Screen space ambient occlusion

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Ouch!
The following article, Screen space ambient occlusion, mentions a feature exclusive to certain versions of the client.
Specifically: This feature was added in 2011.

Also known as ambient shadows, this feature was added in 2011.

How to enable

If the feature is not enabled automatically, then according to Gemlocker, these are the steps to manually enable the feature:

Set graphics mode to Direct3D. The Automatic setting should also work. If the chat button in the top left is wide, then the client is running in Direct3D.
Set antialiasing to Off. The Automatic setting should also work.
Set quality level to 21.

System requirements

Also according to Gemlocker, there are specific system requirements in order for the feature to be automatically enabled. These may also prevent the feature from being manually enabled and some are undocumented.

These include:

ROBLOX quality level is at least 14
Have a certain amount of video memory
Support two or more rendering buffers

How to check your system

Open a place file in the client.
Check the log file that ends in "gfx_d3d.txt"
Look for "RBX device", which needs to be one of the allowed names.
Look for "RBX vidMemSize", which needs to be at least a certain amount.
Look for "Multiple Render Targets", which needs to be 2 or more.

Troubleshooting

NVIDIA

If you see screen space ambient occlusion but the skybox is white, and antialiasing is off in ROBLOX, you need to change 3D settings in the NVIDIA control panel. If, in the global settings, antialiasing mode is override, then you need to change it to something else in the global settings or program settings.