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'''Stereo shading reprojection''' is a technique developed by [[Oculus]] to optimize [[virtual reality]] (VR) rendering. It achieves this by reprojecting rendered pixels from one eye to the other one, and the gaps are filled with an additional rendering pass. <ref name=”1”>Zang, J. and Green, S. (2017). Introducing stereo shading reprojection for Unity. Retrieved from https://developer.oculus.com/blog/introducing-stereo-shading-reprojection-for-unity/</ref>

'''Stereo shading reprojection''' is a technique developed by [[Oculus]] to optimize [[virtual reality]] (VR) rendering. It achieves this by reprojecting rendered pixels from one eye to the other one, and the gaps are filled with an additional rendering pass. <ref name=”1”>Zang, J. and Green, S. (2017). Introducing stereo shading reprojection for Unity. Retrieved from https://developer.oculus.com/blog/introducing-stereo-shading-reprojection-for-unity/</ref>

According to Oculus, this technique has worked well in a Unity sample for scenes that are pixel shader heavy, saving up to 20% on GPU performance. <ref name=”1”></ref> <ref name=”2”>Lang, B. (2017). Oculus’ new ‘stereo shading reprojection’ brings big performance gains to certain VR scenes. Retrieved from https://www.roadtovr.com/oculus-new-stereo-shading-reprojection-brings-big-performance-gains-certain-vr-scenes/</ref>

According to Oculus, this technique has worked well in a Unity sample for scenes that are pixel shader heavy, saving up to 20% on GPU performance. <ref name=”1”></ref><ref name=”2”>Lang, B. (2017). Oculus’ new ‘stereo shading reprojection’ brings big performance gains to certain VR scenes. Retrieved from https://www.roadtovr.com/oculus-new-stereo-shading-reprojection-brings-big-performance-gains-certain-vr-scenes/</ref>

First-generation oculus virtual reality apps render the scene twice - one for each eye. Since the two images normally look similar, the Oculus researchers thought of sharing some pixel rendering work between both eyes. Stereo shading reprojection is a technique that makes this possible. <ref name=”1”></ref>

First-generation oculus virtual reality apps render the scene twice - one for each eye. Since the two images normally look similar, the Oculus researchers thought of sharing some pixel rendering work between both eyes. Stereo shading reprojection is a technique that makes this possible. <ref name=”1”></ref>

Stereo shading reprojection is easy to add to Unity-based projects and could also be integrated with Unreal or other engines. This technology comes after the introduction, in 2016, of Asynchronous Spacewarp, also developed by Oculus. This was also a rendering technology that permitted the company to establish lower minimum specifications for the [[Oculus Rift]]. <ref name=”1”></ref> <ref name=”2”></ref>

Stereo shading reprojection is easy to add to Unity-based projects and could also be integrated with Unreal or other engines. This technology comes after the introduction, in 2016, of Asynchronous Spacewarp, also developed by Oculus. This was also a rendering technology that permitted the company to establish lower minimum specifications for the [[Oculus Rift]]. <ref name=”1”></ref><ref name=”2”></ref>

==Theory==

==Theory==

[[File:Stereo shading reprojection edge ghosting.png|thumb|Figure 2. Edge ghosting. (Image: Oculus)]]

[[File:Stereo shading reprojection edge ghosting.png|thumb|Figure 2. Edge ghosting. (Image: Oculus)]]

When it comes to computational resources, virtual reality has high requirements and is very hard on graphics cards. Indeed, an expensive high-end graphics card is a requirement for most PC VR applications. This has motivated the VR industry to research and develop new techniques that can reduce the load on a GPU and improve its performance, therefore making VR rendering more efficient. Increased efficiency means that the minimum requirements for VR applications are reduced, making virtual reality available to more people. <ref name=”2”></ref> <ref name=”3”>Hills-Duty, R. (2017). Oculus introduce new rendering technology for performance gains. Retrieved from https://www.vrfocus.com/2017/08/oculus-introduce-new-rendering-technology-for-performance-gains/</ref>

Virtual reality has high computational requirements and is very hard on graphics cards. An high-end graphics card is a requirement for most PC VR applications. Increased efficiency is desirable. It means that the minimum requirements for VR applications are reduced, making virtual reality available to more people. <ref name=”2”></ref><ref name=”3”>Hills-Duty, R. (2017). Oculus introduce new rendering technology for performance gains. Retrieved from https://www.vrfocus.com/2017/08/oculus-introduce-new-rendering-technology-for-performance-gains/</ref>

The idea behind Oculus stereo shading reprojection is simple: it uses similarities between the perspective view of each eye to lessen redundant rendering work. Normally for VR apps the images are rendered twice, one for each eye. With stereo shading reprojection, pixels are “rendered once, then reprojected to the other eye, to share the rendering cost over both eyes.” <ref name=”2”></ref> <ref name=”3”></ref>

The idea behind Oculus stereo shading reprojection is simple: it uses similarities between the perspective view of each eye to lessen redundant rendering work. Normally for VR apps the images are rendered twice, one for each eye. With stereo shading reprojection, pixels are “rendered once, then reprojected to the other eye, to share the rendering cost over both eyes.” <ref name=”2”></ref><ref name=”3”></ref>

Since the eyes are separated by a short distance they see the environment from a slightly different perspective. However, the two different perspectives share some visual similarities which the stereo shading reprojection technique takes advantage of to avoid doing redundant rendering work for parts of the scene which have already been calculated for one of the eyes. According to Oculus engineers, stereo shading reprojection uses “information from the depth buffer to reproject the first eye’s rendering result into the second eye’s framebuffer.” The software also needs to take into account pixels that are visible that shouldn’t be, resulting from the different eye perspectives. Stereo shading reprojection has to identify which pixels are valid or not so that the invalid ones can be masked out to avoid artifacts and ghosting effect (Figure 1 and Figure 2). <ref name=”1”></ref> <ref name=”2”></ref> <ref name=”3”></ref> <ref name=”4”>Arguinbaev, M. (2017). Oculus introduces stereo shading reprojection to the Unity engine. Retrieved from https://thevrbase.com/oculus-introduces-stereo-shading-reprojection-to-the-unity-engine/</ref>

Since the eyes are separated by a short distance they see the environment from a slightly different perspective. However, the two different perspectives share some visual similarities which the stereo shading reprojection technique takes advantage of to avoid doing redundant rendering work for parts of the scene which have already been calculated for one of the eyes. According to Oculus engineers, stereo shading reprojection uses “information from the depth buffer to reproject the first eye’s rendering result into the second eye’s framebuffer.” The software also needs to take into account pixels that are visible that shouldn’t be, resulting from the different eye perspectives. Stereo shading reprojection has to identify which pixels are valid or not so that the invalid ones can be masked out to avoid artifacts and ghosting effect (Figure 1 and Figure 2). <ref name=”1”></ref><ref name=”2”></ref><ref name=”3”></ref><ref name=”4”>Arguinbaev, M. (2017). Oculus introduces stereo shading reprojection to the Unity engine. Retrieved from https://thevrbase.com/oculus-introduces-stereo-shading-reprojection-to-the-unity-engine/</ref>

While the idea behind stereo shading reprojection is relatively simple, there are some points - made by Oculus - that need to be achieved so that it can be implemented and provide a comfortable VR experience:

While the idea behind stereo shading reprojection is relatively simple, there are some points - made by Oculus - that need to be achieved so that it can be implemented and provide a comfortable VR experience:

* ''Reprojection pass: using the depth values, reproject the left eye color to the right eye’s position, and generate a pixel culling mask;''

* ''Reprojection pass: using the depth values, reproject the left eye color to the right eye’s position, and generate a pixel culling mask;''

* ''Continue the right eye opaque and additive lighting passes, fill pixels in those areas which aren’t covered by the pixel culling mask;''

* ''Continue the right eye opaque and additive lighting passes, fill pixels in those areas which aren’t covered by the pixel culling mask;''

* ''Reset the pixel culling mask and finish subsequent passes on the right eye.'' <ref name=”1”></ref> <ref name=”2”></ref>

* ''Reset the pixel culling mask and finish subsequent passes on the right eye.'' <ref name=”1”></ref><ref name=”2”></ref>

Oculus has noted that enabling stereo shading reprojection revealed savings of around 20% on rendering time, with the technique being very useful for scenes which are heavy in pixel shading. <ref name=”2”></ref>

Oculus has noted that enabling stereo shading reprojection revealed savings of around 20% on rendering time, with the technique being very useful for scenes which are heavy in pixel shading. <ref name=”2”></ref>