Efficient way of editing vertex positions during runtime in Unreal Engine - unreal-engine4

Question

I’m looking for a way to update the vertex positions of every vertex of a mesh with 65536 vertices from C++ code. It needs to be updated every few frames with values calculated in code, so it needs to be somewhat efficient.
I tried this, with no effect:
if (NewElement->GetStaticMeshComponent()->GetStaticMesh()->RenderData->LODResources.Num() > 0)
{
FPositionVertexBuffer* VertexBuffer = &NewElement->GetStaticMeshComponent()->GetStaticMesh()->RenderData->LODResources[0].VertexBuffers.PositionVertexBuffer;
if (VertexBuffer)
{
const int32 VertexCount = VertexBuffer->GetNumVertices();
for (int32 Index = 0; Index < VertexCount; Index++)
{
VertexBuffer->VertexPosition(Index) += FVector(float(Index), float(100* Index), float(10000 * Index));
}
}
}
I’ll appreciate help with finding a working solution.
As for now, I’m looking for a simple solution, just to start with something. But I know, that updating the mesh CPU side is not the most efficient way, so maybe it would be easier/faster to calculate the position for every vertex and then pass it to Vertex shader? Or generate some pseudo-texture, upload it to GPU and use it in the vertex shader? Does anyone have an example of such a mechanism in UE?
Regards

Answer 1

Your code doesn't actually push any updates to the GPU. You're using a static mesh here which isn't really intended to have vertices modified at runtime , hence the "static" moniker. That's not to say you can't modify that data at runtime but that's not what you're doing here. Your code is only changing data CPU-side.
If you look through the various vertex buffers implemented in engine code, you'll see that ultimately they all extend FRenderResource which provides RHI-management functions or FVertexBuffer, which is an FRenderResource and contains an FBufferRHIRef field, which is the actual GPU-bound vertex buffer.
Because rendering in Unreal Engine is multithreaded, the engine uses the concept of scene proxies which extend from FPrimitiveSceneProxy. Each primitive type that exists on the game thread and needs to be rendered will have some form of a FPrimitiveSceneProxy created and will pass data and updates to its proxy in a thread-safe manner, usually by queuing rendering commands via ENQUEUE_RENDER_COMMAND(...) which you would pass a lamba function of what should be executed when the rendering thread determines its time to run it. This proxy will contain the vertex and index buffers, and is where the "real" updates to your rendered geometry happen.
One example could be the following (excerpt taken from BaseDynamicMeshSceneProxy.h, FMeshRenderBufferSet::TransferVertexUpdateToGPU() function), which shows the render buffer collection in a scene proxy for a UDynamicMeshComponent pushing an update of its vertex positions to the GPU by copying its CPU-bound data directly into its GPU-bound vertex position buffer:
FPositionVertexBuffer& VertexBuffer = this->PositionVertexBuffer;
void* VertexBufferData = RHILockBuffer(VertexBuffer.VertexBufferRHI, 0, VertexBuffer.GetNumVertices() * VertexBuffer.GetStride(), RLM_WriteOnly);
FMemory::Memcpy(VertexBufferData, VertexBuffer.GetVertexData(), VertexBuffer.GetNumVertices() * VertexBuffer.GetStride());
RHIUnlockBuffer(VertexBuffer.VertexBufferRHI);
I won't be providing a full sample here for you because, as you can see from everything described to this point, there is much more to it than a simple snippet of code to achieve what you're looking for, but I wanted to outline the overall concept and patterns of what you'll need to understand to achieve this because if you're going to do this directly in your own code, you must understand these concepts and it can be a bit confusing when you first start digging into Unreal Engine's rendering code.
The best resource to help gain a solid understanding of the patterns the engine expects you to follow would be the official documentation found here: Unreal Engine Graphics Programming.
If you are wanting to modify geometry at runtime, there are also other options available which will make the process mush easier than trying to write it completely yourself, such as the engine-provided Procedural Mesh Component plugin, the third-party RuntimeMeshComponent plugin, and in later versions of Unreal Engine (4 and 5), the UDynamicMeshComponent (aka USimpleDynamicMeshComponent in earlier versions) which is part of the Interactive tools framework and in most recent versions of the engine has become a core part of the engine runtime module GeometryFramework.
I hope this helps you in your journey. Runtime-modifiable geometry is tough to get started but it's definitely worth the journey.