Creating Geometry tutorial - Wave Engine

Objectives

Be able to create a Geometry Object.
For example a Cube.

Resume

In WaveServices.GraphicsDevice you will find a low level API to draw.
You only need to generate a vertices and indices array and send it to the GPU.
There are two ways to do this.

Using Static and Base Primitive

The first one, if you want to create a static and basic primitive, you can do that like our
(WaveEngine.Components.Graphics3D) Model.CreateCube

In this example we are going to create a cube Class and initialices it.

/// 
/// A 3D cube.
/// 
internal sealed class Cube : Geometric
{
    #region Initialize
    /// 
    /// Initializes a new instance of the  class.
    /// 
    /// The size of cube.
    public Cube(float size)
    {
        // A cube has six faces, each one pointing in a different direction.
        Vector3[] normals =
        {
            new Vector3(0, 0, 1),
            new Vector3(0, 0, -1),
            new Vector3(1, 0, 0),
            new Vector3(-1, 0, 0),
            new Vector3(0, 1, 0),
            new Vector3(0, -1, 0)
        };

        Vector2[] texCoord =
        {
            new Vector2(1, 1), new Vector2(0, 1), new Vector2(0, 0), new Vector2(1, 0),
            new Vector2(0, 0), new Vector2(1, 0), new Vector2(1, 1), new Vector2(0, 1),
            new Vector2(1, 0), new Vector2(1, 1), new Vector2(0, 1), new Vector2(0, 0),
            new Vector2(1, 0), new Vector2(1, 1), new Vector2(0, 1), new Vector2(0, 0),
            new Vector2(0, 1), new Vector2(0, 0), new Vector2(1, 0), new Vector2(1, 1),
            new Vector2(1, 0), new Vector2(1, 1), new Vector2(0, 1), new Vector2(0, 0),
        };

        float sizeOverTwo = size / 2;

        // Create each face in turn.
        for (int i = 0, j = 0; i < normals.Length; i++, j += 4)
        {
            Vector3 normal = normals[i];

            // Get two vectors perpendicular to the face normal and to each other.
            Vector3 side1 = new Vector3(normal.Y, normal.Z, normal.X);
            Vector3 side2 = Vector3.Cross(normal, side1);

            // Six indices (two triangles) per face.
            this.AddIndex(this.VerticesCount + 0);
            this.AddIndex(this.VerticesCount + 1);
            this.AddIndex(this.VerticesCount + 3);

            this.AddIndex(this.VerticesCount + 1);
            this.AddIndex(this.VerticesCount + 2);
            this.AddIndex(this.VerticesCount + 3);

            // 0   3
            // 1   2

            // Four vertices per face.
            this.AddVertex((normal - side1 - side2) * sizeOverTwo, normal, texCoord[j]);
            this.AddVertex((normal - side1 + side2) * sizeOverTwo, normal, texCoord[j + 1]);
            this.AddVertex((normal + side1 + side2) * sizeOverTwo, normal, texCoord[j + 2]);
            this.AddVertex((normal + side1 - side2) * sizeOverTwo, normal, texCoord[j + 3]);
        }
    }
}

Getting the Cube
And later in Model class we can just create a static method and get as many cubes we want, only using your new primitive with ModelRenderer component.

public static Model CreateCube(float size)
{
   Model cube = new Model(string.Empty) { InternalModel = new InternalStaticModel() };
   cube.InternalModel.FromPrimitive(WaveServices.GraphicsDevice, new Cube(size));

   return cube;
}

Creating Custom Component

You can hold the model data (vertices and indices) in a new component class (MyPrimitive : Component) and create another new Drawable3D.

To create a vertices and indices:
Vertex and Index
Create and initialise variables needed, for example:

 ushort[] indices = new ushort[this.numIndices];
this.vertices = new VertexPositionColorTexture[this.numVertices];  //You can use different vertex formats

//ToDo: Fill these arrays

Buffer
Now we can upload all data to GPU

this.indexBuffer = new IndexBuffer(indices);
this.GraphicsDevice.BindIndexBuffer(this.indexBuffer);

this.vertexBuffer = new VertexBuffer(VertexPositionColorTexture.VertexFormat);
this.vertexBuffer.SetData(this.numVertices, this.vertices);
this.GraphicsDevice.BindVertexBuffer(this.vertexBuffer);

Draw
After setting buffer, you need to override the Draw method, where you are able to draw any render layer, and set values for the material that you want to use. (for example BasicMaterial)

public override void Draw(TimeSpan gameTime)
{
   base.Draw(gameTime); // Add to render layer (default behavior, but you can manage this to draw each part of you model in a different render layer)
   this.material.Matrices.World = this.localWorld;  //You can write this or in each DrawBasicUnit pass if this changes for each part.

In DrawBasicUnit, you draw each part, if you only have one part it is simple:

currentMaterial.Apply(this.RenderManager);  //You always need a shader to draw anything, but you can use a default material, for example BasicMaterial.

this.GraphicsDevice.DrawVertexBuffer(
                   NumVertices,
                   PrimitiveCount,
                   PrimitiveType.TriangleList,
                   VertexBuffer,
                   IndexBuffer);

Or if the vertices of your primitive change dynamically like a particle system you can hold this in the Drawable3D too, and use DynamicVertexBuffer for better performance.

Updating
You also need to upload indices, vertices or both if this changes in each frame (inside the DrawBasicUnit method)

this.vertexBuffer.SetData(this.numVertices, this.vertices);
this.GraphicsDevice.BindVertexBuffer(this.vertexBuffer);
this.GraphicsDevice.DrawVertexBuffer(this.numVertices,
                                     this.numPrimitives,
                                     PrimitiveType.TriangleList,
                                     this.vertexBuffer,
                                     this.indexBuffer);

Example

Here you have another simple demo about custom geometry (Custom Geometry Sample).