Light Glow occlusion

Posted by the area, 2 May 2007 5:33 am

There is a common confusion with lightglow occlusion in Maya(not to be confused with light fog).

The size of the spotlight "directed disk" shape (or sphere shape for a point light) determines how large the light source is considered to be. Thus if part of this shape is visible to the camera the glow will appear even though the center of the light may be occluded. This can look unnatural if the disk is large because the center glow region appears to show through the surface. This feature is designed to allow the glow to slowly wink out when the light goes behind an object instead of suddenly popping off. However depending on your scene scale the size of the light may be too big( in general the default sizes are way too large ). In the animation I'm showing the size is still too large, but I wished to demonstrate the gradual winking out effect. A little smaller will result in a more sudden on/off of the light during the animation, which will look more natural as the glow in this case simulates a very small light with a very focused center to the glow. If you needed to simulate a large glowing source, like a bright window or large paper lantern then it is better to model the geometry and use glow on the shader node, rather than the light glow. Light glow is best for smaller point glow sources, while the shader glow, being raster based, is better at glowing large areas of the screen.

Simply scale down your light size
or the directed disk size (using the standard transform scale tool... with a point light you can also instead adjust the sphere radius attribute).

Note that shadow settings do not affect this occlusion, although you must have zero transparency on the surface. Also mapping transparency will allow the glow to show through, even in opaque regions. However if you enable raytracing and raytraced refractions on the shader then it will become opaque for the glow.

One final point: With "Light Glow" (as opposed to "Light Fog" the actual glow simulates various scattering effects that happen in the viewing device( be it an eye or camera ). Thus the glow appears in front of the entire scene unless you directly eclipse the source of the glow, in which case the glow entirely vanishes. If you are walking down a street at night the street lights may appear to have large glows around them( especially if you have just been swimming in a chlorinated pool ). Try holding your thumb out and just block the source of the light. The entire glow around the light will disappear( unless it is a foggy or rainy night, in which case one also has some illuminated fog ).


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