/// <summary>
/// Helper function adds a single new billboard sprite to the output geometry.
/// </summary>
private void GenerateBillboard(MeshContent mesh, GeometryContent geometry,
Vector3 position, Vector3 normal)
{
VertexContent vertices = geometry.Vertices;
VertexChannelCollection channels = vertices.Channels;
// First, create a vertex position entry for this billboard. Each
// billboard is going to be rendered a quad, so we need to create four
// vertices, but at this point we only have a single position that is
// shared by all the vertices. The real position of each vertex will be
// computed on the fly in the vertex shader, thus allowing us to
// implement effects like making the billboard rotate to always face the
// camera, and sway in the wind. As input the vertex shader only wants to
// know the center point of the billboard, and that is the same for all
// the vertices, so only a single position is needed here.
int positionIndex = mesh.Positions.Count;
mesh.Positions.Add(position);
// Second, create the four vertices, all referencing the same position.
int index = vertices.PositionIndices.Count;
for (int i = 0; i < 4; i++)
{
vertices.Add(positionIndex);
}
// Third, add normal data for each of the four vertices. A normal for a
// billboard is kind of a silly thing to define, since we are using a
// 2D sprite to fake a complex 3D object that would in reality have many
// different normals across its surface. Here we are just using a copy
// of the normal from the ground underneath the billboard, which can be
// used in our lighting computation to make the vegetation darker or
// lighter depending on the lighting of the underlying landscape.
VertexChannel <Vector3> normals;
normals = channels.Get <Vector3>(VertexChannelNames.Normal());
for (int i = 0; i < 4; i++)
{
normals[index + i] = normal;
}
// Fourth, add texture coordinates.
VertexChannel <Vector2> texCoords;
texCoords = channels.Get <Vector2>(VertexChannelNames.TextureCoordinate(0));
texCoords[index + 0] = new Vector2(0, 0);
texCoords[index + 1] = new Vector2(1, 0);
texCoords[index + 2] = new Vector2(1, 1);
texCoords[index + 3] = new Vector2(0, 1);
// Fifth, add a per-billboard random value, which is the same for
// all four vertices. This is used in the vertex shader to make
// each billboard a slightly different size, and to be affected
// differently by the wind animation.
float randomValue = (float)random.NextDouble() * 2 - 1;
VertexChannel <float> randomValues;
randomValues = channels.Get <float>(VertexChannelNames.TextureCoordinate(1));
for (int i = 0; i < 4; i++)
{
randomValues[index + i] = randomValue;
}
// Sixth and finally, add indices defining the pair of
// triangles that will be used to render the billboard.
geometry.Indices.Add(index + 0);
geometry.Indices.Add(index + 1);
geometry.Indices.Add(index + 2);
geometry.Indices.Add(index + 0);
geometry.Indices.Add(index + 2);
geometry.Indices.Add(index + 3);
}