Nevertheless, there are a few hints in the games and their promotional material which give the impression that Portal and Half Life do indeed take place in the same fictional universe (or at least that 'Black Mesa' and 'Apperture Science' exist in both universes). That's why the game appears with the Half Life application icon. So when you are playing Portal 1, you are technically playing HL2 with custom maps and a custom weapon and bunch of other custom assets.

When you already have HL2 installed, Steam just needs to download an additional 2.3 GB. When you download Portal without having HL2 installed, it's a 7GB download. Interestingly, Valve managed to do it in a way that if you install both games through Steam, they will share the files they have in common. It is important to keep the LUT tickbox enabled. The mod is called ApertureVR Thinking Without Portals and it does a fantastic job recreating the look, feel and sound of Valve’s classic Portal 2 adapted with Half-Life: Alyx’s gravity. Done, you can use the tickboxes to disable / enable effects you want. Select the aperture reshade preset by pressing the small arrows at the top of the menu. Launch the game, press the home key to open reshade menu. They did that by taking the original HL2, removing all the content not needed to run Portal, configure it to start up right into the Portal mod and put it onto Steam. Place the filmlut.png file into Half-Life 2binreshade-shadersTextures. FakeFactorys Cinematic Mod, also simply known as Cinematic Mod, is a mod that aims to improve Half-Life 2 and its episodes, and push the Source engine capabilities as far as possible. When Valve found that Portal was extremely well-received by critics and gamers alike, they decided to release it as a stand-alone game. It was a free extra on the 'Orange Box' - a Half Life 2 compilation which included HL2, the two expansion 'Episode 1' and 'Episode 2', Team Fortress 2 (also 'just' a HL2 extra back then) and of course Portal. In fact originally you couldn't buy Portal 1 on its own.

Portal 1 was originally implemented as a mod for Half Life 2.

Half life 2 portal mod