Will make a tips page for this. In the meantime ...
The user ibl shaders are attached to area lights as "user" type. (In Maya UI, this is labeled as Custom shape type.) This means that the shader controls the sampling rather than any specific shape. [Note that user (Custom) type of area light is required for user_ibl_env, but for user_ibl_rect, please use rectangle type.]
The user ibl env shader attaches to both the environment and to an area light placed anywhere in the scene. It doesn't matter, because its just going to use the texture as a spherical environment texture. Its a lat-long, typically people use a texture that is 2:1 resolution width to height. Width represents 360 degrees, height, 180.
The as_reflection input specifies whether the IBL image was shot as a reflection like a camera shooting a mirror ball, or if not, straight out into the environment, like a camera swiveling about the origin capturing the data. [Drawing a diagram, which I will do soon, should help. A ray that goes from your eye/camera straight out to the environment, is different than a ray that hits a mirror and reflects backwards. Often people take a shot of a mirror ball and convert it to a 2:1 lat-long, and this would need the as_reflection on.]
The samples setting controls the maximum number of samples used from the light. This is affected by an importance mechanism in two ways. One is that more important parts of the texture are used more often in the sampling. And the number of samples may be decreased, if the light path goes down in depth, for example, we are processing a material shader for a material hit after several reflections and refractions.
These are direct area lights, so typically you should specify as visible, if you want to see them. The visible user ibl env, creates your environment, and therefore is not used for FG (indirect) because it is now a direct light representing the environment. Ie, the number of samples it uses is called for every material in the scene. For high res, high dynamic range env images, this should perform much better for lighting than straight up FG. To get the same with basic FG would take thousands of FG rays.
For user ibl rect, just attach to an area light again, [edited later], but not as user, rather as rectangle type. In Maya for example, the UI for the area light should match the rectangle region that will display. This is typically used for large textured light cards, that may be taking the place of lots of rendered objects. For example rendering these objects into the texture you will use.
Last edited by bart; May 9th, 2012 at 20:02.
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