Mesh Builder Local

TRealtimeMeshBuilderLocal is a helper that makes building the most common vertex format simple. Rather than juggling a separate stream builder per stream and a linkage to keep them in sync, the local builder wires all of that up for you: it owns the position, tangents, tex coord, color, triangle, and poly group builders, keeps the vertex streams size-locked through internal linkages, and gives you a single AddVertex / AddTriangle interface. It’s specialized towards the needs of Unreal’s local vertex factory — the format used by ordinary rendered meshes.

You customize the exact stored types through template parameters, and — just like a stream builder — it converts between those stored types and the friendly access types you use in code.

template <
    typename IndexType = uint32,             // per-index type, e.g. uint16 or uint32
    typename TangentElementType = FPackedNormal,   // per-tangent storage, e.g. FPackedNormal or FPackedRGBA16N
    typename TexCoordElementType = FVector2DHalf,  // per-channel tex coord storage, e.g. FVector2f or FVector2DHalf
    int32 NumTexCoords = 1,                   // number of tex coord channels, 1-8
    typename PolyGroupIndexType = uint16>     // poly group index storage
struct TRealtimeMeshBuilderLocal;

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Creating a local mesh builder

using namespace RealtimeMesh;

// The builder operates on a stream set. It creates the required Position and Triangles streams
// immediately; the optional streams are added when you enable them below.
FRealtimeMeshStreamSet StreamSet;

// Configure the stored data types through the template parameters. Here: 16-bit indices,
// FPackedNormal tangents, FVector2DHalf tex coords with one channel.
TRealtimeMeshBuilderLocal<uint16, FPackedNormal, FVector2DHalf, 1> Builder(StreamSet);

// Choose which optional vertex elements are enabled.
Builder.EnableTangents();
Builder.EnableTexCoords();
Builder.EnableColors();

// Poly groups let you pack multiple sections into one set of buffers by tagging each triangle.
Builder.EnablePolyGroups();

// Add a vertex, then set its attributes with a fluent chain. AddVertex returns a vertex builder that
// converts to the new vertex index, so you can capture the index directly.
const int32 V0 = Builder.AddVertex(FVector3f(-50.0f, 0.0f, 0.0f))
    .SetNormalAndTangent(FVector3f(0.0f, -1.0f, 0.0f), FVector3f(1.0f, 0.0f, 0.0f))
    .SetColor(FColor::Red)
    .SetTexCoord(FVector2f(0.0f, 0.0f));

const int32 V1 = Builder.AddVertex(FVector3f(0.0f, 0.0f, 100.0f))
    .SetNormalAndTangent(FVector3f(0.0f, -1.0f, 0.0f), FVector3f(1.0f, 0.0f, 0.0f))
    .SetColor(FColor::Green)
    .SetTexCoord(FVector2f(0.5f, 1.0f));

const int32 V2 = Builder.AddVertex(FVector3f(50.0f, 0.0f, 0.0f))
    .SetNormalAndTangent(FVector3f(0.0f, -1.0f, 0.0f), FVector3f(1.0f, 0.0f, 0.0f))
    .SetColor(FColor::Blue)
    .SetTexCoord(FVector2f(1.0f, 0.0f));

// Add a triangle from the three vertex indices, optionally tagging it with a poly group.
Builder.AddTriangle(V0, V1, V2, 0);

This is the exact pattern used by the ARealtimeMeshExample_Simple_HelloTriangle example (Source/RealtimeMeshExamples/Private/Simple/RealtimeMeshExample_Simple_HelloTriangle.cpp), which is the shortest path to a working mesh.

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What else it can do

Beyond AddVertex / AddTriangle, the builder exposes the operations you’d expect:

• Query state with HasTangents, HasTexCoords, HasVertexColors, HasPolyGroups, HasDepthOnlyTriangles, and NumTexCoordChannels.
• Reserve or resize with ReserveNumVertices / ReserveAdditionalVertices, SetNumVertices, and the matching triangle variants.
• Edit an existing vertex with EditVertex(Index), or set individual attributes by index with SetPosition, SetNormalAndTangent, SetColor, SetTexCoord, and so on.
• Enable a simplified shadow/depth index list with EnableDepthOnlyTriangles() and add to it with AddDepthOnlyTriangle.

If your mesh layout isn’t known until runtime, use FRealtimeMeshDynamicBuilder instead — see Stream Builder.

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Motion vectors with PositionPrev

For a deforming mesh (for example one driven by a compute shader), adding a PositionPrev stream alongside Position turns on per-vertex motion-vector output, which keeps the surface clean under TSR/TAA. The PositionPrev stream isn’t part of the local vertex format the builder manages, so you add it directly to the stream set as a copy of Position — the RMC and the compute driver maintain it from there:

using namespace RealtimeMesh;

// After building your vertices into StreamSet, mirror Position into a PositionPrev stream.
if (const FRealtimeMeshStream* PositionStream = StreamSet.Find(FRealtimeMeshStreams::Position))
{
    FRealtimeMeshStream PrevStream(FRealtimeMeshStreams::PositionPrev, PositionStream->GetLayout());
    PrevStream.Append(*PositionStream);
    StreamSet.AddStream(MoveTemp(PrevStream));
}

You can see this in the ARealtimeMeshExample_Simple_ComputeWave example (Source/RealtimeMeshExamples/Private/Simple/RealtimeMeshExample_Simple_ComputeWave.cpp), where toggling the PositionPrev stream on and off A/B-tests the ghosting that occurs without per-vertex velocity.