Hi, I want to know usually which render engine is used in industry for houdini within Mantra? Micropoly or PBR? What are the difference between these two? Thanks.

PBR rendering is pretty firmly established in rendering pipelines with Arnold, MantraPBR and PRMan19 and a bunch of other render engines that are unbiased.

VRay, Clarisse, Mantra-RaytraceOrMicropolygon, MentalRay are non-physically based render engines are still used but are slowly being pushed out of lighting pipelines. Clarisse is an interesting one as it trades off PBR for performance. It is a very interesting option to watch as it is an all-or-nothing approach with it's own turn-key solution to lighting.

Any of this can change over night btw. Who knows what will be in use in a year. Makes managers of lighting pipelines stay up at night…


The benefits of using a PBR style unbiased render engine is that you get physically plausible rendering with little effort while with other render engines, you have to use various techniques such as Global Illumination, Final Gathering, photon caches, Radiosity, etc to get realistic renders with indirect illumination. Time consuming to implement and tweak but can be highly tuned for specific cases to get very fast renders with excellent look. Not a general approach to lighting and rendering that PBR style render engines offer you as an artist.


As for Mantra, you can choose between three different render engines, although most options will still lead you in to PBR: micro-polygon, raytrace, PBR.

Lump raytrace and micropolygon together as both utilize illuminance loops within the shader to compute illumination, light-by-light where the shader is in complete control.

Contrast this with PBR style render engines where all the lighting computation is done inside the render engine (black box to the artist) and all the material shaders define how the light interacts with the surface and how rays are probably going to be scattered (called BSDF).

- micropolygon rendering where your geometry is diced in to small quad patches (around a pixel in size) and the points on the quad are pre-shaded and then rays are cast against the micro-polygon and the attributes are interpolated. Fantastic in the days of 32 bit computers and 4GB maximum memory as you only needed to load in the micro-polygons you needed and pre-shading the micropolygon vertices saved a lot of time. Good for motion blur as well.
All illumination is done inside illuminance loops inside the shader. Each light is looped through the surface shader and illumination is computed. Shader is in complete control.

- Raytracing is where you cast rays in to the scene and you direct them as you see fit in illuminance loops within the shader. You can control the ray depth and return ray information to do Final Gathering approaches to light scattering to get physically plausible render results.

- PBR is an unbiased render engine where the surfaces contain BSDF's that determine the probability of scattered light rays based on incoming light rays and how the surfaces interact with the lights. No illuminance loops. Render engine does all the computation.


The nice thing about Houdini is that it has good support for Mantra and third party render engines. Mantra itself makes for an excellent platform to study micropolygon, raytrace and PBR render engines all within the same shader environment.

Those skills are fully transportable to other render engines as the theory really doesn't change very much between render engines and the various approaches.

Unfortunately at this time all render engines have their own unique scene description interface and their own shader implementations. OSL is one hope of unifying shader languages among various render engines but who knows if this will fly in the long run. Depends if the large render engines support OSL as well as their own shader implementations.

Hope that helps.

Thanks for your detailed description Jeff. Very helpful.
Another question is when I want do smoke, ocean or liquid sim, then which type of engine will give the best result? Thanks in advance.

Right now, PBR. Straightforward to set up with ok render times and memory usage.

Most lights still come in off the shelf with no light fall-off where the default should be “physically correct” imho. Watch out for that. Set all area/point/spot lights to physically correct fall-offs. Environment/sun/distant lights should have no fall-off. I don't even think the option is available on Environment lights.

jeff
Right now, PBR. Straightforward to set up with ok render times and memory usage.

Most lights still come in off the shelf with no light fall-off where the default should be “physically correct” imho. Watch out for that. Set all area/point/spot lights to physically correct fall-offs. Environment/sun/distant lights should have no fall-off. I don't even think the option is available on Environment lights.

Thanks a ton. Now I am clear why those weird shapes were visible on that smoke. Thanks again.