Now that the beta for Renderman 19 is over and I have access again (Yay, non-commercial licenses!), I can share some renders I've done with RIS. I got onto the beta last fall and was able to play with it, and while RIS is Renderman's answer to Arnold and VRay, it's quite a different beast. Let me start out by saying this was my 1st dip into Renderman, and I am by no means an expert. I've always stayed on the raytracing side of the raytrace / Reyes debate, and now that Renderman has a production level raytracer, I couldn't wait to dive in. With that said, I do use Arnold, VRay, and MR and am very comforable in those renderers, and I feel like I can give the new RIS features a fair shake.
Like Arnold, Pixar is going for the least common denominator with RIS. The less settings to tweak, the better. There are 2 basic sampling setting, and that is it. Min/max samples, and a pixel variance (threshold) setting. There are no light or shadow samples to set. No reflection / refraction / SSS / GI samples to set. (Full brute force GI mind you) Renderman does it all for you. Noise is taken care of in the max samples. This can be a great feature, as it's easy to get rendering almost immediately, but it can also take a lot of extra samples to get noise out of small areas where I feel just a little tweak on a light or shader could make for a faster render.
*** UPDATE ***
As I've been playing with the full / non-beta release of the software, I have found ways to increase sampling on a per light, GI, and shader basis, however out of the box, it's setup to not need to make changes to these controls.
This render is a great example of what I'm talking about. (This setup is very similar to the Arnold render I did on this model several months back, however rendered at 850x850) The overall skin shader was smooth by 1024 samples (about 5-10 mins of rendering), however I let the renderer go to about 2048 samples (20 mins), and you can still see the flickering noise in the ears. I'd imagine another 10-15 minutes and it would be production quality. While Arnold rendered a similar image in ~3 minutes at a higher resolution, it's still hard to fault Renderman here. VRay and MR would probably be a bit faster, but also require a much higher entry level. While the render was slower than Arnold, I really like the way this render looks over my old Arnold render. Perhaps I should go back and revisit it.
I do have to say, I like the shaders. The above render is using the PxrSkin shader. It opporates much like equivilent SSS shaders with fewer settings. The PxrDisney shader is the do-all workhorse for dialectrics. While I'm used to the shader models that the other renderers have been using, the Disney shaders are probably more intuitive to use. I felt like I was able to get the basic look of my materials faster than I would with my typical MIA / Arnold / VRay_Mtl shaders. That being said, Disney and Pixar call these shaders "Physical-like", so there are some attributes that behave... interestingly. There are separate metalic and specular attributes, and when metalic is set to 100%, specular doesn't do anything. I think this is an odd behavior, but once you understand this, it doesn't cause any issues.
One benefit of RIS is the bi-directional path tracer. This is something you can't get in Arnold, and while I've heard Vlado talk about implementing it into VRay, it's not something I've seen in VRay 3. This rendering "integrator" shoots primary rays not just from the camera, but from lights as well, then blending any rays that are within a certain radius and angle. This allows for beautiful caustics without any tweaking, and can resolve renders faster around lights tucked into corners. While this is a really cool intergrator, I don't imagine it would generally be as useful as the uni-directional path tracer due to the rendering overhead.
While technically Arnold wins the above rendering speed battle, and Arnold isn't much more complicated, there is something to be said about the price difference. Rendermain is free for non-commercial use, and is a tad under $600 for a license. As of this writing, Arnold is $1200 and requires you to buy 5 licenses at a time. It's clear who Renderman is targeting, and I think it's worth a good hard look if you're freelance or working on a small project.
More to come as I can now render with my free non-commercial license.