/reference/engine/classes/EditableMesh.yaml

Adds a new triangle to the mesh and returns a stable face ID.

Adds a new triangle to the mesh and returns a stable face ID.

#### Automatic attribute assignment

`AddTriangle` only takes vertex (position) IDs as arguments. For each
vertex of the new triangle, it automatically assigns normal, UV, and color
attributes using the following logic:

- If the vertex **already belongs to another face**, the new triangle
  corner **reuses** the same normal, UV, and color IDs from the existing
  face. This produces smooth shading and continuous UVs across adjacent
  faces by default.
- If the vertex **does not yet belong to any face**, new normal, UV, and
  color IDs are created with default values (auto-computed normal, UV
  coordinate of `(0, 0)`, and white color).

<img src="../../../assets/engine-api/classes/EditableMesh/Shared-Attributes.png" alt="Two adjacent faces sharing vertex attribute IDs at their common vertices" width="690" />

#### Overriding attributes after adding a face

The auto-assigned attributes can be customized after adding a face. The
approach depends on whether you want to change attribute **values** while
keeping the topology intact, or create entirely new attribute IDs to add
or remove **seams** between faces.

**To change values in-place**, retrieve the auto-created IDs with methods
like `Class.EditableMesh:GetVertexFaceUV()|GetVertexFaceUV()` and set
their values directly. This is more performant and avoids leaving unused
IDs in the mesh:

```lua
-- Create a triangle
local faceId = editableMesh:AddTriangle(v0, v1, v2)

-- Get the UV IDs that were automatically created
local uvId0 = editableMesh:GetVertexFaceUV(v0, faceId)
local uvId1 = editableMesh:GetVertexFaceUV(v1, faceId)
local uvId2 = editableMesh:GetVertexFaceUV(v2, faceId)

-- Set UV values on those existing UV IDs
editableMesh:SetUV(uvId0, Vector2.new(0, 0))
editableMesh:SetUV(uvId1, Vector2.new(1, 0))
editableMesh:SetUV(uvId2, Vector2.new(0, 1))
```

**To create a seam** — where the same vertex needs different attribute
values on different faces (for example, a UV seam or a sharp edge) —
create new attribute IDs with `Class.EditableMesh:AddUV()|AddUV()`,
`Class.EditableMesh:AddNormal()|AddNormal()`, or
`Class.EditableMesh:AddColor()|AddColor()`, then assign them to the face
with `Class.EditableMesh:SetFaceUVs()|SetFaceUVs()`,
`Class.EditableMesh:SetFaceNormals()|SetFaceNormals()`, or
`Class.EditableMesh:SetFaceColors()|SetFaceColors()`:

```lua
-- Create a triangle
local faceId = editableMesh:AddTriangle(v0, v1, v2)

-- Create new UV IDs with custom coordinates
local uv0 = editableMesh:AddUV(Vector2.new(0, 0))
local uv1 = editableMesh:AddUV(Vector2.new(1, 0))
local uv2 = editableMesh:AddUV(Vector2.new(0, 1))

-- Replace the face's UV IDs (creates a seam if the vertex had different UVs on another face)
editableMesh:SetFaceUVs(faceId, {uv0, uv1, uv2})
```

#### Normal sharing and smooth vs. sharp edges

Because `AddTriangle` reuses normal IDs from adjacent faces by default,
meshes appear smooth. To create a **sharp edge** (crease) between faces,
create a separate normal ID for each face and assign it with
`Class.EditableMesh:SetFaceNormals()|SetFaceNormals()`:

<img src="../../../assets/engine-api/classes/EditableMesh/Faceted-Normals.png" alt="Two adjacent faces with separate normal IDs producing flat shading" width="690" />

```lua
-- Two triangles sharing vertices v0 and v1, but with a sharp edge between them
local fid1 = editableMesh:AddTriangle(v0, v1, v2)
local fid2 = editableMesh:AddTriangle(v0, v3, v1)

-- Create separate normals for each face to produce a hard edge
local n1 = editableMesh:AddNormal()  -- auto-computed for fid1
local n2 = editableMesh:AddNormal()  -- auto-computed for fid2
editableMesh:SetFaceNormals(fid1, {n1, n1, n1})
editableMesh:SetFaceNormals(fid2, {n2, n2, n2})
```

See also `Class.EditableMesh:SetNormal()|SetNormal()` to manually specify
a normal vector, and `Class.EditableMesh:ResetNormal()|ResetNormal()` to
revert a normal back to automatic computation.


EditableMesh:AddTriangle

vertexId0

vertexId1

ID of the second vertex of the triangle.


vertexId2

Adds a new triangle to the mesh and returns a stable face ID.


Luau

{any}

A data type that represents both a 3D position and orientation.

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EditableMesh:AddTriangle

Parameters

Returns