Renderman has a very useful parameter called Trace displacements that basically turns On/Off displacements from the pov of a ray.
For example, if you have a displaced sphere and a mirror, and trace displacements is Off, the sphere's reflection won't have displacement. edit: The sphere as seen directly has displacements, its reflection does not.
Now if you imagine the mirror is also displaced and/or it has soft reflections, it's easy to see how a reflection of the displaced sphere is completely unnecessary.

This is a very useful thing to have for optimization purposes.
Normally to achieve this in mantra I have to go about creating phantom duplicates of existing geometry with the displacement turned off and set-up complicated scope relationships between objects. This is very necessary because I know from experience that render times can go from an hour to a few minutes, with little if any visual degradation…
However, the setup is a mess… it's difficult for the scene creator to have a good mental picture of what is going on, and as you can imagine way more difficult for another artist to work with.

Mantra should have this… With the addition of being able to turn On/Off Self-trace-displacements. What this would do is allow the displaced sphere to self reflect/refract/shadow/occlude, but any rays from other objects are seen with no-displacement.

90% of the time tracing of displacements isn't needed at all, unless objects are touching, intersecting, or needs to trace itself.

puurrrrlease

cheers

Sergio

Serg
Renderman has a very useful parameter called Trace displacements that basically turns On/Off displacements from the pov of a ray.

puurrrrlease

cheers

Sergio

Take a look at the “True Displacements” property in the render properties.

Thats not what I mean… note I said “trace” not true displacements.

Think of it as a “true displacements” for rays, not what you see through the camera.

S

Serg
Thats not what I mean… note I said “trace” not true displacements.

Think of it as a “true displacements” for rays, not what you see through the camera.

S

In mantra the true displacements are not separable between traced rays
and camera rays. The reason for this is because of the potential
artifacts that arise.

Consider a sphere which has a uniform displacement towards its origin
(i.e. shrinks the sphere). If the rays traced the non-displaced
surface, you would end up self-shadowing the surface.

Yep, its the same artifact you get if the “mp ray shading quality multiplier” is set too low. Thats why I mention a self-trace-displacemnt option. The sphere sees itself as being displaced, but the ground it sits on sees no-displacement from the sphere.

actually just thinking about it now, the ray shading quality multiplier (can't remember what you call it exactly) property shows that this is probably already possible in the current mantra architecture. And its probably a better mechanism than an On/Off switch. All that is needed is a “self ray shading quality multiplier” property.

So, say you have two displaced sphers A and B. Sphere A and B are set to SRSQM of 1 and a RSQM of 0.1. Both spheres see themselves perfectly, but rays traced against each other are tesselated far less.

Setting the RSQM to 0 would stop the displacement shader from being evaluated at all.

edit: just realised this affects how the ray sees the surface shader as well as displacement, so you maybe you wont want to use 0 as a means to switch off displacement completely in raytracing. Maybe a On/Off as in previous post would be a good addition to SRSQM.

Btw its standard procedure here to set the RSQM to 0.5, the vast majority of time we see no artefacts. Render times however drop by about a 3rd.

Hope I'm making sense

cheers
S