Hi!
I’m a former software engineer and now working as an architectural photographer. I’ve been always attracted to the 3d world and started learning Houdini (because of its procedural nature) a year ago for some “artistic” explorations, related to light and material in the most photorealistic way possible. Pretty hard but rewarding experience by now.
But I’m in some sort of crossroad now as my first serious tests are showing me Mantra, as unidirectional pathtracer, struggles to render the type of images I’m creating (think mostly closed spaces with very indirect light).
Then I researched about Renderman and its bidirectional path tracer that could make my task a lot easier but I’m afraid I don’t know what I’ll be missing or losing from not learning Mantra further. I have to note I’ve started learning to make my own shaders using VOP and don’t know how capable is Renderman in that sense.

I won’t earn money from this on the short time so I’m looking for no-cost solutions, And GPU based solutions are not a choice for me as my workstation is CPU-based. But I don’t rule out learning others engines in apprentice mode, such as the watermarked Arnold.

I’m very interested in your experiences and opinions on this topic.
Thanks!

Renderman is a very capable renderer, however I've found since it is lacking a shading language, recreating even simple mantra vop networks is frustratingly complicated as most simple mathematically operations have no analog (add subtract multiply divide, etc). True OSL shaders can be used for some of these roles, however it is missing any pattern generation capabilities such as noise which makes it mostly useless as anything other than a ‘math box’. It also adds a layer of abstraction which makes the network more complicated that it would be with mantra.

While true that mantra struggles with brute-force resolving highly indirect scenes, it's photon light is generally capable of solving these types of scenes. Renderman will still take some time to converge for these scenes, and doesn't have any photon approaches that it could use to speed up rendering.

You might want to look into VRay if you are mostly going to render indoor architectural type scenes on a cpu, as it has several different options for optimized rendering of the indirect lighting.

jsmack
Renderman is a very capable renderer, however I've found since it is lacking a shading language, recreating even simple mantra vop networks is frustratingly complicated as most simple mathematically operations have no analog (add subtract multiply divide, etc). True OSL shaders can be used for some of these roles, however it is missing any pattern generation capabilities such as noise which makes it mostly useless as anything other than a ‘math box’. It also adds a layer of abstraction which makes the network more complicated that it would be with mantra.

While true that mantra struggles with brute-force resolving highly indirect scenes, it's photon light is generally capable of solving these types of scenes. Renderman will still take some time to converge for these scenes, and doesn't have any photon approaches that it could use to speed up rendering.

You might want to look into VRay if you are mostly going to render indoor architectural type scenes on a cpu, as it has several different options for optimized rendering of the indirect lighting.

Thanks for the answer!
I've been testing Renderman (22) for several weeks and I have to say I'm impressed with the speed I'm getting, and the quality of the Pixar Surface shader. Current integration is cumbersome and with lots of rough edges for the newcomer, but it will improve with the coming-soon rewritten plugin.
BUT on the other side: I really miss the integration of Mantra with Houdini, the vast possibilities of creating shaders within the mat context with VOPs, everything mostly “just works”, etc. I have yet to find a thing I can't do in Renderman that I can in Mantra, but the process is never as simple and elegant (Renderman has pxrOSL and Se Expressions and that helps a lot when creating simple shaders, I have found its node selection somewhat limited).
So the summary by now is: I'm completely divided! I love Mantra customizability, integration with Houdini and philosophy, but I love Renderman bidirectional integrator for indirect lighting and speed.
And I have to say Renderman earns one extra point for being open on what's down the road. SideFx has been teasing big changes on Mantra and lighting for a long time, but nothing specific that could help me decide in which tool devote my time.