Get the triangles at run time from a model

The following is based on various other code and was produced with assistance from the XNA forums.

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Vertex Helper XNA 4.0 class (3kb) Oct. 2010

The following reads the vertex buffer of each model mesh part and stores the triangles in a vertex and index array.

public class VertexHelper
{
 public VertexHelper()
 {
 }

 public struct TriangleVertexIndices
 {
  public int A;
  public int B;
  public int C;
 }

 /// 

 /// Extract the vertices and indices from the specified model
 /// 
 /// Output the list of vertices
 /// Output the list of indices
 /// The models world position or use Matrix.Identity for object space
 public static void ExtractTrianglesFrom(Model modelToUse, List vertices, List indices, Matrix worldPosition)
 {
  Matrix transform = Matrix.Identity;
  foreach (ModelMesh mesh in modelToUse.Meshes)
  {
   // If the model has bones the vertices have to be transformed by the bone position
   transform = Matrix.Multiply(GetAbsoluteTransform(mesh.ParentBone), worldPosition);
   ExtractModelMeshData(mesh, ref transform, vertices, indices);
  }
 }

 /// 

 /// Transform by a bone position or Identity if no bone is supplied
 /// 
 public static Matrix GetAbsoluteTransform(ModelBone bone)
 {
  if (bone == null)
  {
   return Matrix.Identity;
  }
  return bone.Transform * GetAbsoluteTransform(bone.Parent);
 }

 /// 

 /// Get all the triangles from all mesh parts
 /// 
 public static void ExtractModelMeshData(ModelMesh mesh, ref Matrix transform,
  List vertices, List indices)
 {
  foreach (ModelMeshPart meshPart in mesh.MeshParts)
  {
   ExtractModelMeshPartData(meshPart, ref transform, vertices, indices);
  }
 }

 /// 

 /// Get all the triangles from each mesh part (Changed for XNA 4)
 /// 
 public static void ExtractModelMeshPartData(ModelMeshPart meshPart, ref Matrix transform,
  List vertices, List indices)
 {
  // Before we add any more where are we starting from
  int offset = vertices.Count;
  
  // == Vertices (Changed for XNA 4.0)

  // Read the format of the vertex buffer
  VertexDeclaration declaration = meshPart.VertexBuffer.VertexDeclaration;
  VertexElement[] vertexElements = declaration.GetVertexElements();
  // Find the element that holds the position
  VertexElement vertexPosition = new VertexElement();
  foreach (VertexElement vert in vertexElements)
  {
   if (vert.VertexElementUsage == VertexElementUsage.Position &&
    vert.VertexElementFormat == VertexElementFormat.Vector3)
   {
    vertexPosition = vert;
    // There should only be one
    break;
   }
  }
  // Check the position element found is valid
  if (vertexPosition == null || 
   vertexPosition.VertexElementUsage != VertexElementUsage.Position ||
   vertexPosition.VertexElementFormat != VertexElementFormat.Vector3)
  {
   throw new Exception("Model uses unsupported vertex format!");
  }
  // This where we store the vertices until transformed
  Vector3[] allVertex = new Vector3[meshPart.NumVertices];
  // Read the vertices from the buffer in to the array
  meshPart.VertexBuffer.GetData(
   meshPart.VertexOffset * declaration.VertexStride + vertexPosition.Offset, 
   allVertex, 
   0, 
   meshPart.NumVertices,
   declaration.VertexStride);
  // Transform them based on the relative bone location and the world if provided
  for (int i = 0; i != allVertex.Length; ++i)
  {
   Vector3.Transform(ref allVertex[i], ref transform, out allVertex[i]);
  }
  // Store the transformed vertices with those from all the other meshes in this model
  vertices.AddRange(allVertex);

  // == Indices (Changed for XNA 4)

  // Find out which vertices make up which triangles
  if (meshPart.IndexBuffer.IndexElementSize != IndexElementSize.SixteenBits)
  {
   // This could probably be handled by using int in place of short but is unnecessary
   throw new Exception("Model uses 32-bit indices, which are not supported.");
  }
  // Each primitive is a triangle
  short[] indexElements = new short[meshPart.PrimitiveCount * 3];
  meshPart.IndexBuffer.GetData(
   meshPart.StartIndex * 2, 
   indexElements, 
   0, 
   meshPart.PrimitiveCount * 3);
  // Each TriangleVertexIndices holds the three indexes to each vertex that makes up a triangle
  TriangleVertexIndices[] tvi = new TriangleVertexIndices[meshPart.PrimitiveCount];
  for (int i = 0; i != tvi.Length; ++i)
  {
   // The offset is because we are storing them all in the one array and the 
   // vertices were added to the end of the array.
   tvi[i].A = indexElements[i * 3 + 0] + offset;
   tvi[i].B = indexElements[i * 3 + 1] + offset;
   tvi[i].C = indexElements[i * 3 + 2] + offset;
  }
  // Store our triangles
  indices.AddRange(tvi);
 }


}

A version of the above working in XNA 3.1 is available from: enchantedage.com