1. Pushin’ Geo to the GPU
As well as colors, normals, and other vertex data
Made by Jeff Chastine, Modified by Chao Mei
Jeff Chastine

2. Triangles and Vertices
A triangle has 3 vertices.
How and when can we describe the triangle to the Graphics Card?
Answers:
3 floats per vertex, pass each vertex individually – many calls to graphics card
deprecated:: glVertex
Immediate mode drawing is deprecated – why?
An array (i.e. a vertex array)
Lets buffer these in advance – benefits?

3. A Mesh Class A mesh is a bunch of triangles.
It contains information about the vertices (e.g., position, color, etc.).
It should be able to
Add a triangle to the mesh
Render the mesh

4. Overview: Draw a triangle in OpenGL
1. Define the vertex data for a mesh: put it in a dynamic array in CPU memory 2. Create buffers in graphics memory: dedicated to hold the mesh’s vertices 3. Define the GLSL vertex shader program to use for rendering the mesh 4. Draw each mesh with one function call

5. This is what we want to make
Jeff Chastine

6. Normalized Device Coordinate System
(-1, 1)
(0, 1)
(1, 1)
Normalized Device Coordinate System
(-1, 0)
(1, 0)
(0, 0)
(-1, -1)
(0, -1)
(1, -1)
Jeff Chastine

7. Coordinates of our triangle
(0.0f, 0.5f, 0.0f)
(-0.5f, -0.5f, 0.0f)
(0.5f, -0.5f, 0.0f)
Jeff Chastine

8. QUICK! Color Theory!
Represent almost any color by adding red, green and blue
Alpha is the transparency
Called the primary colors (RGB or RGBA)
0.0 means “all the way off”
1.0 means “all the way on”
Examples:
Red (1.0, 0.0, 0.0, 1.0)
Blue (0.0, 0.0, 1.0, 1.0)
Purple (1.0, 0.0, 1.0, 1.0)
Yellow (1.0, 1.0, 0.0, 1.0)
White (1.0, 1.0, 1.0, 1.0)
Black (0.0, 0.0, 0.0, 1.0)
Grey (0.5, 0.5, 0.5, 1.0)
Brown (0.7, 0.5, 0.1, 1.0)
Jeff Chastine

9. Colors of our triangle*
(0.0f, 0.0f, 1.0f, 1.0f)
(1.0f, 0.0f, 0.0f, 1.0f)
(0.0f, 1.0f, 0.0f, 1.0f)
*Note the beautiful interpolation of color!
Jeff Chastine

10. Basic Problem Get the geometry and color to the GPU
Typically also need a normal and texture coordinate for each vertex!
Ask the OpenGL driver to create a buffer object
This is just a chunk of memory (e.g. array)
Nothing to be afraid of!
Located on the GPU (probably)
Jeff Chastine

11. Working with Buffers To create a buffer ID:
// This will be the ID of the buffer GLuint buffer;
// Ask OpenGL to generate exactly 1 unique ID glGenBuffers(1, &buffer);
To set this buffer as the active one and specify which buffer we’re referring to:
glBindBuffer(GL_ARRAY_BUFFER, buffer);
Documentation:
Notes:
That buffer is now bound and active!
Any “drawing” will come from that buffer
Any “loading” goes into that buffer
Jeff Chastine

12. Two Approaches to Loading the Buffer with Data
Assume everything is in GLfloat* (called data)
One-shot call to load the buffer with data:
glBufferData(GL_ARRAY_BUFFER, sizeof(data),
data, GL_STATIC_DRAW);
Other drawing types GL_X_Y: X
STREAM for infrequent use and changes
STATIC for frequent use and infrequent change
DYNAMIC for frequent use and frequent change
Y could be DRAW, READ or COPY
Jeff Chastine

13. How We’ll do it Process Create the buffer and pass no data
Load the geometry
Load the colors (if any) after that
Load the normals after that…
Note: some like to interlace their vertex data…
Jeff Chastine

14. A Side-By-SIDE Comparison
Show the relationship between client code and shader code
Assume we’re still trying to draw 1 triangle…
numVertices – the number of vertices (will be 3 - duh!)
verts – the position information of each vertex (XYZ - array of GLfloats)
colors – the color information of each vertex (RGBA - array of GLfloats)
Jeff Chastine

15. Note: buffer “lives” on the graphics card in a nice, two-bedroom loft…
GLuint buffer; glGenBuffers(1, &buffer);
buffer
Note: buffer “lives” on the graphics card in a nice, two-bedroom loft…
Jeff Chastine

16. Hey – I’m active now
GLuint buffer; glGenBuffers(1, &buffer); glBindBuffer (GL_ARRAY_BUFFER, buffer);
buffer
Jeff Chastine

17. Allocate how much space (in bytes) we need
Now I know how big I am!
GLuint buffer; glGenBuffers(1, &buffer); glBindBuffer (GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW);
buffer
Allocate how much space (in bytes) we need
Where’d the 7 come from?
Jeff Chastine

18. Allocate how much space (in bytes) we need
Now I know how big I am!
GLuint buffer; glGenBuffers(1, &buffer); glBindBuffer (GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW);
buffer
Allocate how much space (in bytes) we need
Where’d the 7 come from?
(x, y, z) + (r, g, b, a) = 7
Jeff Chastine

19. it’s pretty big though…
Now I’m putting verts at the beginning
GLuint buffer; glGenBuffers(1, &buffer); glBindBuffer (GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW); glBufferSubData (GL_ARRAY_BUFFER, 0, numVertices*3*sizeof(GLfloat), verts); glBufferSubData (GL_ARRAY_BUFFER, numVertices*3*sizeof(GLfloat), numVertices*4*sizeof(GLfloat), colors);
buffer
verts
Put verts at 0…
it’s pretty big though…
Jeff Chastine

20. Put colors starting right after that!
I’m putting colors next
GLuint buffer; glGenBuffers(1, &buffer); glBindBuffer (GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW); glBufferSubData (GL_ARRAY_BUFFER, 0, numVertices*3*sizeof(GLfloat), verts); glBufferSubData (GL_ARRAY_BUFFER, numVertices*3*sizeof(GLfloat), numVertices*4*sizeof(GLfloat), colors);
buffer
verts
colors
Put colors starting right after that!
It’s pretty big too…
Jeff Chastine

21. What we Have so far… We have a buffer with an ID
That buffer lives on the graphics card
That buffer is full of vertex position/color data
How do we get that info to our shader?
Jeff Chastine

22. How To link to the Shader
Query the shader program for its variables
The code below goes into the shader program and gets the “vPosition” ID
GLuint vpos; vpos = glGetAttribLocation (programID, “vPosition”);
In OpenGL, we have to enable things (attributes, in this case):
glEnableVertexAttribArray(vpos); // turn on vPosition
Finally, we set the location and tell it the format of the data in the buffer
glVertexAttribPointer(vpos, 3, GL_FLOAT, GL_FALSE, 0, 0);
void glVertexAttribPointer(GLuint index, GLint size, Glenum type, GLboolean normalized, GLsizei stride, const GLvoid* offset);
Jeff Chastine

23. Find the variable “vPosition” inside the shader
GLuint loc = glGetAttribLocation(shaderProgramID, "vPosition"); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; }
Find the variable “vPosition” inside the shader
Jeff Chastine

24. Guys! I’m still active, remember?
GLuint loc = glGetAttribLocation(shaderProgramID, "vPosition"); glEnableVertexAttribArray(loc); glVertexAttribPointer(loc, 3, GL_FLOAT, GL_FALSE, 0, 0); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; }
Guys! I’m still active, remember?
buffer
verts
colors
Jeff Chastine

25. Tell vColor where to find its data in me…
GLuint loc2 = glGetAttribLocation(shaderProgramID, "vColor"); glEnableVertexAttribArray(loc2); glVertexAttribPointer(loc2, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(numVertices*3*sizeof(GLfloat))); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; }
buffer
verts
colors
Jeff Chastine

26. GLuint loc = glGetAttribLocation(shaderProgramID, "vPosition"); glEnableVertexAttribArray(loc); glVertexAttribPointer(loc, 3, GL_FLOAT, GL_FALSE, 0, 0); GLuint loc2 = glGetAttribLocation(shaderProgramID, "vColor"); glEnableVertexAttribArray(loc2); glVertexAttribPointer(loc2, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(numVertices*3*sizeof(GLfloat))); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; }
Jeff Chastine

27. One Last thing Vertex Array Objects (VAOs)
“Pure State” - it remembers almost everything about buffers
Set it up once, then just call it before drawing!
glVertexAttribPointer…
Doesn’t bind the VBO though…
Creating a vertex array object:
// This will be the ID of the VAO GLuint vao;
// Ask the driver for exactly 1 unique ID glGenVertexArrays(1, &vao);
// Everything after this will be part of the VAO glBindVertexArray(vao);
Jeff Chastine

28. // Create the "remember all" glGenVertexArrays(1, &vao); glBindVertexArray(vao); // Create a buffer and bind it as active glGenBuffers(1, &vbo); glBindBuffer(GL_ARRAY_BUFFER, vbo); // Create space and load the buffer glBufferData(GL_ARRAY_BUFFER, 7*3*sizeof(GLfloat), NULL, GL_STATIC_DRAW); glBufferSubData(GL_ARRAY_BUFFER, 0, 3*3*sizeof(GLfloat), vertices); glBufferSubData(GL_ARRAY_BUFFER, 3*3*sizeof(GLfloat),3*4*sizeof(GLfloat), colors); // Find the positions of the variables in the shader positionID = glGetAttribLocation(shaderProgramID, "vPosition"); colorID = glGetAttribLocation(shaderProgramID, "vColor"); // Tell the shader variables where to find their data glVertexAttribPointer(positionID, 3, GL_FLOAT, GL_FALSE, 0, 0); glVertexAttribPointer(colorID, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(3*3*sizeof(GLfloat))); glUseProgram(shaderProgramID); glEnableVertexAttribArray(positionID); glEnableVertexAttribArray(colorID);
Jeff Chastine

29. END WITH A UTAH TEAPOT!
Jeff Chastine