SteamVR 2.2 + nvidia Flex errors

I am trying to use the SteamVR Plugin with the nvidia Flex fluid renderer. On their own, they all work fine. But when combined, there are runtime errors.

Setup:

• Unity 2018.3.6f1 (latest at this time)
• SteamVR Plugin 2.2 (latest from asset store)
• nvidia Flex for Unity 1.0.5 (latest from asset store)

Steps to reproduce:

• Create a new Unity project
• Import the SteamVR Plugin from the store
• Import nvidia Flex for Unity from the store
• Turn on your VR system (HTC Vive in my case)
• Open the flex no.9 Fluid Renderer sample scene, press Play and observe the fluid flow fine in VR
• Now disable the Camera in that scene and drop in the [CameraRig] from the SteamVR prefabs
• Press Play now and observe that the fluid stops flowing after a few seconds and the following errors appear:

ArgumentException: ComputeBuffer.SetData() : Accessing 35100 bytes at offset 0 for Compute Buffer of size 35000 bytes is not possible.
UnityEngine.ComputeBuffer.SetData (System.Array data) (at C:/buildslave/unity/build/Runtime/Export/ComputeShader.bindings.cs:124)
NVIDIA.Flex._auxFlexDrawFluid.UpdateMesh (NVIDIA.Flex.FlexContainer+ParticleData _particleData) (at Assets/NVIDIA/Flex/Auxiliary/_auxFlexDrawFluid.cs:247)
NVIDIA.Flex.FlexContainer.UpdateDrawFluid (NVIDIA.Flex.FlexContainer+ParticleData _particleData) (at Assets/NVIDIA/Flex/Assets/FlexContainer.cs:788)
NVIDIA.Flex.FlexContainer.UpdateSolver () (at Assets/NVIDIA/Flex/Assets/FlexContainer.cs:696)
NVIDIA.Flex.FlexContainer.FixedUpdate () (at Assets/NVIDIA/Flex/Assets/FlexContainer.cs:605)
NVIDIA.Flex.FlexScene.FixedUpdate () (at Assets/NVIDIA/Flex/Helpers/FlexScene.cs:68)

LogWarning - Mapping a buffer that was already mapped
File - ..\..\dx\utils.cpp (62)
UnityEngine.Debug:LogError(Object)
NVIDIA.Flex.FlexContainer:ErrorCallback(ErrorSeverity, IntPtr, IntPtr, Int32) (at Assets/NVIDIA/Flex/Assets/FlexContainer.cs:491)
NVIDIA.Flex.FlexExt:MapParticleData()
NVIDIA.Flex.FlexContainer:UpdateSolver() (at Assets/NVIDIA/Flex/Assets/FlexContainer.cs:688)
NVIDIA.Flex.FlexContainer:FixedUpdate() (at Assets/NVIDIA/Flex/Assets/FlexContainer.cs:605)
NVIDIA.Flex.FlexScene:FixedUpdate() (at Assets/NVIDIA/Flex/Helpers/FlexScene.cs:68)

I understand that it seems to be Flex “crashing” here and not SteamVR, which is why I first posted this issue on the nvidia forums, however as SteamVR is implicated (Flex works fine without it) I am here to see if SteamVR Plugin could be improved in some way to not affect other such systems.

[rt974]

Wow i managed to fix this :

//ORIGINAL
//m_indexBuffer.SetData(indices);
//ORIGINAL
//NEW
if(indices.Length != m_indexBuffer.count)
{
	m_indexBuffer = new ComputeBuffer(indices.Length, sizeof(int));
	m_indexBuffer.SetData(indices);
}
else
{
	m_indexBuffer.SetData(indices);
}
//NEW

The flicker happened because the SetData needs to be called in all cases, the code can be simplified by removing the else, and writing only one SetData at the end.

Now for me this works, i dont really know if this doesnt create side effects, i’ve read somewhere on unity forums that recreating ComputeBuffer every frame is not a good idea. Thats as far as my knowledge goes on this topic.

I hope this helps.

[oli414]

The solution by @reforia keeps the number of drawn particles at the maximum. This becomes a problem when deactivating the fluid source, as the fluids will not despawn until every single one of them has reached the end of its lifetime. Instead I propose this workaround.

Start with a fresh (unmodified) _auxFlexDrawFluid.cs, and simply replace:

if (m_indexBuffer != null)
{
    m_indexBuffer.SetData(indices);
}

at line 245 with:

if (m_indexBuffer != null)
{
    m_indexBuffer.SetData(indices, 0, 0, m_indexBuffer.count);
}

The crash happened because indices doesn’t appear to dynamically change size. So if our max particles is 4000, then indices will have a length of 4000 as long as there’re any particles around. indexCount on the other hand directly reflects how many particles are alive, which may for instance be 3672. So when copying the indices we need to limit what we’re copying by the size of m_indexBuffer. Creating a new m_indexBuffer to match the maximum amount of particles on the other hand causes particles to visually stay in the scene.