Implementation: CubeMesh and SingleColoredCube

Overview 

In this post we shall deep dive and understand implementing a mesh and geometric object for cube shape. We will try to draw a single colored cube as shown at the bottom and rotate it along the three axes. The cube looks elongated because aspect ratio is not applied. 

Details

System  class diagram

Every 3D object such as SingleCoredCube derives from BaseGeometry class. It overrides mesh with an instance of CubeMesh to generate geometry.

Cube Geometry

A cube has 8 vertices (0,1,2,3,4,5,6,7) and 6 faces (top, bottom, right, left, back, front), with each face made up of 2 triangles. Its center of the origin is at (0,0,0).

CubeMesh

CubeMesh class generates geometry for the cube shape. As we learnt previously, GPU expects data in the form of vertices of the triangles. Hence the cube is divided into 12 triangles. The geometry consists of geometry data for each of the three vertices of the triangle. In other words, the geometry data consists of 36 vertices.

The geometry data for each vertex consists of

1. X, Y, Z coordinate (mandatory)
2. Color (optional)
3. Normal vector (optional)
4. Texture coordinates (optional)

How this data is packaged depends on if data is sent as indexed or non indexed format.

In non indexed format, all the 4 pieces of data is packaged and sent for all the 36 vertices.

In indexed format, all the 4 pieces of data is packaged and sent for only the 8 vertices.

In addition, an array consisting of indices of the 36 vertices are also sent.

This will be more clear in the discussion below.

The CubeMesh class derives from IGeometryMeshIndexed interface.

 It overrides GenerateVerticesData() method to generate 36 vertices for the 12 triangles, 2 from each of the 6 faces as discussed above. Only Position attribute of the vertex containing X,Y, Z coordinates is stored internally as a vector. 

Members

Name	Description
clrs	Contains individual colors for the six faces. Each color is represented as RGB format with values varying from 0 to 1.
faces	Contains vertices. 3 vertices for each triangle. 2 triangles for each face.  6 faces for entire cube.
normals	Contains individual normal for the six faces. Each normal has X,Y,Z format with values varying from -1 to 1.
texturemap	Contains individual texture coordinate for the six faces. Each texture coordinate  has X,Y format with values varying from 0 to 1.
vertices	Contains 8 vertices of the cube.  Each vertex has X,Y,Z format with values varying from -1 to 1.

Methods

Name	Description
GenerateVerticesData	This method generates vertex data for the GPU to render the geometrical shape. This method is called if the data is sent in  non indexed mode. The parameter att specifies the 4 attributes in the form of their enums ORed together. The enums are defined in BaseGeometry class. The data is stored in the parameter vaoutil by invoking its corresponding method  to update the data. For example, for the enum POS,  AddPosition() method is called etc.
GenerateVerticesDataIndexed	Same as GenerateVerticesData() except this method is called if the data is sent in indexed mode.
GenerateIndicesData	This method is called if the data is sent in indexed mode. It sends  an array consisting of indices of the 36 vertices.
updateColors	Updates the clrs with the array in the parameter clrs.
updateNormals	Updates the normals with the array in the parameter normals.
updateTextureMap	Updates the texturemap with the array in the parameter texturemap.

SingleColoredCube

SingleColoredCube is derived from BaseGeometry class. It assembles vertex data generated by mesh in VBO and EBO buffers. Later  sent to GPU using shader programs.

Members

Name	Description
color	Contains the color of the cube

Methods

Name	Description
Init	This method overrides the base class Init method. It creates the mesh object. Generates the vertices data in non indexed mode and later sets them up in VBO buffers of the GPU.
UpdateUniforms	This method overrides the base class UpdateUniforms method. The base class method updates the "transform" matrix uniform by calling GetTransformationMatrix method. It updates the uniform "color" containing the value from color to the shader.
vertexShaderSource	This method overrides the base class vertexShaderSource method. It returns the vertex shader code to render the cube object.
fragmentShaderSource	This method overrides the base class fragmentShaderSource method. It returns the fragment shader code to render the cube object.

Output