As well as colors, normals, and other vertex data PUSHIN’ GEO TO THE GPU JEFF CHASTINE 1.

JEFF CHASTINE 1

As well as colors, normals, and other vertex data

PUSHIN’ GEO TO THE GPU

JEFF CHASTINE 2

This is what we want to make

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Normalized Device Coordinate System

(-1, 1)

(-1, 0)

(-1, -1)

(0, 1)

(0, -1)

(0, 0)

(1, 1)

(1, 0)

(1, -1)

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Coordinates of our triangle(0.0f, 0.5f, 0.0f)

(0.5f, -0.5f, 0.0f)(-0.5f, -0.5f, 0.0f)

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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)

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Colors of our triangle*(0.0f, 0.0f, 1.0f, 1.0f)

(0.0f, 1.0f, 0.0f, 1.0f)(1.0f, 0.0f, 0.0f, 1.0f)

*Note the beautiful interpolation of color!

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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)

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WORKING WITH BUFFERS• To create a buffer ID:

// This will be the ID of the bufferGLuint buffer;

// Ask OpenGL to generate exactly 1 unique IDglGenBuffers(1, &buffer);

• To set this buffer as the active one and specify which buffer we’re referring to:

glBindBuffer(GL_ARRAY_BUFFER, buffer);

• Notes:

• That buffer is now bound and active!

• Any “drawing” will come from that buffer

• Any “loading” goes into that buffer

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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

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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…

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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)

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GLuint buffer;

glGenBuffers(1, &buffer);

buffer

Note: buffer “lives” on the graphics card in a nice, two-bedroom loft…

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GLuint buffer;


glBindBuffer (GL_ARRAY_BUFFER, buffer);

buffer

Hey – I’m active now

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GLuint buffer;



glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW);

buffer

Now I know how big I am!

Where’d the 7 come from?

Allocate how much space (in bytes) we need

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GLuint buffer;




buffer

Now I know how big I am!

(x, y, z) + (r, g, b, a) = 7

Where’d the 7 come from?

Allocate how much space (in bytes) we need

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GLuint buffer;




glBufferSubData (GL_ARRAY_BUFFER, 0, numVertices*3*sizeof(GLfloat), verts);

glBufferSubData (GL_ARRAY_BUFFER, numVertices*3*sizeof(GLfloat), numVertices*4*sizeof(GLfloat), colors);

buffer

Now I’m putting verts at the beginning

verts

Put verts at 0…

it’s pretty bigthough…

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GLuint buffer;




glBufferSubData (GL_ARRAY_BUFFER, 0, numVertices*3*sizeof(GLfloat), verts);

glBufferSubData (GL_ARRAY_BUFFER, numVertices*3*sizeof(GLfloat), numVertices*4*sizeof(GLfloat), colors);

buffer

I’m putting colors next

verts

Put colors startingright after that!

It’s pretty big

too…

colors

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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?

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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);

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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

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GLuint loc = glGetAttribLocation(shaderProgramID, "vPosition");glEnableVertexAttribArray(loc);glVertexAttribPointer(loc, 3, GL_FLOAT, GL_FALSE, 0, 0);

#version 130



}

buffer

Guys! I’m still active, remember?

verts colors

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GLuint loc2 = glGetAttribLocation(shaderProgramID, "vColor");glEnableVertexAttribArray(loc2);glVertexAttribPointer(loc2, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(numVertices*3*sizeof(GLfloat)));

#version 130



}

buffer

Tell vColor where to find its data in me…

verts colors

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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



}

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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 VAOGLuint vao;

// Ask the driver for exactly 1 unique IDglGenVertexArrays(1, &vao);

// Everything after this will be part of the VAOglBindVertexArray(vao);

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// 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);

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END WITH A UTAH TEAPOT!

As well as colors, normals, and other vertex data PUSHIN’ GEO TO THE GPU JEFF CHASTINE 1.

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Transcript of As well as colors, normals, and other vertex data PUSHIN’ GEO TO THE GPU JEFF CHASTINE 1.