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.
Within the test chambers, portal-able surfaces take on the appearance of concrete slabs and tiles. Aside from a few alternate color choices, all of the wall surfaces are identical in size. The "behind the scenes" areas of the game feature the same slabs, only looking more worn down and orange tinted; tiled floors are replaced with solid concrete flooring.
Non-portal-able surfaces in the test chambers of Portal are made up of metallic dark brown, blackish cubes and squares. It's stated in the Commentary Mode of the game that this design choice was made because it made the material easy to spot from a distance. Metallic flooring is a lighter color.
The "behind the scenes" areas have rougher metallic walls as well as less smooth metal flooring when present.
Being constructed out of panels, the portal-able surfaces in the test chambers (whether during the reconstruction phase at the beginning of the game or during Wheatley's takeover towards the end), tend to be white in appearance (rather than gray as in Portal) to contrast better with the updated metallic textures.
In Old Aperture, portal-able surfaces with often feature a checker-board pattern.
The non-portal-able surfaces in Portal 2 don't have any 3D effect or shininess like the ones in Portal. They've been changed from a bronze-like color to black with grey lines to better contrast with the updated portal-able textures.
In Old Aperture, non-portal-able surfaces can include a variety of materials including green, black, brown metals and wood.
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.
- Glass, despite being a flat surface in-game, automatically repels both Portals and Gels.
- As a result of this, glass panels are often used in Aperture Tag: The Paint Gun Testing Initiative as a way to limit gel usage and flesh out puzzles, in much the same way as non-portal-able surfaces are used in the official series.