Surfaces
Each level within the Portal series consists of surfaces. Typically, a portal can be placed upon any light-colored flat surface, whereas an uneven or dark surface cannot be used in this way. In this way, the placement and location of portal-conductive surfaces is a fundamental design choice, and determines the way in which the puzzle(s) need to be solved.
In some Test Chambers, surfaces are blocked by doors or other obstacles, or must be activated before a portal can be placed upon them. In Portal 2, the only way to overcome some puzzles that lack portal-conductive surfaces is through the use of Gels.
Appearance
Portal
Portal-able
Portal-able surfaces in Portal are white plates that seem to be made of some kind of stone. They are easy to tell apart from dark surfaces.
Non-portal-able
Non-portal-able surfaces in Portal are made out of dark brown/black metal plates that reflect the light slightly. Valve has chosen this design because the darkness and the shininess properties of the material are easy to spot from a distance.
Portal 2
Portal-able
The portal-able surfaces in Portal 2 seem to be the a slightly dirtier version of those seen in Portal. This could be because Aperture Science withered in the time between Portal and Portal 2.
Non-portal-able
The non-portal-able surfaces in Portal 2 don't have any 3D effect or shininess like the ones in Portal.
Gels
Introduced in Portal 2, Gels can be applied on most surfaces in order to modify its attributes. There are 3 Gels which can be applied:
- Repulsion Gel is a blue gel that bounces a player or object away from the surface.
- Propulsion Gel is an orange gel that removes/reduces friction on the surface on which it is applied, allowing players to move on it at high speeds.
- Conversion Gel is a white gel that allows portals to be placed on surfaces that portals normally cannot be placed.