update readme

Author: julienkay

readme.md

@@ -11,12 +11,25 @@ This repository contains the source code for a Unity port of the web viewer from
 After cloning the project you can simply use the menu *MobileNeRF -> Asset Downloads* to download any of the sample scenes available.
 In each scene folder there will be a convenient prefab, that you can then drag into the scene and you're good to go.
 
+## Updating
+
+Since the initial release a small number of features have been added to the automatic shader generation code.
+That means, that if you have already downloaded some scenes before, you'll have to regenerate the source files by going to *MobileNeRF -> Asset Downloads* again.
+(This will not actually redownload Assets unless necessary, so this shouldn't take too long)
+
 ## Details
 
 The project was created with Unity 2021.3 LTS using the Built-in Render Pipeline.
 
-The biggest deviation from the official viewer is that this project doesn't use Deferred Rendering, but uses Forward Rendering instead.
-Part of the reason was just me being more comfortable implementing custom shaders for forward rendering pipelines in Unity.
-But I also think this makes it more practical to add VR support for this, by following [these steps](https://docs.unity3d.com/Manual/SinglePassInstancing.html). (I haven't had time for that yet).
+The biggest deviation from the official viewer is, that this project doesn't use Deferred Rendering, but uses Forward Rendering instead.
+This has certain implications on performance.
+While the MobileNeRF representation itself greatly reduces the cost to render NeRFs, it still requires evaluating a small, view-dependent MLP (Multi Layer Perceptron) per fragment. Whenever the bottleneck is in the fragment shader, Deferred Rendering has obvious advantages, as each pixel only needs to run a single fragment shader. 
+
+Forward Rendering however gives us MSAA, which is important for VR use cases. Additionally, in VR the image has to be rendered twice, once for each eye, with a fairly large resolution. The larger the G-buffer, the smaller the benefit of Deferred Rendering. Still, MobileNeRFs mesh representations have a fairly large poly count which works against us here.
+
+Some things to possibly look into:
+* Forward Rendering, but do a Depth Prepass to reduce overdraw (might require URP, see [here](https://forum.unity.com/threads/need-clarification-on-urps-use-of-the-depth-prepass.1004577/) and [here](https://gist.github.com/aras-p/5e3aa6f81c543ca74e3ae296c72ffcaf))
+* Implement Deferred Rendering and compare performance in various scenarios
+
 
 For more details read the official paper here: https://mobile-nerf.github.io/