New-Tech Europe | November 2016 | Digital edition

We can obviously see a bit of a pattern here as we are emitting two draw calls, and sending the same geometries twice. If Vertex Buffer Objects can mitigate the latter, doubling the draw calls is still a major issue as it is adding an important overhead on your CPU. That is where multiview kicks in, as it allows you in that case, to render the same scene with multiple points of view with one draw call. Multiview Double Action Extension Before going into the details of the expected improvements, I would like to have a quick look at the code needed to get multiview up and running. Multiview currently exists in two major flavors: OVR_multiview and OVR_ multiview2. If they share the same underlying construction, OVR_multiview restricts the usage of the gl_ViewID_OVR variable to the computation of gl_ Position. This means you can only use the view ID inside the vertex shader position computation step, if you want to use it inside your fragment shader or in other parts of your shader you will need to use multiview2. As antialiasing is one of the key requirements of VR, multiview also comes in a version with multisampling called OVR_multiview_multisampled_render_to_ texture . This extension is built against the specification of OVR_multiview2 and EXT_multisampled_render_to_texture . Some devices might only support some of the multiview extensions, so remember to always query your OpenGL ES driver before using one of them. This is the code snippet you may want to use to test if OVR_multiview is available in your driver:

Fig. 1: Stereo camera setup

ViewID_OVR to make it correspond to the view currently being processed. For more in depth information on how to implement multiview, see the sample code and article "Using Multiview Rendering". Why using Multiview? Now that you know how to implement multiview, I will try to give you some insights as to what kind of performance improvements you can expect. The Multiview Timeline Before diving into the numbers, let’s discuss the theory. In this timeline, we can see how our CPU-GPU system is interacting in order to render a frame using regular stereo. For more in depth information on how GPU scheduling works on Mali, please see Peter Harris’ blogs. First the CPU is working to get all the information ready, then the vertex jobs are executed and finally the fragment jobs. On this timeline the light blue are all the jobs related to the left eye, the dark blue to the right eye and the orange to the composition (rendering our two eyes side by side on a buffer). In comparison, this is the same

In your code multiview manifests itself on two fronts; during the creation of your frame buffer and inside your shaders, and you will be amazed how simple it is to use it.

That is more or less all you need to change in your engine code. More or less, because instead of sending a single view matrix uniform to your shader you need to send an array filled with the different view matrices. Now for the shader part:

Simple isn’t it? Multiview will automatically run the shader multiple times, and increment gl_

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