OpenGL
Geometry/Rendering
Institute of Computer Graphics and Algorithms
Vienna University of Technology
Rendering Geometry
• Old (< 2.0) way:
Without VBO:
Init()
Load model data from file
Render()
Send model data to GPU
Render model
16.03.2016
CGUE 2016 - Steiner/Plank
2
Rendering Geometry
• Modern way
Without VBO:
With VBO:
Init()
Load model data from file
Init():
Load model data from file
Send model data to GPU and
store it in VBOs
Render()
Send model data to GPU
Render model
Render():
Enable VBOs
Render model
• VBOs send data only once to the GPU -> Faster
– Use VBOs wherever possible
16.03.2016
CGUE 2016 - Steiner/Plank
3
VBO
• Generate VBO
glGenBuffers(1, &handle);
• Bind VBO and upload data
glBindBuffer(target, handle);
glBufferData(target, size, data, usage);
• Delete VBO
glDeleteBuffers(1, &handle);
16.03.2016
CGUE 2016 - Steiner/Plank
4
VBO Data Upload
• Upload Data
glBindBuffer(target, handle);
glBufferData(target, size, data, usage);
• target
– GL_ARRAY_BUFFER for vertex data
• Positions, Normals, UV-Coordinates
– GL_ELEMENT_ARRAY_BUFFER for index data
16.03.2016
CGUE 2016 - Steiner/Plank
5
VBO Data Upload
• Upload Data
glBindBuffer(target, handle);
glBufferData(target, size, data, usage);
• size
– Memory size of data array (in Bytes!!!!!)
eg. array_lenght * sizeof(float)
• data
– Address of Data-Array
16.03.2016
CGUE 2016 - Steiner/Plank
6
VBO Data Upload
• Upload Data
glBindBuffer(target, handle);
glBufferData(target, size, data, usage);
• Usage
– Spezifies how data will be used by the application
– Several values possible
• GL_{ACCESS_FREQUENCY}_{ACCESS_NATURE}
16.03.2016
CGUE 2016 - Steiner/Plank
7
• GL_{ACCESS_FREQUENCY}_{ACCESS_NATURE}
GL_STREAM_...
You will modify the data once, then use it once, and repeat this
process many times.
GL_STATIC_...
You will specify the data only once, then use it many times
without modifying it.
GL_DYNAMIC_...
You will specify or modify the data repeatedly, and use it
repeatedly after each time you do this.
..._DRAW
The data is generated by the application and passed to GL for
rendering.
..._READ
The data is generated by GL, and copied into the VBO to be used
for rendering.
..._COPY
The data is generated by GL, and read back by the application. It
is not used by GL.
• GL_STATIC_DRAW most usefull for CGUE
16.03.2016
CGUE 2016 - Steiner/Plank
8
VAO
• VAOs are a collection of VBOs and attribute pointers
Render_without_VAO()
Render_with_VAO():
Enable vertex attribute 1
Enable vertex attribute 2
...
Enable vertex attribute n
Enable VAO
Render model
Disable VAO
Render model
Disable vertex attribute 1
Disable vertex attribute 2
...
Disable vertex attribute n
16.03.2016
CGUE 2016 - Steiner/Plank
9
VAO
• Generate VAO
glGenVertexArrays(1, &vao);
• Bind VAO
glBindVertexArray(vao);
• Unbind VAO
glBindVertexArray(0);
• Delete VAO
glDeleteVertexArrays(1, &vao);
16.03.2016
CGUE 2016 - Steiner/Plank
10
VAO – Shader Attributes
• Must attach each buffer to a shader attribute
– Shader
in vec3 position;
– Query location
glGetAttribLocation(program, "position");
– Enable attribute
glEnableVertexAttribArray(positionIndex);
glVertexAttribPointer(positionIndex, 3,
GL_FLOAT, GL_FALSE, 0, 0);
16.03.2016
CGUE 2016 - Steiner/Plank
11
VAO – Shader Attributes
• Alternative: Specify locations in shader
– Shader
layout(location = 1) in vec3 pos;
– No need to query location anymore
– Enable attribute
glEnableVertexAttribArray(1);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, 0);
16.03.2016
CGUE 2016 - Steiner/Plank
12
VAO initialisation
//Bind VAO
glBindVertexArray(vao);
//Bind VBO
glBindBuffer(GL_ARRAY_BUFFER, positionBuffer);
//Enable Attribute
GLint positionIndex = glGetAttribLocation(program, "position");
glEnableVertexAttribArray(positionIndex);
glVertexAttribPointer(positionIndex, 3, GL_FLOAT,
GL_FALSE, 0, 0);
//Bind Index Buffer
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer);
//Unbind VAO and VBOs
glBindVertexArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
16.03.2016
CGUE 2016 - Steiner/Plank
13
VAO - Hints
• Per combination of Shader and Model (VBOs) one
VAO is needed
– Also possible to fix the attribute location in shader
– VAO works as long as attributes have same location
• Don't call
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0)
when a VAO is bound, or the VAO will loose the
current set index vbo
16.03.2016
CGUE 2016 - Steiner/Plank
14
VAO rendering
//Activate shader
glUseProgram(programHandle);
//Bind VAO
glBindVertexArray(vao);
//Draw array
glDrawArrays(GL_TRIANGLES, 0, 3);
//or with index buffer
glDrawElements(GL_TRIANGLES, 0, GL_UNSIGNED_INT, 0);
//Unbind VAO
glBindVertexArray(0);
//Deactivate shader
glUseProgram(0);
16.03.2016
CGUE 2016 - Steiner/Plank
15
DEMO 04
VBO/VAO