Revit 2016's New Physical-Realistic Rendering EngineAECbytes Tips and Tricks Issue #72 (April 13, 2015)
Dan Stine, CSI, CDT
Registered Architect and Author
The 2016 version of Revit that was just announced has a number of new features, several of which have already found their way into Revit 2015 for subscription customers via the 2015 R2 update. One unique feature in the 2016 version is the new Autodesk Raytracer rendering engine. Well, technically parts of this feature are already in the RaytraceVisual Style. The technology has been enhanced and incorporated into the Rendering Dialog for 3D views. This article will cover what is new and different about this new rendering engine for high quality still images from within Revit. At the end, we will also take a quick look at a new tool which creates a Section Box around the selected elements.
The images shown in this article are from:
- North Dakota State University (NDSU) graduate student, Logan Diehl
- Interior Design Using Autodesk Revit 2016 by Daniel John Stine and Aaron Hansen
Physical Based Rendering
Physically based techniques attempt to simulate reality; that is, they use principles of physics to model the interaction of light and matter. In physically based rendering, realism is usually the primary goal. This approach is in contrast to non-photorealistic rendering, which strives for artistic freedom and expressiveness. The result is less tweaking and fewer parameters to get the correct results. For example, the Autodesk engine does not have the Daylight Portal option found in mental ray.
Rendered scenes are based on physically accurate lights, materials, and light transport (i.e., Global Illumination). There is a clear trend towards physically based rendering as seen from CPU and GPU manufacturers as well as Games and other rendering engines (e.g. Maxwell, Octane Render).
Given the fundamental differences between the two rendering engines, it is not a good comparison to take a model/view that has been optimized for mental ray and render it in Autodesk Raytracer.
Autodesk Owned Technology
Some readers may be surprised to find out that the current rendering engine in Revit, called mental ray is owned by NVIDIA – the graphics card company. In addition to Autodesk likely having to pay to use this technology, they are not at liberty to change or enhance it. The new Autodesk Raytracer engine is owned by Autodesk and can be modified and enhanced as needed to parallel other changes in Autodesk’s offerings. This new technology is also available in AutoCAD, Navisworks and Showcase.
CPU Based, not GPU
For stability and scalability, this new engine is CPU based, rather than relying on the graphics card’s GPU and memory which has been popular for rendering engines. The contemporary high-end Intel cards have Ray Trace functions used by Autodesk Raytracer to streamline compute power and improve performance. Other CPUs are also supported. The process is scalable from laptops to large HPC clusters, with the ability to handle complex scenes and lighting. Similar to mental ray, when a rendering is being processed, all the cores of the CPU are being pegged at 100%.
The Autodesk Raytracer can use more than sixteen CPU cores, which had been the limit for rendering in Revit 2015 (as per Autodesk’s website). This makes the new Autodesk engine very fast! For example, the first image below (Figure 1) only took 3 minutes to render with the following settings: Medium, 150dpi and Interior Sun and Artificial Light. Compare the detail in this image with the next one (Figure 2), rendered using the mental ray engine which took 2.75 hours with the following settings: Medium, 300dpi and Interior Sun and Artificial Light. The two images are fairly comparable in quality despite the significant time difference.
Increasing the Autodesk Raytracer settings to Best and 300dpi takes 11 hours (Figure 3).
In comparing the three images, the difference can be most obviously spotted by the quality of the light and shadow on the table in the lower right corner of the scene.
Perhaps the best thing about this new engine is that it works when the mental ray engine does not. For example, Logan’s current thesis project, shown in the image below (Figure 4), was rendered using Autodesk Raytracer in Revit 2016. This project has a massive adaptive component-based space frame and glass roof assembly. This same model will not render at all using the mental ray engine – even with the lowest settings. I am not sure if it is a limitation with mental ray or how it is implemented within Revit, but that really does not matter to me. What matters is that one works and the other does not.
The testing done around this same project showed mental ray used nearly all 16GB of system RAM while Autodesk Raytracer peaked at about 5GB. Also, due to the “physical” nature of the process, portions of the rendered image will appear within minutes.
Material Color and Quality
In my experience, the selected colors and materials tend to shift too much when rendered using the mental ray engine. For example, the first image below (Figure 5) shows a carpet material from Shaw flooring used in a Revit material. The second image (Figure 6) shows how the carpet material was rendered in the Autodesk Raytracer engine. The same scene was then rendered in mental ray (Figure 7). The color is more accurate in the Autodesk Raytracer engine.
Another thing to notice in the two images above is how Autodesk Raytracer handles specular materials better—compare the aluminum finish for the curtain wall mullions. Also, a common issue with the mental ray engine is a blotchy look on walls. This does not appear to be an issue with the Autodesk Raytracer engine.
How It Works
Using the new rendering engine is very easy as it is totally integrated into the existing Revit rendering workflow. Figure 8 shows the only obvious change to the Rendering dialog when it is first opened. The default option selected is NVIDIA mental ray for each view.
Selecting the Autodesk Raytracer engine changes a few of the options in the Quality and Background drop-down menus (Figure 9). The Quality option only has four settings: Draft, Medium, High and Best. The Background option has three settings: Sky, Color and Image. The ability to customize the quality settings is not available. Likewise, there are no options for simulating clouds. The steps for setting the Color or specifying an Image for the Background are identical to the mental ray engine.
The only other change in the Rendering dialog is the Exposure Control dialog (Figure 10). Ideally, these settings should not have to be adjusted as often due to the physically accurate nature of the rendering engine. However, when adjustments are needed, the scene has to be re-rendered. This is different than the mental ray engine where clicking Apply will instantly update the rendered image while still on screen.
Once the rendering starts, the familiar Rendering Progress dialog appears (Figure 11). The only difference here is there is no row to list Daylight Portals as when using the mental ray engine.
It should be noted that Revit still does not support Absolute Photometry which represents an LED light source. I believe this relates more to fundamental Revit functionality rather than the rendering engines.
When exporting a PNG file for a mental ray rendering, the background is transparent. This is not the case with the Autodesk Raytracer engine.
New Selection Tool
Another cool visualization-related tool in the new Revit 2016 is the Selection Tool found in the View panel on the Modify tab. Simply select some elements in the model (as shown in Figure 12) and then click the Selection Tool icon. The result is a Section Box applied to your default 3D view (Figure 13).
With the new 2016 release, rendering in Revit is making a step in a new, and I think better, direction which should make the process of developing presentation materials directly in the application faster and more realistic.
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