Lab 3 Vertex Array Objects and Vertex Buffer Objects

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Lab 3 Vertex Array Objects and Vertex Buffer Objects

Introduction
Getting started
OpenGL client program
Things to do
More things to do
How to submit

1. Introduction

Now is the time to look at our geometry, add colors, and examine the way we draw things on the screen.
Things to consider:
Here is the source code of the vertex and fragment shaders we are using in this lab:

Vertex shader


#version 150

in vec4 vPosition;
in vec4 vColor;
out vec4 passColor;

void main () {
    gl_Position = vPosition;
    passColor = vColor;
}

Fragment shader


#version 150

in vec4 passColor;
out vec4 fColor;

void main () {
    fColor = passColor;
}

Consider the setup of these two shaders:
1. vPosition and vColor are input varaibles in the vertex shader.
2. gl_Position is the global variable that the vertex shader must set.
3. The vertex shader also sets the output variable passColor, which becomes the input variable to the fragment shader.
4. Once again, the fragment shader passes the color to the output variable fColor
5. OpenGL draws pixels interpolated between coordinates set in gl_Position with colors specified in fColor.
For the above plan to work, we need to find a way to feed the vertex shader two sets of data:
- (a) vertex coordinates that fill each vPosition
- (b) vertex colors that fill the vColor

2. Getting started

Download c262_lab03.zip and unzip the file under the labs directory. You should get the directory structure
```
    C:\bcc\GL262Labs\labs\c262_lab03
```

Go ahead and open Visual Studio project


    C:\bcc\GL262Labs\labs\c262_lab03\c262_lab03.sln

Compile it:
```
    Build -> Build Solution
```
(or simply press F7.)

Notice there are two subdirectories,


    C:\bcc\GL262Labs\labs\c262_lab03\Shaders
    C:\bcc\GL262Labs\labs\c262_lab03\src

one for the shaders and another for C++ source code.

3. OpenGL client program

The structure of our C++ program is

App init
- init GLUT and GLEW
- Compile and link shaders
- init and upload each model (sky, terrain, objects)
- glutMainLoop

App render
- glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT)
- render each model
- glutSwapBuffers

Model init
- glGenVertexArrays and get your vao
- glGenBuffers and get your vbo
- for vertices, normals, colors, and texture:
  - load data (e.g. from a file)
  - glGetAttribLocation ***
Model upload
- glBindVertexArray( vao )
- glBindBuffer( GL_ARRAY_BUFFER... )
- glBufferData( GL_ARRAY_BUFFER... )
- for vertices, normals, colors, and texture:
  - glBufferSubData( GL_ARRAY_BUFFER... )
  - glVertexAttribPointer

Model render
- glBindVertexArray( vao )
- glUseProgram( shader->program_ID )
- for vertices, normals, colors, and texture:
  - glEnableVertexAttribArray ***
- glDrawArrays( GL_TRIANGLES... )
- for vertices, normals, colors, and texture:
  - glDisableVertexAttribArray ***
Note: functions marked *** use vertex attribute IDs in the vertex shader.

App shutdown
- shutdown each model

Model Shutdown
- for vertices, normals, colors, and texture:
  - glDisableVertexAttribArray ***
- glDeleteBuffers( ...vbo )
- glDeleteVertexArrays( ...vao )
- delete any dynamically allocated memory

4. Things to do

The idea of this lab is to add the Model C++ class to our application. This will allow us to draw multiple models (sky, terrain, and other objects) easily.
The Model class has already been added to the project for you. Take a look at Model.h and Model.cpp.
Look at the constructor of the Model class, understand the purpose of each data member intialized there.
Find implementation of Model::upload_2_server() function in Model.cpp file. Follow the instructions provided there and write code in places indicated by /*YOUR ANSWER*/ comments.
Compile and run the project. If everything's okay, you should see this:

5. More things to do

Find implementation of Model::upload_2_server() function in Model.cpp file and add this line at the beginning of the function:
```
    glPolygonMode( GL_FRONT_AND_BACK, GL_LINE );
```
Compile and run the program. What do you see now?

Go to your main() function and replace vertices[] and colors[] with


    GLfloat vertices[] = {-0.5f, -0.5f, 0.0f, 
                           0.0f, -0.5f, 0.0f, 
                         -0.25f,  0.0f, 0.0f,
                         -0.25f,  0.0f, 0.0f,
                          0.25f,  0.0f, 0.0f,
                          0.0f,   0.5f, 0.0f,
                          0.0f,  -0.5f, 0.0f,
                          0.5f,  -0.5f, 0.0f,
                          0.25f,  0.0f, 0.0f
    };
    GLfloat colors[] = {1.0f, 0.75f, 0.0f, 1.0f, 
                        1.0f, 0.63f, 0.0f, 1.0f,
                        1.0f, 1.0f,  0.0f, 1.0f,
                        1.0f, 0.75f, 0.0f, 1.0f, 
                        1.0f, 0.63f, 0.0f, 1.0f,
                        1.0f, 1.0f,  0.0f, 1.0f,
                        1.0f, 0.75f, 0.0f, 1.0f, 
                        1.0f, 0.63f, 0.0f, 1.0f,
                        1.0f, 1.0f,  0.0f, 1.0f
    };

Compile and test your program again.

6. How to submit

There are no files to submit.
Demonstrate a working c262_lab03 program to your instructor.