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Shadows in an image convey a lot of information. Everything from time of= day to how many lights there are can be inferred from shadows. Soft shadow= s from indirect lighting and shadows from objects off-screen can also be pl= easing and realistic. The below guide explains how shadowing works in Rende= rMan as well as types of shadowing and quality.
The type of shadowing produced in RenderMan is based on controls in the = lights as well as what's available = to the integrator. Beyond the de= scriptions below, it's important to know that shadow softness is controlled= by the size (or apparent size in an HDRI) of the light source. Larger ligh= t sources create softer shadows.
Typically users render with the PxrPathTracer Integrator and the default light settings of "Thin Shado= w". This combination produces a quick and somewhat plausible image for tran= sparent/refractive objects. It also obeys the object color such as transpar= ent red glass or maybe green emeralds, etc. You can manipulate the shadow f= or art direction including shadow color, distance, falloff and gamma (see e= ach light's page for examples).
If you were to disable Thin Shadows on the light by sel= ecting Trace Light Paths, and render with the PxrPathTrace= r, you would see a solid shadow.
The integrator would render a physically correct shadow now, but the Pxr= PathTracer doesn't generate photons by default and would make this very dif= ficult to resolve using Allow Caust= ics as seen below.
If using Trace Light Paths on the light, you should com= bine this with the PxrVCM or PxrUnifi= ed integrators and selectively enable this parameter to generate photo= ns for realistic shadows. Below the dome light is using Trace Light Paths.<= /p>
Tracing photons for caustics can be very expensive and difficult to art = direct. In some cases your hero object may not get enough photons as they a= re scattered around the scene. Other times it may come into the path of a v= ery bright light and suddenly there are caustics scattered all over the sce= ne which may be a distraction. By selectively choosing which lights use Tra= ce Light Paths, the artist can better direct where they want photons and at= what intensity.
The PxrVCM and PxrUnified integrators also have a Guiding= strong> parameters that allows artists to direct where they want to concent= rate the photons in the scene. This parameter can bias the render (make it incorrect physically) if driven wi= th high values, but for art direction purposes it can greatly improve conve= rgence. Below are two images, both with 32 samples. Notice the image on the= left is much less converged, Photon Guiding was set to 0.00 while the righ= t image used the glass bounding box information as the Photon Guiding BBox = and Photon Guiding set to 0.900.
Scene courtesy of Julien Duval.
PxrUnified has a novel way to generate caustics selectively using the Manifold Walk technique.= p>
When using some lights, you may be tempted to alter the shadow falloff p= arameter. In many cases it's pure artistry, but for the PxrDomeLight it can be a useful hack for quick int= erior renderings. Since this operates as an overall environment light, you = can reduce the shadow falloff (based on your scene extent) to get more ligh= t into an interior in a way that might resemble ambient occlusion. Below ar= e some examples. Note how the shadowing on the right side of the image simp= ly goes away.
Typical render at default with no falloff = or portal lights
Shadow falloff set to 500 units
Shadow falloff set to 50 units