Implementation: Pipeline

Overview

In this post we shall deep dive and understand mechanics behind rendering 3D objects on the screen.
So far we discussed  FrameBuffers,Vertex Array Object containing VBO and EBO buffers and  Program Object containing Vertex Shader and Fragment Shaders.

The following diagram combines all the above and represents it,

Details

 Every 3D object such as cube derives from BaseGeometry class. The following describes this in details.

BaseGeometry

BaseGeometry class implements base class for all the 3D objects. All the 3D objects such as cube must derive from this class.  

Members

Name	Description
bindexed	Indexed if the data is VBO or EBO. True if EBO.
kount	Number of elements in VBO or EBO. It's populated by derived classes.
mesh	Geometry provider. Supplies the vertices and other data.
ProjectionMatrix	Contains project matrix computed based on mouse and keyboard inputs.
MM	Contains 3D angles and other info based on keyboard/Mouse inputs.
ViewMatrix	Contains view matrix computed based on mouse and keyboard inputs.
shader	Contains instances of vertex shader and fragment shaders.
vaoutil	Contains vertex data containing VBO and EBO

Methods

Name	Description
Cleanup	Releases resources associated with VAO, EBO, shader programs and others. This is overridden by derived classes.
Draw	Calls  OpenGL APIs such as glDrawArrays or glDrawElements are used to draw the 3D objects from VAO buffers using shaders.
Init	Loads and attach shaders to program object.  This is overridden by derived classes. This is shown in the diagram below.
UpdateUniforms	Updates uniform transform containing Transformation Matrix  This is overridden by derived classes,
GetTransformationMatrix	Returns transformation matrix after combining with view and model matrix.
vertexShaderSource	Returns vertex shader string. This is overridden by derived classes.
fragmentShaderSource	Returns fragment shader string. This is overridden  by derived classes.

Operations

The following describes the key operations that happens during the lifetime of a scene object.

Init

In a typical usage scenario, a 3D object first loads all the vertex data such as position by calling  GenerateVerticesData method of the mesh. Then it calls SetupVBO method for all the vertex data with location number of the the vertex data. Note that the location number has to match with the location number defined in the vertex shader. First time, this will generate a VAO and bind it. VBO is created, data is loaded for GL_STATIC_DRAW, bound and associated with the location and the location is enabled.

In cases of indices, the indices are loaded by calling  GenerateIndicesData method. Later BindEBO method is called. This will  generate a EBO, data is loaded for GL_STATIC_DRAW, bound.

Next  the shader code is attached and linked. 

DrawScene

At the time of drawing, uniforms are updated and program is set for use. Draw method is called to draw the 3D object. finally program is reset for use. 

In DrawTriangles function, to draw a VBO cube glDrawArrays API is used where as for EBO or Indexed, glDrawElements is used. Here the EBO contains indices data.

Cleanup

Releases the resources.

In the next post we will draw a single colored cube.