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In theatre it is common to create visual effects on stage by using shape masks to cast shadows across the scene. These masks are called "cucoloris" or "cookies". Creating a light cookie in WebGL using PlayCanvas is achieved using a texture or a cubemap that is applied to the Light Component. Light Cookies are applicable to spot lights and omni lights.
A simple grid cookie gives the impression of a window
A cookie texture is just a regular texture asset which is applied to a spot light component in the Cookie section. When a cookie is applied you can choose which channels of the texture to use. For example, if you are doing a simple mask, you can use just the Alpha channel. Or if you would like a full color effect like a stained glass window you can use the full RGB value.
For an omni light the cookie needs to be a cubemap so that it can be projected in 6 directions around the point of the light.
Shaped Lights - cookies are useful for masking the shape of a light source. For example, if you're scene contains a window or a fluorescent strip light using a spot light will create a light shape that is rounded. You can mask the shape of the light to be long and thin, or square in order to simulate the shape of the light.
Torches - Torches and headlights don't light a surface evenly instead they have light and dark "caustic" areas. You can use a light cookie to create this effect.