Arnold Vs Mental Ray: which is the best render engine for Maya?

INTRO

After the NVIDIA anticipations about the departure of Mental Ray and iray rendering as integrated render engines, Autodesk confirms official rumors that wanted Arnold to be the new 3ds Max 2018 integrated render engine.

The integration will follow the one already carried out last year for Maya 2017. Right now Arnold is free when used directly via Maya interface on a single workstation and limited to still frames (so no batch render for animations). The Price start at $1,220 for a single license, with a range of discounts for the bundle. If the system does not find valid licenses, however, batch renders will display a watermark.

Arnold for Maya is one of the most widely used rendering engines in the CG Industry, which often constituted a valid and simple alternative to the old Mental Ray for Maya.

Regarding Mental Ray, you can use it for free of charge for use on a single machine, with the same restrictions for Arnold (no network render and animations). You have to download it directly from the official website, it will no longer be included in the Autodesk installer.

What are the main characteristics and differences between the two? 

MENTAL RAY

Mental Ray is a feature-rich, programmable, biased ray-tracing render engine. It has been used by the industry and professionals for over 28 years (the first release have been built in 1989 by Mental Images), Mental Ray has become a standard for photo realistic rendering across the film, visual effects, and design industries (Hulk, The Matrix Reloaded & Revolutions, Star Wars: Episode II – Attack of the Clones, The Day After Tomorrow and Poseidon are just few examples of products made using Mental Ray). In 2003 Mental Images was awarded an Academy Award for their contributions to the mental ray rendering software for motion pictures.

Mental Images was bought in December 2007 by NVIDIA, trying to bring some freshness to the old Mental Ray. We saw concrete results just in 2015 when the new GI NEXT was implemented. With this new GI model, Mental Ray can use the power of CUDA to compute the indirect/global illumination on GPU. Anyway a big part of the render computing is still CPU based, I hope to see some big steps toward the full GPU utilization.

ARNOLD

Arnold is an unbiased (so it's similar to Maxwell, Octane, Cycles), physically based, ray tracing 3D rendering application created by the company Solid Angle. Arnold is an advanced Monte Carlo ray tracing render engine built for the demands of feature-length animation and visual effects. Originally co-developed with Sony Pictures Imageworks and now their main render engine, Arnold is used at over 300 studios worldwide including ILM, Framestore, MPC, The Mill and Digic Pictures. Arnold was the primary render engine on dozens of films from Monster House and Cloudy with a Chance of Meatballs to Pacific Rim and Gravity. On 4 January 2017, the Academy of Motion Picture Arts and Sciences recognized Fajardo with a Scientific and Engineering award (Academy plaque) for "the creative vision and original implementation of the Arnold Render engine."

These are the main attractives of Arnold Render: is simple and fast to learn, is photo realistic with no efforts, is memory efficient and can handle extreme complex assets without problems (did you saw Gravity?!), alShaders (a powerful toolset for artists).

THE TEST

I tried to make the overall scene and light setup very simple, just to reduce the number of factors that could enter in the game. Whenever possible I used the same Maya light primitives, I've also put any light source in the same position and with the same intensity. Obviously the rendered images are not identical because any render engine interprets the lights and materials in its own way. No post processing. The first render is always Arnold (mtoa 1.4.2.0), the second is Mental Ray (2017.12, 3.14.4.6).

HARDWARE SETUP

I used an average pc: CPU I7 2600k @ 4.4GHz (to give you and idea of its compute power the Cinebench R15 Score is 758), 32GB DDR3, Nvidia GTX 1060 6GB, SSD 500gb. Win 10 pro 64 bit

FIRST TEST: INTERIOR RENDER

Same scenario for both render engines: Sun & Sky + area light. The scene consists of 3M vertices, plain polygons, 3 diffuse bounces, no caustics, SSS, dispersion effects, hair or fur. I've put just few textures, keeping the environment pretty flat to make us clearly see and compare noise and overall render quality.

Between the images above I didn't see any significant difference in terms of quality, the render seems a little sharper in Arnold Render, but the noise is also more prominent (especially in the direct shadows areas) . The new Mental Ray GI NEXT is pretty amazing: it can compete with the brute force unbiased Arnold's approach (that uses Montecarlo's algorithms) maintaining a lot of details, especially regarding tiny contact shadows. Look at these closeups:

Huge difference in render time: Mental Ray 1h 22min vs Arnold 18h 26min (sill noisy)

SECOND TEST: HAIR RENDER

I put just one area light on the left. GI on with 1 diffuse bounce (Final Gather for Mental Ray since GI NEXT didn't work with hair).

Again I don't notice any significant difference between the two renders, I just wonder why they render different colors.
Render time: Mental Ray 4h 36min vs Arnold 2h 5min

THIRD TEST: BACKPLATE + HDRI RENDER

This is another very common render scenario: a dome light HDRI + Shadow catcher. Diffuse bounces 3.

Very similar images, anyway Arnold delivers a little more detail in a shorter render time. 
Render time: Mental Ray 39min vs Arnold 32min


FOURTH TEST: VOLUMETRIC RENDER (MAYA FLUIDS)

 I loaded a Maya fluid fx preset to test Arnold and Mental Ray in volumetric render.  FG on for Mental Ray, 2 diffuse bounces.

The result is quite different, the correct representation is the Mental Ray one: it renders identically to Maya software, while Arnold interprets the fluids in its own way (it seems to be a common Arnold problem, I watched a lot of comparisons where Arnold renders volumetric effects in a wrong way). Here Mental Ray is the king, it renders the correct fluid appearance in a much shorter time :

Render times: Mental Ray 0min 17s | Arnold 6min 48sec

CONCLUSIONS:

The quality of the 2 render engines is very similar, Arnold can give sharper results and a flicker free render. Mental Ray is superior for architectural renders (interior renders) and special effects renders (fluids and volumetrics).

In the other hand Arnold is a little easier to learn and the official documentation is also very well written. Arnold have a great shaders library (including the amazing ALShaders), but Mental Ray have the revolutionary MILA shader, which is an amazing shader for pro artists. 

Arnold is much faster when doing full GI IPRs, Mental Ray suffers of long precalculation render times when GI is on (FG or GI-Next) so you can basically use IPR only disabling GI. 

Mental Ray is a much better render engine for architectural and VFX purposes (due to its better performance in these particular scenarios), Arnold is for artists that wants a very fast and solid IPR, flicker-free detailed renders: I advise Arnold to Character designers and artists more oriented to the cinematography industry.

Mental Ray is cheaper (especially if you own a Nvidia Quadro). 

Arnold is also a must if you are planning to work in big VFX studio, in fact today Arnold is one of the most used render engines in the VFX industry. Anyway Mental Ray 2017 is a very good render engine (Nvidia did huge steps toward modernization), probably a better choice for individual artists, especially for architects and designers.

Fell free to share your opinion and your experience in the comments below!
Antonio