Seamless Portals is a mod that, as the name suggests, includes seamless portals. The mod is said to have completely broken the bounds and limitations of the Source Engine. Unfortunately, the add-on functions quite poorly on outdated hardware. Using this add-on on outdated hardware could cause frame drops, stutters, and crashes.
Mechanics[]
The portals work identically to the ones from the Portal franchise, letting you gain massive amounts of momentum if you jump into a portal from a high space.
Bugs[]
All of the bugs below have been tested in the x86-64 branch of Garry's Mod launched in 64-bit mode.
Seamless Portals is incompatible with anti-aliasing. If the player has anti-aliasing enabled, then the portals with still not have anti-aliasing but outside the portals, there will still be anti-aliasing. If the player gets through portal linked to itself, the view gets flipped. This is also incompatible with anti-aliasing and anti-aliasing will be useless by then.
There are also glitches with the rendering of those portals due to visibility optimization with the only known way to reduce the problem being to disable visibility optimizations with console commands like "r_novis 1" and "r_frustumcullworld 0" without quotes. Those commands still don't completely disable visibility optimizations and visual glitches still may occur but at a lower frequency/severity. If the map has a 3D skybox, you'll have to use the "r_3dsky 0" command before using the "r_novis 1" command otherwise the 3D skybox's camera will be rendering the entire map instead of just the skybox which causes other rendering glitches. Disabling visibility optimizations will greatly decrease the FPS.
Seamless Portals is incompatible with Ray-Trace as in if a portal is open in the player's PVS while attempting to render a path-traced screenshot, then the rendered screenshot will be taken from the portal but it's not meant to happen so the path-tracing adds garbage.
Seamless Portals also has issues with TARDIS where if a portal gets made in a TARDIS, then another portal gets made outside of it, then when going through the portal in the TARDIS, the player will teleport right next to the TARDIS instead of to the other portal. The player can also create a portal in the TARDIS, and when trying to link it, it will create a line that traces to the portal. This trace doesn't break when exiting the TARDIS and therefore can be used to noclip to the TARDIS, which is invisible outside as the inside of the TARDIS must take up some physical space in the map. The portals can also cause graphical glitches with the TARDIS when the door of it is open.
Many entities can't go through a portal. Instead those explode off the surface of the portal (if it's an RPG round) or go through the portal without teleporting to the other portal. Some weapons can't shoot through portals such as the Tau Cannon, which it's projectiles are not entities. The Physgun, Gravity Gun, and many melee weapons can't hit through portals. The Physgun will instead attempt to move the portal itself. If a prop or other physics based entity gets selected by the Physgun, then gets moved to the portal, then entity wouldn't teleport to the other portal. If a portal gets moved to a prop that it's physics is in sleep mode (didn't move in a while), then the prop wouldn't teleport to the other portal.
Explosions, light and sound can't go through portals, although sounds can't get through portals in Portal anyways. Many tools don't work well with portals. For instance, when making balloons right next to portals through the Balloons tool, if a balloon gets through a portal, the rope will not get through the portal and instead will teleport to the closest non-portal path to the balloon, since it's usually being much farther than the max limit on rope length, the balloon will get back quickly to the portal and teleport through it again, repeating the cycle.
In Infinite Map Base, portals can't collide with the ground or parts of the "world" even if the map is an OBJ, although portals can collide with brushes and displacements in infmap.