OnEndTouch for trigger_ entities), so we wouldn't be able to decrease the math_counter's value. Any more ideas...?

Mek wrote:
I would do this by using math_counter. Everytime a portal is created inside the portal detector, the math_counter's value would be increased by one. But hey, it has a glitch - the portal detectors don't have output for NOT detecting portals (like OnEndTouch for trigger_ entities), so we wouldn't be able to decrease the math_counter's value. Any more ideas...?

Which just so happens to unfortunately be the entire point of this topic, the fact that I can't detect when portals are NOT there.

CassataGames wrote:
Which just so happens to unfortunately be the entire point of this topic, the fact that I can't detect when portals are NOT there.

And so, you fix this by placing a massive func_portal_detector, covering all the rooms other than the room(s) that need to know when nothing is there, and because the detectors detect anything in their volume, you can subtract from the counter whenever both portals are outside the room(s). (use OnStartTouchBothLinked, I believe.) once this is triggered, have it disable itself. When the detector inside of the room(s) discused a few lines up is triggered, have it reactivate the supermassive trigger. (You need a supermassive trigger. This is so that nobody just runs out of the otherwise limited trigger area and gets a free ride, so to speak. UNLESS you have the area contained with grills and such.)
It's late here, so please feel free to get another persons opinion on this. I don't think I've missed anything....