# addTangentBasis(forTextureCoordinateAttributeNamed:tangentAttributeNamed:bitangentAttributeNamed:) Generates surface tangent and bitangent data for the mesh based on its vertex position and texture coordinate data. ``` func addTangentBasis(forTextureCoordinateAttributeNamed textureCoordinateAttributeName: String, tangentAttributeNamed tangentAttributeName: String, bitangentAttributeNamed bitangentAttributeName: String?) ``` ## Parameters `textureCoordinateAttributeName` The name of the vertex attribute from which to read texture coordinate data for use in generating tangent and bitangent vectors. `tangentAttributeName` The name of the vertex attribute for storing surface tangent vector data. `bitangentAttributeName` The name of the vertex attribute for storing surface bitangent vector data. ## Discussion Surface-space tangent and bitangent vectors can be used to produce shading effects that follow the “flow” of a surface or to generate normal map textures. Model I/O calculates tangent and bitangent vectors based on vertex positions and texture coordinates using a common definition: The tangent vector at a point is tangent to the surface and parallel to the texture coordinate s-axis, and the bitangent vector is tangent to the surface and parallel to the texture coordinate t-axis. For this method to calculate surface tangent vectors, the mesh must contain vertex data for both the [`MDLVertexAttributePosition`](/documentation/ModelIO/MDLVertexAttributePosition) attribute and the texture coordinate attribute specified in the `textureCoordinateAttributeName` parameter. Calling this method on a mesh that does not contain the specified vertex attributes raises an exception. This method saves calculated normal data in the vertex attributes named in the `tangentAttributeName` and `bitangentAttributeName` parameters. If the mesh already contains either attribute, this method overwrites the contents of the corresponding vertex buffers. If the mesh does not contain an attribute, this method creates a new attribute and updates the mesh’s [`vertexDescriptor`](/documentation/ModelIO/MDLMesh/vertexDescriptor) object accordingly. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)