Portal Glitch Fix

Overview

Type “-dxlevel 95” into the launch options for the game.

How to fix the portal glitch:

If your portals look like this, then this this simple trick should fix those portals.
Go to your steam library and find Portal. Right click on the game and select properties. In the properties menu, select Set Launch Options. In the new window, type in this:

-dxlevel 95

This sets up the game to use DirectX 9 instead of 8 and your portals should look beautiful again.