Noob mapping question - why sheets?

[Pileyrei]

On some maps like Deck16 and HyperBlast there are sheets in windows/doorways which are obviously not visible in game.
I can understand that for HyperBlast to separate zone info (gravity for example).

So why is the lift area in Deck16 "sheeted" off?
Does this help computers with CPU processing or something?

[Dr.Flay]

Yes this is usually done to make maps work faster.
Zoning a big map into smaller areas can make a huge difference.

If you change the view to show zones/portals you will see the lift area is a different colour from the main area, and the liquid zones.

Deck-16 does not have any Zoneinfo actors.
Adding a Zoneinfo allows you to change various settings within that zone, eg. Lighting, fog, gravity... (like a mini Level-info actor).
Splitting into zones also makes it easy to use the new EFX audio ambients, so you can make each area sound like it should.

[Pileyrei]

Great!

I've learnt something today.

Big thanks

[sektor2111]

It's a whole document about zoning, but not many people bother to gather information and to do things properly, I gotta admit you should not follow entirely default examples as long as some default levels are badly screwed. But document is more relevant, some people have learned lesson and they made Levels, others are only living with this feeling.
Let's see what was written:
_______________

• Unreal Zones
  Tim Sweeney
  Epic MegaGames, Inc.
  http://www.epicgames.com/
  
  Audience: Level Designers.
  Last Updated: 07/21/99
  The concept of zones
  
  Unreal levels can be huge; many of our game levels are pushing 20,000 world polygons. At any time, the player only sees a tiny fraction of those 20,000 polygons, typically 100-200 polygons. The engine spends a significant amount of CPU time figuring out which 200 polygons, out of a potential 20,000, are visible in each scene. Unreal employs several techniques to speed up this "visible surface determination" process. Most of these techniques are automatic, requiring no designer intervention. However, one of the most powerful techniques, zoning, requires level designers to manually place invisible polygons called "zone portals" in their levels. Zones greatly accelerate the engine's visibility calculations, and they also enable designers to create many special effects that are specific to a region of space, such as ambient lighting or modified gravity.
  
  A zone is an non-convex interior region of connected 3D space, for example a room, or a group of several complex, interconnected rooms.
  
  By dividing your map up into zones, you gain several benefits:
  
  Speed. A well-zoned level typically speeds up the engine's "visible surface determination" process by over 100% in a complex map, compared to an unzoned level.
  Ambient lighting. Each zone can have its own ambient lighting value, which is added to the regular lighting on each surface. Ambient lights are very useful for outdoors areas, where pure black shadows are unnatural, as well as in diffuse environments where one would not expect dark shadows.
  Reverberation. Each zone can have its own aural reverberation properties; see the audio document for more information.
  Zone-based physics properties, such as friction and gravity.
  Water.
  
  Seeing zones
  
  UnrealEd has a special mode that enables you to see your level's zones graphically, which each zone rendered in a distinct color. In UnrealEd, in the 3D viewport's menu, select "Zone/Portal view". You can also see this in the game by typing "RMODE 2" at the console.
  
  In UnrealEd, zone portals are normally visible, and you normally can't see through them. To hide them, press the joystick button in the viewport, which turns on realtime updating of the viewport. You should only use this for previewing your architecture, as this realtime updating slows down the rest of UnrealEd's user interface.
  Partitioning a level into zones
  
  Level designers can partition their levels into zones by using special kinds of brushes, zone portals, to seal up regions of their maps and make them "water tight".
  
  There are two tools in UnrealEd that you need to use to create zone portals:
  
  The "sheet brush" tool. This is represented by the bottom brush builder icon on the toolbar, which lets you build a flat square polygon. You need to build zone portal polygons using this tool; you need to create polygons that are big enough to block the passages you are sealing up.
  The "add special brush" tool. This is represented by the solid green square icon on the toolbar, below the "add" and "subtract" buttons, which enables you to add a polygon to the world with special properties to it. When you click on the "add special brush" tool, a little dialog box comes up with several pull-down options. Choose the "Zone Portal" pulldown option. Then position your sheet brush and click "add" when you're ready to add the portal.
  
  To zone your map, first you add a bunch of portal polygons (using the "sheet brush" tool to build them, and the "add special brush" tool to add them). Next, bring up the "rebuilder" dialog and rebuild you map. The rebuilder will go through your map and detect regions of space which are sealed up "water-tight" and assign a unique zone to each region. You can go into the "Zone/Portal" view to verify, graphically, that your level is properly zoned.
  
  If, in Zone/Portal view, you see that the engine hasn't recognized your zones, you have probably not made the portals "water tight". You need to add in enough portals that the separate zones are completely sealed-off from each other.
  
  If, in the game, you see "hall of mirror" effects where you placed portals, make sure your portals completely seal up the regions of space they separate. If you have a portal that only partially fills up a passageway, you might see through it into nothingness.
  Where to place zone portals
  
  For special effects zones like water zones you will, of course, place the portals wherever you need the effect.
  
  For zones whose purpose is only to speed up the engine, your goal is to divide your level up into zones while introducing portals whose overall area is as small as possible. The smaller the portals, the less work the engine must do to process the portals. Therefore, we place our portals in passageways which naturally separate large areas, such as doorways and hallways. This approach causes the level to be divided up well, without adding too much portal area.
  
  There is no benefit (and often a performance penalty) to placing portals in the middle of large rooms. These huge portals usually eat up more CPU time being processed than they save. Stick portals in doorways, hallways, and other narrow passages.
  Adding ZoneInfo actors to specify zone properties
  
  If you want to assign individual properties to a zone, such as water, ambient lighting, or reverb, you need to add a "ZoneInfo" actor to the zone. You only need to do this if you want to assign custom properties to the zone; if all you want it the speed advantage of portalization, you don't need to add ZoneInfo's.
  
  In the class browser, expand the "Info" class and you'll see "ZoneInfo" right there. You can add the ZoneInfo actor anywhere in your zone; as long as it's in empty space and there aren't more than one ZoneInfo's in your zone, the ZoneInfo will work. For the engine to recognize your ZoneInfo actor, you need to rebuild your map again.
  
  See the ZoneInfo property sheet for a complete list of the ZoneInfo properties. There are many cool adjustable parameters, such as friction, gravity, ambient lighting, pain, water, etc.
  Water and translucent zone portals
  
  To create a water zone, just set the ZoneInfo's bWaterZone variable to True. The Unreal physics code (in Pawn.uc and PlayerPawn.uc) automatically recognizes these water zones and transitions between walking and swimming physics modes.
  
  To make transparent water, just select the zone portal's surface in UnrealEd, bring up the "Surface Properties" and turn its "Translucent" option on.
  Caveats
  
  A level can only have 64 zones. If you create more than 64 zones, the engine will merge some zones together, causing a loss of performance. This limit might be extended in the future.
  
  End

Capish ?

[Hellkeeper]

Pretty much all you need to know is contain in sektor's post (also saved on OldUnreal's wiki).
Basically, zoning the level allows you to tell the engine not to load and display anthing but the zone where the player is currently located AND the small parts of other zones currently visible through zone portal. Thus, less polygons and to be computed in real-time and better performances.

The 64 zone limit is really 63 because the LevelInfo counts as one.