Vertex Specification

Vertex Specification
In "Modern" OpenGL

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Important Concepts

Vertex Stream:
- The stream of data values that define the vertices
Vertex Attributes:
- A description of the elements in the vertex stream
Primitive:
- The shape that is encapsulated by the vertex stream

The Vertex Stream

Makeup:
- Everything that is needed to encapsulate a collection of vertices (e.g., positions, colors, normals)
Organization:
- Can be interleaved or not (e.g., the position of the first vertex can be followed by the color of the first vertex or the positions of all vertices can come first, followed by the colors of all vertices)

The Vertex Stream (cont.)

A Scene Object Consisting of Two Triangles with a Non-Interleaved Stream

openglexamples/modern/multiple.cpp

(Fragment: stream)

          // The positions of the vertices of two triangles
  GLfloat positions[] = {
    -0.5f, -0.5f, 0.0f, // Vertex 0 of triangle 0
     0.5f, -0.5f, 0.0f, // Vertex 1 of triangle 0
     0.0f,  0.5f, 0.0f, // Vertex 2 of triangle 0
    -0.5f, -0.5f, 0.0f, // Vertex 0 of triangle 1
     0.5f, -0.5f, 0.0f, // Vertex 1 of triangle 1
     0.0f, -1.0f, 0.0f, // Vertex 2 of triangle 1
  };

  // The colors of the vertices of two triangles
  GLfloat colors[] = {
    1.0f,  0.0f, 0.0f, // Vertex 0 of triangle 0
    0.0f,  1.0f, 0.0f, // Vertex 1 of triangle 0
    0.0f,  0.0f, 1.0f, // Vertex 2 of triangle 0
    1.0f,  0.0f, 0.0f, // Vertex 0 of triangle 1
    0.0f,  1.0f, 0.0f, // Vertex 1 of triangle 1
    0.0f,  0.0f, 1.0f, // Vertex 2 of triangle 1
  };

The Vertex Stream (cont.)

A Scene Object Consisting of Two Triangles with an Interleaved Stream

openglexamples/modern/interleaved.cpp

(Fragment: stream)

          // The interleaved positions and colors of the vertices of two triangles
  GLfloat vertices[] = {
    -0.5f, -0.5f, 0.0f, // Vertex 0 of triangle 0: Position
     1.0f,  0.0f, 0.0f, // Vertex 0 of triangle 0: Color
     0.5f, -0.5f, 0.0f, // Vertex 1 of triangle 0: Position
     0.0f,  1.0f, 0.0f, // Vertex 1 of triangle 0: Color
     0.0f,  0.5f, 0.0f, // Vertex 2 of triangle 0: Position
     0.0f,  0.0f, 1.0f, // Vertex 2 of triangle 0: Color
    -0.5f, -0.5f, 0.0f, // Vertex 0 of triangle 1: Position
     1.0f,  0.0f, 0.0f, // Vertex 0 of triangle 1: Color
     0.5f, -0.5f, 0.0f, // Vertex 1 of triangle 1: Position
     0.0f,  1.0f, 0.0f, // Vertex 1 of triangle 1: Color
     0.0f, -1.0f, 0.0f, // Vertex 2 of triangle 1: Position
     0.0f,  0.0f, 1.0f, // Vertex 2 of triangle 1: Color
  };

Vertex Specification

Vertex Array Objects:
- Describe the data associated with each vertex
Vertex Buffer Objects:
- Store the data associated with each vertex

Vertex Buffer Objects

Buffer Objects:
- Store an array of unformatted memory allocated by the OpenGL context (typically on the GPU)
Vertex Buffer Objects (VBOs):
- A buffer object that is used as a source for vertex data

Vertex Buffer Objects (cont.)

Creation:
- void glGenBuffers(GLsizei n, GLuint* buffers) where n is the inbound number of buffers to create and buffers is the outbound array of names/handles/IDs
Deletion:
- void glDeleteBuffers(GLsizei n, const GLuint* buffers) where n is the inbound number of buffers to create and buffers is the inbound array of names/handles/IDs to delete
Setting State:
- void glBindBuffer(enum target, uint bufferName) binds the buffer object to the context so its state can be changed (where target is typically GL_ARRAY_BUFFER)
- Subsequent calls to glBufferStorage(), glBufferData(), glClearBufferData(), etc... then change the state

Vertex Buffer Objects (cont.)

The VBOs for the Non-Interleaved Stream

openglexamples/modern/multiple.cpp

(Fragment: vbo)

          // Create a vertex buffer object (VBO) for the positions
  glGenBuffers(1, &positionBufferID);
  // Bind the VBO so it's state can be changed
  glBindBuffer(GL_ARRAY_BUFFER, positionBufferID);
  // Change the state of the VBO
  glBufferData(GL_ARRAY_BUFFER, sizeof(positions), positions, GL_STATIC_DRAW);
  
  // Create a vertex buffer object (VBO) for the colors
  glGenBuffers(1, &colorBufferID);
  // Bind the VBO so it's state can be changed
  glBindBuffer(GL_ARRAY_BUFFER, colorBufferID);
  // Change the state of the VBO
  glBufferData(GL_ARRAY_BUFFER, sizeof(colors), colors, GL_STATIC_DRAW);

Vertex Buffer Objects (cont.)

The VBO for the Interleaved Stream

openglexamples/modern/interleaved.cpp

(Fragment: vbo)

          // Create a vertex buffer object (VBO) for the vertices
  glGenBuffers(1, &vertexBufferID);
  // Bind the VBO so it's state can be changed
  glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID);
  // Change the state of the VBO
  glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

Vertex Array Objects (VAOs)

Recall:
- VBOs are unformatted
Purpose of VAOs:
- Store all of the state needed by vertices (including the format and the VBOs)

Vertex Array Objects (VAOs)

Creation:
- void glGenVertexArrays(GLsizei n, GLuint* arrays); where n is the inbound number of buffers to create and arrays is the outbound array of names/handles/IDs
Deletion:
- void glDeleteVertexArrays(GLsizei n, const GLuint *arrays); where n is the inbound number of buffers to create and arrays is the inbound array of names/handles/IDs to delete
Setting State:
- void glBindVertexArray(GLuint array); binds the VAO to the context so it's state can be changed
- void glEnableVertexAttribArray(GLuint index); allows the attribute numbered index to be changed
- glVertexAttribPointer() can then be used to set the format of the VBO and associate the currently bound VBO with the currently bound VBA
- void glDisableVertexAttribArray( GLuint index); prevents the attribute numbered index from being changed

Vertex Array Objects (cont.)

The VAOs for the Non-Interleaved Stream

openglexamples/modern/multiple.cpp

(Fragment: vao)

          // Bind the VAO so that it's state can be changed
  // (by calls to glEnableVertexAttribArray() and glVertexAttribPointer())
  glBindVertexArray(vertexArrayID);
  
  // Describe the positions in the vertex array buffer (changing the state
  // of the VAO)
  glEnableVertexAttribArray(0);                                   
  glBindBuffer(GL_ARRAY_BUFFER, positionBufferID); // Doesn't modify the VAO's state                
  glVertexAttribPointer( // Modify's the VAO's state
    0,                          // attribute 0
    3,                          // size                             
    GL_FLOAT,                   // type                             
    GL_FALSE,                   // normalized or not?                      
    0,                          // stride (0 because not interleaved)
    reinterpret_cast<void*>(0)  // offset into the buffer
  );                                                              

  // Describe the colors in the vertex array buffer (changing the state
  // of the VAO)
  glEnableVertexAttribArray(1);
  glBindBuffer(GL_ARRAY_BUFFER, colorBufferID);
  glVertexAttribPointer( // Modify's the VAO's state
    1,
    3,
    GL_FLOAT,
    GL_FALSE,
    0,       
    reinterpret_cast<void*>(0)
  );

Vertex Array Objects (cont.)

The VAOs for the Interleaved Stream

openglexamples/modern/interleaved.cpp

(Fragment: vao)

          // Bind the VAO so that it's state can be modified
  // (by calls to glEnableVertexAttribArray() and glVertexAttribPointer())
  glBindVertexArray(vertexArrayID);

  // Describe the positions in the vertex array buffer (changing the state
  // of the VAO)
  glEnableVertexAttribArray(0);                                   
  glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID); // Doesn't modify the VAO's state
  glVertexAttribPointer( // Modify's the VAO's state
    0,                          // attribute 0
    3,                          // size
    GL_FLOAT,                   // type
    GL_FALSE,                   // normalized or not
    sizeof(GLfloat)*6,          // stride (6 GLfloat values are interleaved)
    reinterpret_cast<void*>(0)  // offset (positions start at 0)
  );                                                              
  
  // Describe the colors in the vertex array buffer (changing the state
  // of the VAO)
  glEnableVertexAttribArray(1);
  glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID); // Doesn't modify the VAO's state
  glVertexAttribPointer( // Modify's the VAO's state
    1,                                          // attribute 1
    3,
    GL_FLOAT,
    GL_FALSE,
    sizeof(GLfloat)*6,
    reinterpret_cast<void*>(3*sizeof(GLfloat)) // offset (colors start at 3)
  );