What is Variable Rate Shading (VRS)

What is Variable Rate Shading (VRS)

Variable Rate Shading a term that in the coming months will enter the lexicon of gamers. We heard about it for the first time from Nvidia, at the presentation of the Turing GPU-based video cards, and later from Microsoft, who first implemented the technology in the DirectX 12 API on PC and subsequently announced the support from Xbox Series X .

"Our patented form of VRS allows developers to use the full power of Xbox Series X more efficiently. Instead of spending GPU computing cycles for every single pixel on the screen, developers can give priority to individual effects on game characters or important environmental objects. This technique leads to more stable frame rates and higher resolution, without impacting the final image quality. "

AMD subsequently confirmed full support for Variable Rate Shading (VRS) with the RDNA 2 graphics architecture, which will be the basis of the company's future GPUs arriving later this year and of the new consoles from Microsoft and Sony. The technical explanation given by Microsoft regarding Xbox Series X still "rose water", but on the occasion of the implementation in DirectX 12 the house of Redmond was certainly more detailed. So let's explain what this Variable Rate Shading is and how it can change the life of developers and players.

Variable Rate Shading explained in an easy way

Variable Rate Shading a new API that allows developers to use GPUs smarter. Developers can allocate the GPU power to specific areas of the game image, for example certain characters, a vehicle and the track on which it moves or other, and less on other areas that can be defined as "secondary".

Click to enlarge

The rest of the image out of focus of the gamer can be reproduced with a slightly lower graphic levelthus allowing GPUs to reach higher frame rates per second. It should not be forgotten that in fact, on a game screen, only a fraction of the areas require maximum detail, while the others may also lose some "nuance" which is not important for the overall quality, but important for gaining performance.

Take for example a car racing game: it is necessary to reproduce the cars and the road at the maximum graphic detail, but at the same time not important if the audience in the stands is slightly less defined. On the other hand, cars move quickly and our eyes pay attention to other aspects. The visual impact for the gamer is consequently null or almost, but the performance improves significantly.

Variable Rate Shading explained in a difficult way

For many, the article may end here, but let's move on to a slightly more technical explanation. On the other hand, the Variable Rate Shading (also known as coarse pixel shading) the union of precise terms, and the translation "variable rate shading". Shading means "shading", and it is an operation at the base of computer graphics that describes how surfaces (polygonal meshes) respond to light based on their characteristics, the angle of impact of light, the vision of the observer and other aspects. In conclusion, shading defines the appropriate levels of light, darkness and color in an image rendered by your GPU.

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Here comes the term "rate", i.e. rate or speed. Together with"variable", VRS describes a variable shading rate, ie the resolution at which the shaders are "called", not to be confused with the screen resolution. A shader is a set of algorithms that perform the shading operation during the rendering process.

For each pixel on the screen, shaders are called to calculate what a pixel's color should look like. The higher the shading rate, the higher the visual fidelity, but also the impact on the calculation performance, which the GPU does. A lower shading rate means the opposite, i.e. less visual fidelity and lower performance impact.

Traditionally, the shading speed set by developers for a game is applied to all pixels in an image. The problem, if we want to define it, that "not all pixels are equal". With VRS, developers have the option of selectively reduce the shading rate in areas of the image that do not affect visual quality, thus gaining performance. In this way, not only a GPU can reach higher speeds, for example 90 instead of 65 fps, but at the same time also lower-end hardware (less powerful) able to run a game faster than the same title without VRS support.

Illustrating integration into the DirectX 12 API, Microsoft spoke of three different ways to set the shading rate: "per draw", "in a draw (within-draw) using a screenspace image" and "inside a draw, for primitive". A "draw" is nothing more than the operation of drawing objects on the screen. A screenspace image the coordinate space of the 2D image resulting during 3D rendering, i.e. the result of 3D projection on geometry in the camera space. The primitive, on the other hand, represents the simplest geometric object that a system can manage.

There are also two versions, called Tier, to identify the different types of hardware and support for the Variable Rade Shading (VRS). The hardware that can support in hardware the VRS per draw indicated as Tier 1. Tier 2 instead the hardware compatible also with the VRS per-draw and within-draw.

Tier 1 implementation. One part has VRS, the other doesn't: which one? The imperceptible difference – Click to enlarge

By allowing developers to establish the shading rate per draw, different draw calls – calls that are made to the graphical API (Direct3D in this case) in order to reproduce objects – can have different shading rates. This allows a developer to create large environmental assets or other elements with a lower shading rate, while maintaining a higher shading rate for more detailed assets than a scene.

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As written before, Tier 2 hardware gives developers additional flexibility. "By allowing developers to indicate the shading rate using a screenspace image, the possibilities are opened up for a variety of techniques. For example," explains Microsoft, "the foveated rendering renders much of the detail in the area where the user pays attention and gradually reduces the shading rate outside that area to improve performance. In an FPS, the user probably pays close attention to the viewfinder and little to the extreme edges of the screen, which makes FPS an ideal candidate for this technique. "

The shading rate per primitive means that developers can indicate, within a draw, the shading rate per triangle. This technique can be applied by developers who know that they will apply a depth of field blur to render triangles beyond a certain distance with a lower shading rate. This will not lead to a reduction in visual quality but to an increase in performance, since distant triangles will still be blurred.

Tier 2 implementation. The left side the one with active VRS – Click to enlarge

Developers don't have to choose between these techniques, as Microsoft allows you to combine them at the same time. Microsoft illustrated the impact of VRS on games by collaborating with Firaxis and leveraging existing Nvidia hardware. The software house has added both draw and screenspace image support to Civilization.

Before implementing VRS, the system with a GeForce RTX 2060 at 4K has reached a frame rate of approximately 53 fps. By adding Tier 1 support, reproducing soil and water with a lower shading rate and other smaller assets such as vehicles, user interface and buildings with a higher shading rate, the performance is grown by 20% with an almost zero deterioration in quality.

Red indicates areas where the shading rate set to 1×1 (more shading, more detail), blue areas where 2×2 (less shading, less detail)

With Tier 2, Firaxis implemented the screenspace image greatly reducing the negative impact on image quality and registering a 14% increase in FPS. VRS will be supported not only by Nvidia and AMD but also by Intel. The company has already shown a first test with the Gen11 integrated graphics hardware and a demo called Sun Temple created with the Unreal Engine 4, so let's expect support also on the new Xe architecture of future dedicated video cards.

As for game engines and titles, realities such as Ubisoft, Activision, Epic Games, 343 Industries, Unity, Playground Games, Massive Entertainment and Turn10 have already been working on VRS for months, which will therefore become, thanks to the new consoles, a feature almost standard games in the coming years.

VRS in Nvidia sauce? Nvidia Adaptive Shading (NAS)

Nvidia was the first with Turing GPUs to implement Variable Rate Shading in the form of Nvidia Adaptive Shading (NAS), through two algorithms that are called Content Adaptive Shading (CAS) and Motion Adaptive Shading (MAS). In the first case the shading rate linked to the spatial and temporal coherence of the colors between the frames, in the second to the amount of movement present in a particular part of the scene.

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NAS was implemented in Wolfenstein: Youngblood with three quality settings called Quality, Balanced and Performance, with each having different compromises in terms of quality and performance. Using Balanced or Quality the limited quality drop, which instead becomes slightly more aggressive in Performance, even if visible to a trained eye and mainly by comparing static images. NAS, according to Nvidia, can speed up GameGames' game performance by up to 15%.

Content Adaptive Shading predicts the impact of shading at reduced speed and does so only if the impact is expected to be unnoticeable. Thanks to the VRS function on the Turing GPUs, the shading rate can be set to vary independently for each 16×16 frame on the screen (think of the screen as a checkerboard), allowing a careful distribution of the shading on the tens of thousands of tiles that make up the screen. In general, the lower the contrast and variation of the objects in a scene, the lower the shading rate that can be done without reducing the visual quality.

The Motion Adaptive Shading takes advantage of motion blur effects to reduce the shading rate when moving and spinning within a game. Nvidia explained that Motion Adaptive Shading is inspired by a phenomenon called "LCD persistence blur" related to monitors. Until recently, all screens could only be updated at fixed intervals (fixed refresh rate), so the objects were displayed in jerky movements (albeit fast and generally not perceptible). At the same time, our eyes tried to follow the object with a linear movement and on our retina the image "trembled" quickly, leading us to perceive a blurred image. In addition to blurring the screen, several game engines added motion blur to the rendering, thus doubling the blur effect.

LCD persistence blur and motion blur hid image imperfections related to a reduction in the shading rate. The blur did not conceal similarly for all levels of image imperfection, scaling the loss proportionally. Using theories of signal processing and running simulations to make sure that you apply the best scaling factors at every speed of movement, Nvidia can reduce the shading rate of the blurred elements of the game with MAS without the eyes noticing any difference, thus ensuring better performance.


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