Hello, Arnold GPU

Arnold is one of the most popular renderers in animation and film production, the software and renderers supported by Fox Renderfarm including Arnold, we are also an Arnold render farm. With the recent release of Arnold-5.3.0.0, the first version of the Arnold GPU (beta) has been officially released. We can share some of the results and thoughts about the pre-test of the Arnold GPU.
The principle of Arnold GPU
More than two years ago, Marcos Fajardo (author of the Arnold) talked about the possibility of Arnold GPUs for the first time at SIGGRAPH2015. At the time, a full-platform-supported GPU renderer based on the OpenCL computing framework was designed. However, the current Arnold GPU is based on NVIDIA Optix rendering architecture, the bottom layer is CUDA language, CUDA is much more friendly than OpenCL, the development progress is certainly faster, and the most important thing is that CUDA is faster and more stable. With the release of the RTX series of graphics cards, vector intersection processing is getting faster and faster, and it is more suitable for GPU ray tracing algorithms.

Arnold CPU and GPU effects
The original purpose of Arnold GPU rendering was to keep the CPU and GPU rendering results close to each other, which is the effect and speed of the different rendering modes of the beta version.
Test environment:

Katana3.0 KtoA 2.3.0.0 gpu19 GeForce GTX 1080
Using 8 render threads
Parameter settings:
AA samples = 6
GI diffsue samples = 3
GI specular samples = 3
GI transmisson samples = 3
GI diffsue depth = 2
GI specular depth = 3
GI transmisson depth = 8
Light sampling is the default
Metal
Metal (CPU on the left, GPU on the right)
Rendering time:
CPU - 2m13s
GPU - 10s
In summary, the metal BRDF material does not require much computing performance, and can be well calculated on the GPU, so the CPU and GPU noise is less, GPU rendering is much faster. When there are many hard surface materials in the scene, the GPU will show more powerful speed and performance without losing the rendering quality.
Glass

Glass (CPU on the left, GPU on the right)
Rendering time:
CPU - 31m57s
GPU - 17s
Glass BTDF material, ray tracing renderer is too slow and too inefficient. Although the Arnold GPU renders extremely fast, it is obvious that the rendering details are lost and the noise is dense. Still using the CPU to render slowly is more reliable.
Subsurface scattering

Subsurface scattering (CPU on the left, GPU on the right)
Rendering time:
CPU - 6m39s
GPU - 36s
Subsurface scattering(SSS), also known as BSSRDF, is the biggest surprise for Arnold GPU in so many tests. The Arnold GPU implements the random_walk BSSRDF in standard_surface.
First, the Arnold 5 comes with two types of SSS:
Dif
Random_walk
Diffusion is an empirically based SSS model that renders results faster because many of the parameters are obtained by looking up the table. This empirical model is also commonly used today. Random_walk is a complete light reflection calculation based on a real physical model.
Going back to the Arnold GPU, it doesn't support diffusion but uses random_walk directly. The results are perfect, the noise is small, and the rendering is very similar to the CPU. In actual production, this is a CG production requirement that is fully qualified for film.
Cornell Box

Cornell_box (CPU on the left, GPU on the right)
Rendering time:
CPU - 23m37s
GPU - 1m27s
The result is perfect, although the rendering time is slow, but the effect is amazing. Compared with Redshift, the speed is fiasco, and the effect is over. The biggest problem with Redshift is that indirect lighting tends to be too bright and dark details are not enough. The effect of the Arnold GPU is basically the same as that of the CPU. At this point, it is the best known in the current renderer (prman doesn't know).
Please note here that the reason why the rendering time of the GPU is significantly faster than the CPU is because the sampling is the same, and the sampling result is the same, the noise of the GPU rendering result is significantly more than the noise of the CPU rendering result.
Arnold Beta Edition Limitations
Because Arnold is still in the early beta version, many features are not available. It should be a little early to use the Arnold GPU for production. Here are some of the bigger drawbacks.
General restrictions
GPU rendering, based on the same sampling, more noise than CPU
GPU rendering will read all textures into memory and video memory, not supporting streaming texture
GPU rendering does not support bucket rendering, all supported AOVs are left in memory
GPU rendering does not support OpenVDB
Shaders restrictions
OSL Shaders is not supported yet
Third-party Shaders are not supported at this time
Does not support AOVs write, does not support write_aov
Lights restrictions
Cylinder_light is not supported yet
Disk_light is not supported at this time
Mesh_light is not supported yet
Light_links is not supported at this time
Light_filters is not supported at this time
The future of Arnold GPU
Arnold wants to use GPU rendering for movie-level projects, but because of the GPU's own shortcomings, many renderings must be calculated using the CPU, which is more efficient. Therefore, it is very important to switch between rendering between CPU and GPU freely. Based on this, Arnold can provide high quality CPU unidirectional ray tracing rendering while providing efficient GPU Optix ray tracing rendering.
A set of APIs supports CPU and GPU
Arnold was originally designed to use a set of APIs that are compatible with CPU and GPU rendering. Now Arnold GPUs are beginning to be compatible with some of the Maya native materials available in MtoA, at least for this, Arnold has made some progress.
Support for OSL Shaders
The latest news, from the developer forum, NVIDIA is working with Sony to develop OSL GPU-based compatibility, including several important features of OSL: Closure and LPEs. Will join the Arnold GPU soon.
Rendering consistency
Now it seems that Arnold still does a good job of restoring effects. GPU rendering tries to move closer to CPU rendering because the two rendering architectures are different and no longer extend.
Reference: MIYAZAKI

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In the last two years, the game art industry has ignited a fierce PBR boom, which is as hot as the "secondary era" of the year. For the PBR process that almost redefines the game art, many friends have a lot of doubts. For example, why does the effect of PBR look so good, whether it is necessary to learn and use the PBR process in the film and television industry. If so, how do we use it? This article will discuss these issues in a very detailed and professional manner from the perspective of film and television practitioners, so that everyone can understand the ins and outs of all this. How to use the logic of film and television to understand PBR? PBR is an abbreviation for physically based rendering, which is based on physical rendering. However, this is just a name. It can only be said that it is close to physics to some extent, and there is still a gap from real physics. But the PBR process has been very good at describing most of the materials we encounter in daily life. 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However, this difference is completely within the scope of acceptance. And you can make up for it by using the iray GPU renderer when previewing. 2) Use PBR maps in vray Similarly, first use vray prefabrication from the material painter to export a set of textures. Most of the content is similar, except that reflection replaces specular, both of which are used to describe the color of the reflection, with different names and meanings. Glossiness is the opposite of Roughness. It is a description of the degree of roughness of the reflection, but one refers to the degree of clarity, and one refers to the degree of ambiguity. The most puzzling texture is a gadget called ior. The first contact with a friend will definitely be embarrassing. The word Ior is an abbreviation for index of refraction, which means refractive index. According to the physics knowledge of junior high school, the refractive index is something that is transparent glass, and should be the inherent property of a transparent object. How can it become a texture? What role can this texture play? Comparing with the previous arnold output texture, you will find that the ior map here corresponds to the F0 of the arnold output. Then make a simple guess, are there any intrinsic links between the two textures? The answer is also yes. We can find such a formula in the help document of allegorithmic: Don't be afraid to see the formula, at least this one is very simple. This formula describes the relationship between F0 and ior. The left side of the equation is of course F0, while the n on the right refers to ior. Then through this formula, we can roughly feel that F0 and ior have an intrinsic correspondence. Now let's try to do a simple operation. When ior is equal to 1.5, it is very easy to calculate, F0 is equal to 0.04. Hey, it seems that this 0.04 is a bit familiar? What is this sense of sight? In fact, the number of 0.04 has already appeared in the previous article. "When the metal value is 0, the value of F0 is 0.04". 0.04 is the value that is mandatory for the F0 of all non-metallic objects in the PBR process. Correspondingly, the value of ior is 1.5. According to the knowledge system of film and television rendering, we know that the ior attribute can also be used to describe the intensity of reflection. Generally we will use the ior between 1.3 and 1.6 to set the non-metallic material, and the ior above 20 to set the metal material. Put a little knowledge here. Seriously seeing friends here may have a question, that is, we usually do rendering, ior may give more change, but according to what I said before, it seems that the non-metallic substance in PBR, the constant use of 1.5 ior does not change ? Isn't the reflection intensity the same? Can the material from this be correct? You can take a look at the scene of the opening of Shenhai 4, they also use this system to do, and nothing wrong. Why is there a problem with the constant ior of 1.5? Because the change of F0 in all non-metallic substances in nature is nothing more than between 0.02-0.05 [2], the difference is very small, even if all use 0.04, there is not much change. Some software also considers this problem, adding a parameter to the basic PBR parameters to slightly control the intensity of F0, so that the artist has more room for operation. For example, the material of the Unreal 4 game engine has a specular attribute, but this specular is completely different from the traditional specular. It actually plays a role in making F0 change from 0.02 to 0.06. You can click on the link at the back of this article. There is a slider on the help file of Unreal 4 to drag the observation effect. The PBR texture is connected to the vray material, which is almost identical to the arnold method except that the ior texture is connected to the material's ior channel. And here is a very noteworthy thing, here is the ior texture we have been talking about, strictly speaking, it should be 1/ior texture. You can look at the output of this 1/ior texture, you can imagine, if the output is ior texture, because the value of ior, non-metal is 1.5, metal is also greater than 20, are greater than 1, the value is reflected in If it is on the map, it must be a pure white piece. It will not be such a grayscale image. Then, do we want to invert the 1/ior texture in the 3D software, and then insert the ior channel after getting the real ior value? If you are using a vray renderer, there is no need for this, because vray will automatically recognize the value of the ior channel inserted. If it is less than 1, it will automatically be treated as 1/ior. The above, through the use of PBR textures in the arnold and vray renderers to sort out the use of F0 textures and ior textures, and other various renderers are nothing more than one of these two methods. One interesting thing is that the redshift renderer borrows a lot of other renderers, so the shader inherits the interfaces of F0 and ior. At the same time, in the new version of the rsMaterial material (that is, the open source alsurface material upgrade version), actually made the metalness attribute, which can be directly connected to the PBR process like the game engine, it is also quite tidal. Source of PBR material realism Many friends have been wondering why the game's picture will be greatly improved after using the PBR process, and even has begun to surpass the film-level effect. Is the process used in film and television more than the PBR process? Through the previous article, we will find that the textures output in the PBR process are also used in various renderers, and the rendering effect is close to or even better. That is to say, the effect of the game PBR process, the production method of film and television can be completely re-enacted or even surpassed, but the reverse is not possible. The PBR process is only a good definition of the two types of materials, dielectric and metal, but there are big gaps in the transparent material 3s material. So, why is the PBR process still giving the public a more realistic impression than the film and television process? Through the above description, whether you find one thing, the PBR process has only two modifications to the reflection parameters - F0 and Roughness. It seems to be very limited, it is very stifling the artist's creative space. In contrast, the material we usually use has unlimited modification rights to the reflection, not only can adjust the intensity of F0, but also adjust the intensity of F90, and can also adjust the body. Intensity, you can also adjust the transition curve from F0 to F90. This makes our work seem more free and more creative, but in fact, It is the software that gives us too much freedom that makes us make a mistake. If our aim is to create truth, then this freedom runs counter to us. In the PBR process, even the opportunity to adjust the intensity of the reflection is not given to you. No matter how you adjust it, you can never break the conservation of energy, can't eliminate the Fresnel reflection, and can't create the material that doesn't exist in nature. It is this seemingly tough restriction that enhances the accuracy of the PBR process and is more in line with industry needs in most cases. Many times, it is not that the level of art in the game is higher now, but that there is almost no chance of making mistakes. The rendering artists engaged in the film and television industry want to achieve the same quality, but they need higher learning costs. Only by understanding all the knowledge behind it will you control yourself and not make mistakes. Hope it helps you.
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