Resolved HoloLens 2 + DrawMeshInstancedIndirect + Single Pass Instanced Rendering

Hello,
I have project where I need to render a large collection of objects on the hololens 2. For this purpose I experimented with Graphics.DrawMeshInstanced and Graphics.DrawMeshInstancedIndirect. Due to performance I chose the latter.

My project is configured with the "old" XR-Settings (Settings -> Player -> XR-Settings, Windows Mixed Reality).
When setting the stereo rendering mode to "Single Pass", everything looks fine, but when I switch to "Single Pass Instanced" the data rendered via DrawMeshInstancedIndirect does not align. Otherwise everything else looks correct. It only affects the meshes rendered by DrawMeshInstancedIndirect.

I assume it has something to do with an incorrect setup for the right and left eye in my shader, but I have no clue what I need to do to fix this issue. I am using Unity 2019.4.8 + URP + MRTK Toolkit.

Custom shade code (partially):

Code (CSharp):

1.  
2. [...]
3. struct appdata
4. {
5.    float4 vertex : POSITION;
6.    float4 normal : NORMAL;
7.  
8.    UNITY_VERTEX_INPUT_INSTANCE_ID
9. };
10.  
11. struct v2f
12. {
13.    float4 vertex : SV_POSITION;
14.    fixed4 color : COLOR0;
15.  
16.    UNITY_VERTEX_OUTPUT_STEREO
17. };
18.  
19. struct MeshProperties
20. {
21.    float4x4 mat;
22.    float4 color;
23. };
24.  
25. StructuredBuffer<MeshProperties> _Properties;
26.  
27. v2f vert(appdata v, uint instanceID: SV_InstanceID)
28. {
29.    v2f o;
30.  
31.    UNITY_SETUP_INSTANCE_ID(v);
32.    UNITY_INITIALIZE_OUTPUT(v2f, o);
33.    UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
34.  
35.    // position
36.    float4 position =  mul(_Properties[instanceID].mat, v.vertex);
37.    o.vertex  = mul(UNITY_MATRIX_VP, position);
38. // I used UnityObjectToClipPos(position); before, but that gave the same result
39.  
40.    // color
41. [...] // code for the color, basicly I adjust the color from the properties array to add a simple lighting effect
42.  
43.    return o;
44. }
45.  
46. fixed4 frag(v2f i) : SV_Target
47. {
48.    return i.color;
49. }
50.  

The MeshProperties Struct correspondeds to the one I use in C# Code and contains the TRS Data of the mesh.

Code (CSharp):

1.  
2. internal struct MeshProperties
3.         {
4.             public Matrix4x4 trsData;
5.             public Vector4 color;
6.  
7.             public static int Size()
8.             {
9.                 return
10.                     sizeof(float) * 4 * 4 + // matrix;
11.                     sizeof(float) * 4;      // color;
12.             }
13.         }
14.  

https://docs.unity3d.com/Manual/SinglePassStereoRenderingHoloLens.html states that "... you will have to manually double the instance count contained in your compute buffers." if using Graphics.DrawProceduralIndirect().
I do not really know how this function is used or what it does, but maybe it also applies to DrawMeshInstancedIndirect, as this approach uses compute buffers, too?

 

Hey, I ran into this same issue, and that blurb from the documentation is sparse and confused me too. What I found out is that they are expecting interleaved values, so I had to double the size of my computebuffer and then double the entries one by one so they are in memory like:
start with 5 instances like so
1,2,3,4,5
change to:
1,1, 2,2,3,3,4,4,5,5

EDIT: In fact when I ran into this problem this post was the first thing I came across in my searches, so wanted to share the answer.

 

Thank you very much for your help!

 

@ferrous @lady_magnetic any chance either of you have code samples you could share?

 

I just came across this problem today and would like to provide a code example for future reference:

as mentioned in the docs and explained by @ferrous, an easy solution is to duplicate your data in the buffer. In my code I introduced a boolean to declare if the data should be duplicated to support stereo rendering:

Code (CSharp):

1. // Here, you may want to check if stereo rendering is enabled
2. protected bool UseStereoBuffer => true;
3.  
4. // Use x2 factor for stereo rendering
5. protected int StereoBufferSizeFactor => (UseStereoBuffer ? 2 : 1);

When it comes to building the buffer I duplicate the data accordingly:

Code (CSharp):

1. // Double your buffer size if stereo rendering is enabled
2. var size = bufferSize * StereoBufferSizeFactor;
3. meshPropertiesBuffer = new ComputeBuffer(size, MeshProperties.Size());
4. MeshProperties[] meshProperties = new MeshProperties[size];
5.  
6. // Set up the args buffer
7. uint[] args = new uint[5] {0, 0, 0, 0, 0};
8. args[0] = (uint) mesh.GetIndexCount(0);
9. args[1] = (uint) size;
10. args[2] = (uint) mesh.GetIndexStart(0);
11. args[3] = (uint) mesh.GetBaseVertex(0);
12. argsBuffer = new ComputeBuffer(1, args.Length * sizeof(uint), ComputeBufferType.IndirectArguments);
13. argsBuffer.SetData(args);
14.  
15. // Set up the mesh data buffer
16. for(int i = 0; i < size; i += StereoBufferSizeFactor)
17. {
18.     meshProperties[i] = GetMyBufferData(i);
19.     // Duplicate data for stereo rendering
20.     if (UseStereoBuffer) meshProperties[i + 1] = meshProperties[i];
21. }

There may be better ways to solve this that do not rely on actually duplicating the data in memory but it works and is relatively easy to implement.