BannerLeft BannerRightToymaker

 

XNA 4.0 Bounding Box from a Model

Updated for XNA 4.0, 13th March 2011

XNA provides a means of obtaining a bounding sphere from a model but not a bounding box. Depending on the shape of the 3D object a bounding box can be much more useful for collisions than a sphere.

Axis Aligned Bounding Box in Model Space

To obtain a bounding box around our model in model space (the space the model is created in i.e. with 0,0,0 normally at the centre of the model in your art package, for more information see : XNA Matrix) we need to go through all the vertices in the model keeping a track of the minimum and maximum x, y and z positions. This gives us two corners of the box from which all the other corners can be calculated since the box is aligned along the axis (hence it is known as an Axis Aligned Bounding Box or AABB for short).

Since each model is made from a number of mesh we need to calculate minimum and maximum values from the vertex positions for each mesh. The ModelMesh object in XNA is split into parts which in turn provides access to the buffer holding the vertex data (VertexBuffer) from which we can obtain a copy of the vertices using the GetData call. (Note that the reason a mesh has parts is because it may use differerent materials per part but for bounding box creation we are not bothered about that so we look through all parts in the mesh).

We can access the vertex data via the GetData call however we have no way of knowing what type of data it is. This is because a vertex can have a number of elements like position, colour, normal, texture coordinate etc. The good thing though is that we can find out the size in bytes of one vertex (the stride) and we also know that every vertex has the position element specified first. We can therefore extract the position by grabbing the first 3 floats per vertex and using the stride value to advance to the next one.

public BoundingBox CalculateBoundingBox()
{

// Create variables to hold min and max xyz values for the model. Initialise them to extremes
Vector3 modelMax = new Vector3(float.MinValue, float.MinValue, float.MinValue);
Vector3 modelMin = new Vector3(float.MaxValue, float.MaxValue, float.MaxValue);

foreach (ModelMesh mesh in m_model.Meshes)
{
  //Create variables to hold min and max xyz values for the mesh. Initialise them to extremes
   Vector3 meshMax = new Vector3(float.MinValue, float.MinValue, float.MinValue);
   Vector3 meshMin = new Vector3(float.MaxValue, float.MaxValue, float.MaxValue);

  // There may be multiple parts in a mesh (different materials etc.) so loop through each
  foreach (ModelMeshPart part in mesh.MeshParts)
   {
     // The stride is how big, in bytes, one vertex is in the vertex buffer
     // We have to use this as we do not know the make up of the vertex
     int stride = part.VertexBuffer.VertexDeclaration.VertexStride;

     byte[] vertexData = new byte[stride * part.NumVertices];
     part.VertexBuffer.GetData(part.VertexOffset * stride, vertexData, 0, part.NumVertices, 1); // fixed 13/4/11

     // Find minimum and maximum xyz values for this mesh part
     // We know the position will always be the first 3 float values of the vertex data
     Vector3 vertPosition=new Vector3();
     for (int ndx = 0; ndx < vertexData.Length; ndx += stride)
      {
         vertPosition.X= BitConverter.ToSingle(vertexData, ndx);
         vertPosition.Y = BitConverter.ToSingle(vertexData, ndx + sizeof(float));
         vertPosition.Z= BitConverter.ToSingle(vertexData, ndx + sizeof(float)*2);

         // update our running values from this vertex
         meshMin = Vector3.Min(meshMin, vertPosition);
         meshMax = Vector3.Max(meshMax, vertPosition);
     }
   }

   // transform by mesh bone transforms
   meshMin = Vector3.Transform(meshMin, m_transforms[mesh.ParentBone.Index]);
   meshMax = Vector3.Transform(meshMax, m_transforms[mesh.ParentBone.Index]);

   // Expand model extents by the ones from this mesh
   modelMin = Vector3.Min(modelMin, meshMin);
   modelMax = Vector3.Max(modelMax, meshMax);
}


// Create and return the model bounding box
return new BoundingBox(modelMin, modelMax);

}

Note that after calculating the minimum and maximum vertex position values for the mesh we also need to take into account any bone transformations that may need to be applied. This can be seen in the code above where the bounding box just calculated for a mesh is transformed by the bone matrix.

For more accurate collisions you may also want to store the individual mesh bounding boxes. A first collision check would then check the model bounding box and if this proved true go through each mesh bounding box. This would also have the benefit of giving you the part of the model where the collision occurred.

There is one issue with the above code though which is that we are not sure that every vertex in the vertex buffer is used by the mesh. You might think it would naturally however it is not guaranteed. This could be solved by using the index buffer to look up the vertices.

Ultimately you may wish to create a custom content processor as by writing code that hooks into the actual model loading we can deal with the vertex data prior to it being put into a vertex buffer and optimised and reorganised by XNA.

Optimisation Issues

It takes some time to go through all the vertices finding min and max values therefore you should only do this once at model load time and not during the game loop. For maximum speed you would do this in advance of the game running.

Handling Animation

If our model is animated then we need to modify our bounding boxes at run time. We do this by storing each mesh min and max values we calculated and at game loop time transform them using the bone matrix. Note that we don't need to extract the vertex again and calculate a mesh box again to do this as we are just transo. A bounding box for the whole model can then be created by looping through all the transformed mesh bounding boxes finding the min and max values.

Further Reading



© 2004-2009 Keith Ditchburn  (A lecturer on the Games Programming Course at the Teesside University)