The Hidden Area Meshes are used to skip rendering the corners of the displays (since they aren't visible through the lenses anyway). This can improve the GPU performance in applications that use them, as well as reduce glare.

The meshes are usually applied when a game starts, so you'll need to reboot your game for any setting changes to apply.

Test Mode inverts the hidden area mesh, only showing you the parts of the screen that will be hidden when Test Mode is later disabled.

The last "Mesh Detail" setting controls how many points are used for each corner. A higher value will result in smoother edges.

Your displays will now look like this:

In Game (text added)	In HMD (note that displays are rotated)

Restart SteamVR Home by toggling the "SteamVR Home" option in SteamVR Settings to see the hidden area mesh changes.

You may also want to adjust your FoV settings at this point, as the Test Mode gives you a good indication of the horizontal and vertical limits.

Example of modified settings:

In Game	In HMD

In Game	In HMD
With mesh	With mesh*
Without mesh	Without mesh

* The line you see in the bottom and right corners in the HMD view here is a bug in how SteamVR Home applies the mesh. I have not noticed it in other games.

1. UEVR does not support hidden area meshes. Games need to explicitly use hidden area meshes and the UEVR mod does not yet have a way to do that (see this open UEVR issue).

2. With a uniform 0.5 radius mesh, the amount of pixels used by the game's pixel shader is theoretically ~21% lower than with no hidden area mesh (area of a square minus the area of circle: 1 - π×0.5² = 0.21). This can have a notable and positive impact on your frame rate if you are GPU-limited, but the scale of that impact will vary from game to game.

3. The performance impact (if any) from the "Mesh Detail" setting has not been tested.

4. While using Test Mode, you may be able to see a corner lit up in your peripheral vision, but not when looking directly towards it. This is due to your iris's position relative to the edge limits of the HMD lenses; When looking left, your iris moves to the left and is able to see further past the right HMD lens edge than when looking right.

5. Only the "Standard" hidden area mesh type is implemented in the driver (this seems to be the one most games use). There are also "Inverse" and "LineLoop" styles that the compositor itself can use and apply to the distortion mesh (effectively clipping out anything the game might've sent outside of that area). These compositor mesh styles are not implemented as they were buggy, required full SteamVR compositor reboots to apply, and had a negligible performance impact (the compositor stage that runs after the game has already rendered its output).