Tuesday, November 12th, 2019 Posted by Jim Thacker

Tech focus: Modo’s new mPath render engine

Foundry has unveiled mPath, a new path tracing renderer described as a “ground-up redesign” of the default Modo renderer, capable of making use of either the CPU or Nvidia GPUs for ray tracing.

The first iteration of the renderer, which promises increased render speed, more flexible output and more advanced material properties, shipped in Modo 13.2, released late last week.

You can read more about the new features in Modo 13.2 itself in a separate story, but we thought that mPath was a big enough feature to warrant a closer look.

A GPU-capable replacement for Modo’s legacy CPU renderer
Although it’s still a work in progress, mPath is ultimately intended as a full replacement for Modo’s ageing default renderer, which was added to the software in 2006.

Whereas the default renderer is purely CPU based, mPath features a modular architecture, making it possible to build in support for a wider range of hardware.

In a livestream hosted by Modo news and training site Pixel Fondue – the recording of which is embedded above – Foundry head of rendering Allen Hastings noted that mPath is not a pure GPU renderer.

“It’s able to leverage the GPU in specific areas, like ray casting,” he commented. “But the way it has been architected allows it to be extended further.”

Support for older Nvidia GPUs, including Maxwell cards
Unlike traditional path tracers, mPath uses a non-recursive approach, in which the process of tracing camera paths is decoupled from that of shading the surfaces in the scene struck by those camera rays.

Hastings said that the approach would enable Foundry to support different hardware backends more easily, and to support separate hardware backends for ray tracing and shading.

In the initial implementation, there are two ray tracing engines: Foundry SSE, which is CPU-based, and a GPU-based alternative, using Nvidia’s OptiX framework.

Although Foundry’s press release describes mPath as providing “accelerated ray tracing via Nvidia’s RTX technology”, the implementation doesn’t actually require a current-gen RTX GPU.

In the Pixel Fondue stream, Hastings confirmed that while the renderer has “significant gains on RTX hardware”, GPU ray tracing is also supported on older Nvidia graphics cards.

That includes cards with the Maxwell architecture, rolled out in the GeForce GTX 800 series in 2014 and the Quadro M series in 2015.

So how much faster is mPath than the default Modo renderer?
The obvious advantage of mPath over its predecessor is speed: one interior test scene shown by Foundry in the Pixel Fondue livestream rendered in 100 minutes in mPath versus 360 minutes in the default renderer.

In other test scenes, the time saving was smaller: Hastings commented that the improvement is most significant in “complex scenes with lots of geometry, or … a lot of [bounced] light”.

Depth of field calculations also benefit – they take “about 50% longer” with the old default renderer – and distribution of noise is more even, which should benefit denoising workflows.

Other benefits: ease of use, flexibility of output, and more advanced material handling
Another benefit is ease of use: mPath features a much smaller set of control parameters than its predecessor, shown in the workflow video above.

As well as settings for the number of light bounces used to render the scene, there are essentially two key parameters: Noise Threshold and Maximum Quality.

The two determine where mPath focuses computational effort while rendering: in later iterations, only those parts of the image in which noise remains continue to be refined.

mPath also supports light path expressions, making it possible to split out the contributions of individual light sources to a render more flexibly.

Instead of needing to being rendered separately, light passes can now be generated in a single render: according to Foundry product designer Greg Brown, with “virtually no [computational] overhead”.

Foundry also says that the new engine handles refraction better that its predecessor with many beta testers “surprised at how much better glass looks” in renders.

Other functionality that may be added to mPath in future includes the ability to stream texture data to improve performance when rendering on the GPU, support for Open Shadling Language – mPath is “inspired by OSL syntax”, according to Hastings – and more physically based subsurface scattering.

Current limitations
However, in its initial implementation, mPath is rather more limited: the renderer currently only supports Modo’s default physical material, not the Traditional or Energy Conserving material models.

It supports all of Modo’s procedural materials and gradients, but not the Skin or Hair material, or the Principled Shader, and can not currently render volumetrics.

The initial implementation is bucket-based, which Foundry describes as having advantages for animation workflows, although fully progressive rendering with the option to stop a render at any point is planned.

Pricing and availability
Modo 13.2 is available for 64-bit Windows 10, RHEL and CentOS 7+ Linux, and macOS 10.12+. New perpetual licences of the software cost $1,799, while rental starts at $599/year.

Read more about mPath in Foundry’s online documentation