stride3d/stride · error · NotImplementedException

The mesh Data needs to have index buffer

Error message

The mesh Data needs to have index buffer

What it means

SortMeshPolygons sorts a mesh's triangles back-to-front and requires an index buffer to reorder polygons without touching vertex data. This error (NotImplementedException) is thrown when the supplied MeshDraw has no IndexBuffer; indexless triangle-list meshes are not supported by the sorter.

Solutions

  1. Generate an index buffer for the mesh (one index per vertex: 0..Count-1) before sorting
  2. Convert the mesh to an indexed triangle list at import/generation time
  3. Skip polygon sorting for unindexed meshes or sort by rebuilding a triangle soup with a new index buffer

Example fix

// before
sort.SortMeshPolygons(mesh, viewDir); // mesh.IndexBuffer == null
// after
if (mesh.IndexBuffer == null)
    mesh.IndexBuffer = BuildIdentityIndexBuffer(mesh.VertexBuffers[0].Count);
sort.SortMeshPolygons(mesh, viewDir);
Defensive patterns

Strategy: validation

Validate before calling

if (mesh.IndexBuffer == null)
    mesh.IndexBuffer = BuildIdentityIndexBuffer(mesh.VertexBuffers[0].Count);

Type guard

static bool CanSortPolygons(MeshDraw mesh) =>
    mesh.IndexBuffer != null && mesh.VertexBuffers != null && mesh.VertexBuffers.Length == 1;

Try / catch

try { mesh.SortMeshPolygons(viewDir); }
catch (NotImplementedException) { /* mesh lacks index buffer: build one or skip sorting */ }

Prevention

When it happens

Trigger: Calling SortMeshPolygons on a MeshDraw whose IndexBuffer property is null (mesh stores vertices without an index list).

Common situations: Sorting unindexed procedurally generated meshes; meshes generated by tools that emit raw triangle soups; forgetting to build an index buffer after generating vertex data.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/a8d464f8a0e1e75e. Report an issue: GitHub.

Appendix: source

Thrown at sources/engine/Stride.Rendering/Extensions/PolySortExtensions.cs:23

using System.Linq;
using Stride.Core;
using Stride.Core.Mathematics;
using Stride.Graphics;
using Stride.Graphics.Data;
using Stride.Rendering;

namespace Stride.Extensions
{
    public static class PolySortExtensions
    {
        public static unsafe void SortMeshPolygons(this MeshDraw meshData, Vector3 viewDirectionForSorting)
        {
            // need to have alreade an vertex buffer
            if (meshData.VertexBuffers == null)
                throw new ArgumentException();
            // For now, require a MeshData with an index buffer
            if (meshData.IndexBuffer == null)
                throw new NotImplementedException("The mesh Data needs to have index buffer");
            if (meshData.VertexBuffers.Length != 1)
                throw new NotImplementedException("Sorting not implemented for multiple vertex buffers by submeshdata");

            if (viewDirectionForSorting == Vector3.Zero)
            {
                // By default to -Z if sorting is set to null
                viewDirectionForSorting = -Vector3.UnitZ;
            }

            const uint PolySize = 3; // currently only triangle list are supported
            var polyIndicesSize = PolySize * sizeof(int);
            var vertexBuffer = meshData.VertexBuffers[0];
            var oldIndexBuffer = meshData.IndexBuffer;
            var vertexStride = vertexBuffer.Declaration.VertexStride;

            // Generate the sort list
            var sortList = new List<KeyValuePair<int, Vector3>>();

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