OCCL

Occlusion is a feature in Vice City and San Andreas that helps make the game run smoother in certain parts of the game. The occlusions are created using invisible boxes. These boxes disable models that are behind the boxes from rendering. This feature is not really effective and Rockstar did not use this feature efficiently. There are not much improvements on occlusions in San Andreas compared to Vice City. Deleting all entries in the occlu section doesn't have a major effect on your games.

Location

Occlusions are controlled by the occl section of the IPL files. Rockstar created the occlusions through the occlu.ipl in Vice City and the occlu.ipl, occluint.ipl, occluLA.ipl, occlusf.ipl, and the occluveg.ipl in San Andreas. You can open these files using Microsoft Notepad. You do not need to use these files to create your own occlusions as long as the entry is in a defined IPL file.

Format

Section identifier:

occl
...
end

Format of the occlu section of the IPL file:

direct mid x, direct mid y, bottom height z, width x, width y, height from bottom height to top,
rotation

• direct mid x, direct mid y: The middle of the occlusion box in X and Y coordinates.
  
• bottom height z: The bottom Z coordinate of the occlusion box.
  
• width x, width y: The width of the occlusion box in units.
  
• height from bottom height to top: The difference in height from the top of the occlusion box to the bottom.
  
• rotation: The rotation of the occlusion box in angles.
  

Viewing It

Vice City's occlusions can only be viewed through KEd. So far, no program can view San Andreas's occlusions with its models. You can copy San Andreas's occlusions and paste it into Vice City's and view it through KEd. KEd sees occlusion boxes as green, transparent boxes with a small triangular hole in the 3D View. KEd does not support editing the occlusions through the program so you have to create the boxes yourself.
http://img.photobucket.com/albums/v41/spaceeinstein/Occlusion.png
Occlusion boxes in KEd

How 2.0

The following way to make occlusion boxes is practiced by spaceeinstein. No other people are modifying the occlusions. First find a building that you want to make occlusion boxes and align it to the X- and Y-axis. Get the coordinates of the lower left corner of the building and the upper right corner of the building. The coordinates for the lower left and upper right corner of the building can be inputted as X1, Y1, and Z1, and X2, Y1, and Z2, respectively, into the occlusion tool. If you are not bothered to get the lower Z coordinate of the box, you can input any number that is lower than the foundation of your building. Making the boxes with angles is pretty tough to do and probably needs some trial and error to fit it into the building.

Problems

If you delete the buildings in Vice City or San Andreas without deleting the occlusions, you would see pop ups behind the occlusions. These occlusions had created problems for a few modifications, most notably in the Liberty City modification. In the Liberty City mod, the team experienced unexpected pop ups caused by the occlusion boxes. These boxes are loading and unloading the models behind boxes where Vice City's buildings used to be at. Someone realized that the occlu.ipl was the problem and deleted every line in that file. The pop ups disappeared.

Current Usage

The only person who is known to modify the occlusions for modifications is spaceeinstein. The modifications that currently use modified occlusions are the Liberty City mod and the Myriad Islands mod for Vice City.

Backups

If you did not backup the files and need the original files, get it here.
Vice City's occlu.ipl
San Andreas's occlu.ipl
San Andreas's occluint.ipl
San Andreas's occluLA.ipl
San Andreas's occlusf.ipl
San Andreas's occluveg.ipl

References

Occlusion Tool - Temporary site to host Opius's occlusion tool