lectureAll-OpenGL-complete-Guide-Tutorial.pdf

University of Texas at Austin CS384G - Computer Graphics Fall 2010 Don Fussell
Introduction to Modern OpenGL
Programming
Adapted from SIGGRAPH 2012 slides by
Ed Angel
University of New Mexico
and
Dave Shreiner
ARM, Inc

 Evolution of the OpenGL Pipeline
 A Prototype Application in OpenGL
 OpenGL Shading Language (GLSL)
 Vertex Shaders
 Fragment Shaders
 Examples
Outline

 OpenGL is a computer graphics rendering API
 With it, you can generate high-quality color images
by rendering with geometric and image primitives
 It forms the basis of many interactive applications
that include 3D graphics
 By using OpenGL, the graphics part of your
application can be
 operating system independent
 window system independent
What Is OpenGL?

 We’ll concentrate on the latest versions of OpenGL
 They enforce a new way to program with OpenGL
 Allows more efficient use of GPU resources
 If you’re familiar with “classic” graphics pipelines,
modern OpenGL doesn’t support
 Fixed-function graphics operations
 lighting
 transformations
 All applications must use shaders for their graphics
processing
This is the “new” OpenGL

The Evolution of the OpenGL
Pipeline

 OpenGL 1.0 was released on July 1st, 1994
 Its pipeline was entirely fixed-function
 the only operations available were fixed by the
implementation
 The pipeline evolved, but remained fixed-function
through OpenGL versions 1.1 through 2.0 (Sept. 2004)
In the Beginning …
Primitive
Setup and
Rasterization
Fragment
Coloring and
Texturing
Blending
Vertex
Data
Pixel
Data
Vertex
Transform and
Lighting
Texture
Store

 OpenGL 2.0 (officially) added programmable shaders
 vertex shading augmented the fixed-function transform and
lighting stage
 fragment shading augmented the fragment coloring stage
 However, the fixed-function pipeline was still available
The Start of the Programmable Pipeline
Primitive
Setup and
Rasterization
Fragment
Coloring and
Texturing
Blending
Vertex
Data
Pixel
Data
Vertex
Transform and
Lighting
Texture
Store

 OpenGL 3.0 introduced the deprecation model
 the method used to remove features from OpenGL
 The pipeline remained the same until OpenGL 3.1
(released March 24th, 2009)
 Introduced a change in how OpenGL contexts are used
An Evolutionary Change
Context Type Description
Full
Includes all features (including those marked deprecated)
available in the current version of OpenGL
Forward Compatible
Includes all non-deprecated features (i.e., creates a context
that would be similar to the next version of OpenGL)

 OpenGL 3.1 removed the fixed-function pipeline
 programs were required to use only shaders
 Additionally, almost all data is GPU-resident
 all vertex data sent using buffer objects
The Exclusively Programmable Pipeline
Primitive
Setup and
Rasterization
Fragment
Shader
Blending
Vertex
Data
Pixel
Data
Vertex
Shader
Texture
Store

 OpenGL 3.2 (released August 3rd, 2009) added an
additional shading stage – geometry shaders
More Programmability
Primitive
Setup and
Rasterization
Fragment
Shader
Blending
Vertex
Data
Pixel
Data
Vertex
Shader
Texture
Store
Geometry
Shader

 OpenGL 3.2 also introduced context profiles
 profiles control which features are exposed
 currently two types of profiles: core and compatible
More Evolution – Context Profiles
Context Type Profile Description
Full
core All features of the current release
compatible All features ever in OpenGL
Forward Compatible
core All non-deprecated features
compatible Not supported

 OpenGL 4.1 (released July 25th, 2010) included
additional shading stages – tessellation-control and
tessellation-evaluation shaders
 Latest version is 4.3
The Latest Pipelines
Primitive
Setup and
Rasterization
Fragment
Shader
Blending
Vertex
Data
Pixel
Data
Vertex
Shader
Texture
Store
Geometry
Shader
Tessellation
Control
Shader
Tessellation
Evaluation
Shader

OpenGL ES and WebGL
 OpenGL ES 2.0
 Designed for embedded and hand-held devices such as cell
phones
 Based on OpenGL 3.1
 Shader based
 WebGL
 JavaScript implementation of ES 2.0
 Runs on most recent browsers

OpenGL Application
Development

A Simplified Pipeline Model
Vertex
Processing
Rasterizer
Fragment
Processing
Vertex
Shader
Fragment
Shader
GPU Data Flow
Application

Framebuffer

Vertices

Vertices

Fragments

Pixels

 Modern OpenGL programs essentially do the
following steps:
1. Create shader programs
2. Create buffer objects and load data into them
3. “Connect” data locations with shader variables
4. Render
OpenGL Programming in a Nutshell

 OpenGL applications need a place to render into
 usually an on-screen window
 Need to communicate with native windowing
system
 Each windowing system interface is different
 We use GLUT (more specifically, freeglut)
 simple, open-source library that works everywhere
 handles all windowing operations:
 opening windows
 input processing
Application Framework Requirements

 Operating systems deal with library functions
differently
 compiler linkage and runtime libraries may expose
different functions
 Additionally, OpenGL has many versions and
profiles which expose different sets of functions
 managing function access is cumbersome, and
window-system dependent
 We use another open-source library, GLEW, to
hide those details
Simplifying Working with OpenGL

 Geometric objects are represented using vertices
 A vertex is a collection of generic attributes
 positional coordinates
 colors
 texture coordinates
 any other data associated with that point in space
 Position stored in 4 dimensional homogeneous
coordinates
 Vertex data must be stored in vertex buffer objects
(VBOs)
 VBOs must be stored in vertex array objects
(VAOs)
Representing Geometric Objects
x
y
z
w
⎛ ⎞
⎜ ⎟
⎜ ⎟
⎜ ⎟
⎜ ⎟
⎝ ⎠

 All primitives are specified by vertices
OpenGL’s Geometric Primitives
GL_TRIANGLE_STRIP

GL_TRIANGLE_FAN

GL_LINES
GL_LINE_LOOP

GL_LINE_STRIP

GL_TRIANGLES

GL_POINTS

A First Program

 We’ll render a cube with colors at each vertex
 Our example demonstrates:
 initializing vertex data
 organizing data for rendering
 simple object modeling
 building up 3D objects from geometric primitives
 building geometric primitives from vertices
Rendering a Cube

 We’ll build each cube face from individual
triangles
 Need to determine how much storage is required
 (6 faces)(2 triangles/face)(3 vertices/triangle)

const
int
NumVertices
=
36;

 To simplify communicating with GLSL, we’ll use a
vec4 class (implemented in C++) similar to GLSL’s
vec4 type
 we’ll also typedef it to add logical meaning
typedef

vec4

point4;

typedef

vec4

color4;

Initializing the Cube’s Data

 Before we can initialize our VBO, we need to stage the
data
 Our cube has two attributes per vertex
 position
 color
 We create two arrays to hold the VBO data
point4

points[NumVertices];

color4

colors[NumVertices];

Initializing the Cube’s Data (cont’d)

//
Vertices
of
a
unit
cube
centered
at
origin,
sides
aligned

with
axes

point4
vertex_positions[8]
=
{

point4(
-‐0.5,
-‐0.5,

0.5,
1.0
),

point4(
-‐0.5,

0.5,

0.5,
1.0
),

point4(

0.5,

0.5,

0.5,
1.0
),

point4(

0.5,
-‐0.5,

0.5,
1.0
),

point4(
-‐0.5,
-‐0.5,
-‐0.5,
1.0
),

point4(
-‐0.5,

0.5,
-‐0.5,
1.0
),

point4(

0.5,

0.5,
-‐0.5,
1.0
),

point4(

0.5,
-‐0.5,
-‐0.5,
1.0
)

};

Cube Data

//
RGBA
colors

color4
vertex_colors[8]
=
{

color4(
0.0,
0.0,
0.0,
1.0
),

//
black

color4(
1.0,
0.0,
0.0,
1.0
),

//
red

color4(
1.0,
1.0,
0.0,
1.0
),

//
yellow

color4(
0.0,
1.0,
0.0,
1.0
),

//
green

color4(
0.0,
0.0,
1.0,
1.0
),

//
blue

color4(
1.0,
0.0,
1.0,
1.0
),

//
magenta

color4(
1.0,
1.0,
1.0,
1.0
),

//
white

color4(
0.0,
1.0,
1.0,
1.0
)

//
cyan

};

Cube Data

// quad() generates two triangles for each face and assigns
colors to the vertices
int Index = 0; // global variable indexing into VBO arrays
void quad(int a, int b, int c, int d) {
colors[Index] = vertex_colors[a]; points[Index] =
vertex_positions[a]; Index++;
colors[Index] = vertex_colors[b]; points[Index] =
vertex_positions[b]; Index++;
colors[Index] = vertex_colors[c]; points[Index] =
vertex_positions[c]; Index++;
colors[Index] = vertex_colors[a]; points[Index] =
vertex_positions[a]; Index++;
colors[Index] = vertex_colors[c]; points[Index] =
vertex_positions[c]; Index++;
colors[Index] = vertex_colors[d]; points[Index] =
vertex_positions[d]; Index++;
}
Generating a Cube Face from Vertices

//
generate
12
triangles:
36
vertices
and
36

colors

void

colorcube()
{

quad(
1,
0,
3,
2
);

quad(
2,
3,
7,
6
);

quad(
3,
0,
4,
7
);

quad(
6,
5,
1,
2
);

quad(
4,
5,
6,
7
);

quad(
5,
4,
0,
1
);

}

Generating the Cube from Faces

 VAOs store the data of a geometric object
 Steps in using a VAO
 generate VAO names by calling
glGenVertexArrays()

 bind a specific VAO for initialization by calling
glBindVertexArray()

 update VBOs associated with this VAO
 bind VAO for use in rendering
 This approach allows a single function call to
specify all the data for an objects
 previously, you might have needed to make many calls
to make all the data current
Vertex Array Objects (VAOs)

//
Create
a
vertex
array
object

GLuint
vao;

glGenVertexArrays(1,
&vao);

glBindVertexArray(vao);

VAOs in Code

 Vertex data must be stored in a VBO, and
associated with a VAO
 The code-flow is similar to configuring a VAO
 generate VBO names by calling glGenBuffers()

 bind a specific VBO for initialization by calling
glBindBuffer(GL_ARRAY_BUFFER,
…)

 load data into VBO using
glBufferData(GL_ARRAY_BUFFER,
…)

 bind VAO for use in rendering
glBindVertexArray()

Storing Vertex Attributes

//
Create
and
initialize
a
buffer
object

GLuint
buffer;

glGenBuffers(1,
&buffer);

glBindBuffer(GL_ARRAY_BUFFER,
buffer);

glBufferData(GL_ARRAY_BUFFER,
sizeof(points)
+

sizeof(colors),
NULL,

GL_STATIC_DRAW);

glBufferSubData(GL_ARRAY_BUFFER,
0,

sizeof(points),
points);

glBufferSubData(GL_ARRAY_BUFFER,
sizeof(points),

sizeof(colors),
colors);

VBOs in Code

 Application vertex data enters the OpenGL
pipeline through the vertex shader
 Need to connect vertex data to shader
variables
 requires knowing the attribute location
 Attribute location can either be queried by
calling glGetVertexAttribLocation()

Connecting Vertex Shaders with Geometry

//
set
up
vertex
arrays
(after
shaders
are
loaded)

GLuint
vPosition
=
glGetAttribLocation(program,

"vPosition”);

glEnableVertexAttribArray(vPosition);

glVertexAttribPointer(vPosition,
4,
GL_FLOAT,

GL_FALSE,
0,
BUFFER_OFFSET(0));

GLuint
vColor
=
glGetAttribLocation(program,

"vColor”);

glEnableVertexAttribArray(vColor);

glVertexAttribPointer(vColor,
4,
GL_FLOAT,

GL_FALSE,
0,
BUFFER_OFFSET(sizeof(points)));

Vertex Array Code

 For contiguous groups of vertices
 Usually invoked in display callback
 Initiates vertex shader
Drawing Geometric Primitives
glDrawArrays(GL_TRIANGLES,
0,
NumVertices);

Shaders and GLSL

Scalar types: float, int, bool
Vector types: vec2, vec3, vec4
ivec2, ivec3, ivec4
bvec2, bvec3, bvec4
Matrix types: mat2, mat3, mat4
Texture sampling: sampler1D, sampler2D, sampler3D,
samplerCube
C++ style constructors: vec3 a = vec3(1.0, 2.0, 3.0);
GLSL Data Types

 Standard C/C++ arithmetic and logic operators
 Operators overloaded for matrix and vector operations
Operators
mat4
m;

vec4
a,
b,
c;

b
=
a*m;

c
=
m*a;

For vectors can use [ ], xyzw, rgba or stpq
Example:
vec3
v;

v[1],
v.y,
v.g,
v.t
all refer to the same element
Swizzling:
vec3
a,
b;

a.xy
=
b.yx;

Components and Swizzling

 in, out
 Copy vertex attributes and other variables to/from
shaders
 in
vec2
tex_coord;

 out
vec4
color;

 Uniform: variable from application
 uniform
float
time;

 uniform
vec4
rotation;

Qualifiers

 if
 if else
 expression ? true-expression : false-
expression
 while, do while
 for
Flow Control

 Built in
 Arithmetic: sqrt, power, abs
 Trigonometric: sin, asin
 Graphical: length, reflect
 User defined
Functions

 gl_Position: output position from vertex
shader
 gl_FragColor: output color from fragment
shader
 Only for ES, WebGL and older versions of GLSL
 Present version use an out variable
Built-in Variables

Simple Vertex Shader for Cube
in
vec4
vPosition;

in
vec4
vColor;

out
vec4
color;

void
main()
{

color
=
vColor;

gl_Position
=
vPosition;

}

The Simplest Fragment Shader
in
vec4
color;

out
vec4
FragColor;

void
main()
{

FragColor
=
color;

}

 Shaders need to be compiled
and linked to form an
executable shader program
 OpenGL provides the compiler
and linker
 A program must contain
 vertex and fragment
shaders
 other shaders are optional
Getting Shaders into OpenGL
Create
Shader
Load Shader
Source
Compile
Shader
Create
Program
Attach Shader
to Program
Link
Program
glCreateProgram()
glShaderSource()
glCompileShader()
glCreateShader()
glAttachShader()
glLinkProgram()
Use Program glUseProgram()
These
steps need
to be
repeated
for each
type of
shader in
the shader
program

 We’ve created a routine for this course to make it
easier to load your shaders
 available at course website
GLuint
InitShaders(
const
char*
vFile,
const
char*

fFile);
 InitShaders
takes two filenames
 vFile for the vertex shader
 fFile for the fragment shader
 Fails if shaders don’t compile, or program doesn’t
link
A Simpler Way

 Need to associate a shader variable with an OpenGL data
source
 vertex shader attributes → app vertex attributes
 shader uniforms → app provided uniform values
 OpenGL relates shader variables to indices for the app to
set
 Two methods for determining variable/index association
 specify association before program linkage
 query association after program linkage
Associating Shader Variables and Data

Assumes you already know the variables’ name
GLint idx =
glGetAttribLocation(program, “name”);
GLint idx =
glGetUniformLocation(program, “name”);
Determining Locations After Linking

Uniform Variables
glUniform4f(index,
x,
y,
z,
w);

Glboolean
transpose
=
GL_TRUE;

//
Since
we’re
C
programmers

Glfloat
mat[3][4][4]
=
{
…
};

glUniformMatrix4fv(index,
3,
transpose,
mat);

Initializing Uniform Variable Values

int
main(int
argc,
char
**argv)
{

glutInit(&argc,
argv);

glutInitDisplayMode(GLUT_RGBA
|
GLUT_DOUBLE
|

GLUT_DEPTH);

glutInitWindowSize(512,
512);

glutCreateWindow("Color
Cube”);

glewInit();

init();

glutDisplayFunc(display);

glutKeyboardFunc(keyboard);

glutMainLoop();

return
0;

}

Finishing the Cube Program

void
display(void)
{

glClear(GL_COLOR_BUFFER_BIT
|
GL_DEPTH_BUFFER_BIT);

0,
NumVertices);

glutSwapBuffers();

}

void
keyboard(unsigned
char
key,
int
x,
int
y)
{

switch(
key
)
{

case
033:
case
'q':
case
'Q':

exit(
EXIT_SUCCESS
);

break;

}

}

Cube Program GLUT Callbacks

 A vertex shader is initiated by each vertex output by
glDrawArrays()

 A vertex shader must output a position in clip
coordinates to the rasterizer
 Basic uses of vertex shaders
 Transformations
 Lighting
 Moving vertex positions
Vertex Shader Examples

3D is just like taking a photograph (lots of
photographs!)
Camera Analogy
camera

tripod

model

viewing

volume

" Transformations take us from one “space” to
another
" All of our transforms are 4×4 matrices
Transformations
Model-View
Transform"
Projection
Transform"
Perspective
Division"
(w)"
Viewport
Transform"
Modeling
Transform"
Modeling
Transform"
Object Coords.
World Coords. Eye Coords. Clip Coords.
Normalized
Device
Coords.
Vertex
Data
2D Window
Coordinates

 Modeling transformations
 assemble the world and move the objects
 Viewing transformations
 define position and orientation of the viewing
volume in the world
 Projection transformations
 adjust the lens of the camera
 Viewport transformations
 enlarge or reduce the physical photograph
Camera Analogy Transform Sequence

⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎣
⎡
=
15
11
7
3
14
10
6
2
13
9
5
1
12
8
4
0
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
M
 matrices are always
post-multiplied
 product of matrix and
vector is
 A vertex is
transformed by 4×4
matrices
 all affine operations
are matrix
multiplications
 all matrices are stored
column-major in
OpenGL
 this is opposite of
what “C”
programmers expect
3D Homogeneous Transformations
v

M

 Set up a viewing frustum to specify how much
of the world we can see
 Done in two steps
 specify the size of the frustum (projection transform)
 specify its location in space (model-view transform)
 Anything outside of the viewing frustum is
clipped
 primitive is either modified or discarded (if entirely
outside frustum)
View Specification

 OpenGL projection model uses eye coordinates
 the “eye” is located at the origin
 looking down the -z axis
 Projection matrices use a six-plane model:
 near (image) plane and far (infinite) plane
 both are distances from the eye (positive values)
 enclosing planes
 top & bottom, left & right
View Specification (cont’d)

 Position the camera/eye in the scene
 To “fly through” a scene
 change viewing transformation and
redraw scene
 LookAt(eyex,
eyey,
eyez,

lookx,
looky,
lookz,

upx,
upy,
upz)

 up vector determines unique orientation
 careful of degenerate positions
Viewing Transformations

Move object or change
frame origin
Translation
!
!
!
!
!
!
"
#
$
$
$
$
$
$
%
&
=
1
0
0
0
1
0
0
0
1
0
0
0
1
)
,
,
(
z
y
x
z
y
x
t
t
t
t
t
t
T

Stretch, mirror or decimate a
coordinate direction
Scale
Note, there’s a translation applied here to
make things easier to see

!
!
!
!
!
!
"
#
$
$
$
$
$
$
%
&
=
1
0
0
0
0
0
0
0
0
0
0
0
0
)
,
,
(
z
y
x
z
y
x
s
s
s
s
s
s
S

Rotate coordinate system about an axis in space
Rotation
Note, there’s a translation applied
here to make things easier to see

Vertex Shader for Cube Rotation
in
vec4
vPosition;

in
vec4
vColor;

out
vec4
color;

uniform
vec3
theta;

void
main()
{

//
Compute
the
sines
and
cosines
of
theta
for

//
each
of
the
three
axes
in
one
computation.

vec3
angles
=
radians(theta);

vec3
c
=
cos(angles);

vec3
s
=
sin(angles);

//
Remember:
these
matrices
are
column-‐major

mat4
rx
=
mat4(
1.0,

0.0,

0.0,
0.0,

0.0,

c.x,

s.x,
0.0,

0.0,
-‐s.x,

c.x,
0.0,

0.0,

0.0,

0.0,
1.0
);

mat4
ry
=
mat4(
c.y,
0.0,
-‐s.y,
0.0,

0.0,
1.0,

0.0,
0.0,

s.y,
0.0,

c.y,
0.0,

0.0,
0.0,

0.0,
1.0
);


mat4
rz
=
mat4(
c.z,
-‐s.z,
0.0,
0.0,

s.z,

c.z,
0.0,
0.0,

0.0,

0.0,
1.0,
0.0,

0.0,

0.0,
0.0,
1.0
);

color
=
vColor;

gl_Position
=
rz
*
ry
*
rx
*
vPosition;

}


//
compute
angles
using
mouse
and
idle
callbacks

GLuint
theta;

//
theta
uniform
location

vec3

Theta;

//
Axis
angles

void
display(void)
{

glClear(GL_COLOR_BUFFER_BIT
|
GL_DEPTH_BUFFER_BIT);

glUniform3fv(theta,
1,
Theta);

0,
NumVertices);

glutSwapBuffers();

}

Sending Angles from Application

Vertex Lighting

 Lighting simulates how objects reflect light
 material composition of object
 light’s color and position
 global lighting parameters
 Lighting functions deprecated in 3.1
 Can implement in
 Application (per vertex)
 Vertex or fragment shaders
Lighting Principles

 Computes a color or shade for each vertex using a
lighting model (the modified Phong model) that takes
into account
 Diffuse reflections
 Specular reflections
 Ambient light
 Emission
 Vertex shades are interpolated across polygons by the
rasterizer
Modified Phong Model

 The model is a balance between simple computation
and physical realism
 The model uses
 Light positions and intensities
 Surface orientation (normals)
 Material properties (reflectivity)
 Viewer location
 Computed for each source and each color component
Modified Phong Model

 Modified Phong lighting model
 Computed at vertices
 Lighting contributors
 Surface material properties
 Light properties
 Lighting model properties
OpenGL Lighting

 Normals define how a surface reflects light
 Application usually provides normals as a vertex atttribute
 Current normal is used to compute vertex’s color
 Use unit normals for proper lighting
 scaling affects a normal’s length
Surface Normals

 Define the surface properties of a primitive
 you can have separate materials for front and back
Material Properties
Property Description
Diffuse Base object color
Specular Highlight color
Ambient Low-light color
Emission Glow color
Shininess
Surface
smoothness

//
vertex
shader

in
vec4
vPosition;

in
vec3
vNormal;

out
vec4
color;

uniform
vec4
AmbientProduct,
DiffuseProduct,

SpecularProduct;

uniform
mat4
ModelView;

uniform
mat4
Projection;

uniform
vec4
LightPosition;

uniform
float
Shininess;

Adding Lighting to Cube

void
main()
{

//
Transform
vertex

position
into
eye
coordinates

vec3
pos
=
(ModelView
*
vPosition).xyz;

vec3
L
=
normalize(LightPosition.xyz
-‐
pos);

vec3
E
=
normalize(-‐pos);

vec3
H
=
normalize(L
+
E);

//
Transform
vertex
normal
into
eye
coordinates

vec3
N
=
normalize(ModelView
*
vec4(vNormal,
0.0)).xyz;


//
Compute
terms
in
the
illumination
equation

vec4
ambient
=
AmbientProduct;

float
Kd
=
max(dot(L,
N),
0.0);

vec4

diffuse
=
Kd*DiffuseProduct;

float
Ks
=
pow(max(dot(N,
H),
0.0),
Shininess);

vec4

specular
=
Ks
*
SpecularProduct;

if(dot(L,
N)
<
0.0)

specular
=
vec4(0.0,
0.0,
0.0,
1.0)

gl_Position
=
Projection
*
ModelView
*
vPosition;

color
=
ambient
+
diffuse
+
specular;

color.a
=
1.0;

}


Shader Examples

 A shader that’s executed for each “potential” pixel
 fragments still need to pass several tests before making it to
the framebuffer
 There are lots of effects we can do in fragment shaders
 Per-fragment lighting
 Bump Mapping
 Environment (Reflection) Maps
Fragment Shaders

 Compute lighting using same model as for per
vertex lighting but for each fragment
 Normals and other attributes are sent to vertex
shader and output to rasterizer
 Rasterizer interpolates and provides inputs for
fragment shader
Per Fragment Lighting

 Vertex Shaders
 Moving vertices: height fields
 Per vertex lighting: height fields
 Per vertex lighting: cartoon shading
 Fragment Shaders
 Per vertex vs. per fragment lighting: cartoon shader
 Samplers: reflection Map
 Bump mapping
Shader Examples

 A height field is a function y = f(x, z) where the
y value represents a quantity such as the height
above a point in the x-z plane.
 Heights fields are usually rendered by sampling
the function to form a rectangular mesh of
triangles or rectangles from the samples yij =
f(xi, zj)
Height Fields

 Form a quadrilateral mesh
 Display each quad using
Displaying a Height Field
for(i=0;i<N;i++)
for(j=0;j<N;j++)
data[i][j]=f(i,
j,
time);

vertex[Index++]
=
vec3((float)i/N,
data[i][j],
(float)j/N);

vertex[Index++]
=
vec3((float)i/N,
data[i][j],
(float)(j+1)/N);

vertex[Index++]
=
vec3((float)(i+1)/N,
data[i][j],
(float)(j+1)/N);

vertex[Index++]
=
vec3((float)(i+1)/N,
data[i][j],
(float)(j)/N);

for(i=0;i<NumVertices
;i+=4)
glDrawArrays(GL_LINE_LOOP,
4*i,
4);

Time Varying Vertex Shader
in
vec4
vPosition;

in
vec4
vColor;

uniform
float
time;
/*
in
milliseconds
*/

uniform
mat4
ModelView,
ProjectionMatrix;

void
main()

{

vec4

v
=
vPosition;

vec4

t
=
sin(0.001*time
+
5.0*v);

v.y
=
0.1*t.x*t.z;

gl_Position
=
ModelViewProjectionMatrix
*
t;

}

 Solid Mesh: create two triangles for each
quad
 Display with
0,
NumVertices);

 For better looking results, we’ll add lighting
 We’ll do per-vertex lighting
 leverage the vertex shader since we’ll also use it to
vary the mesh in a time-varying way
Adding Lighting

uniform
float
time,
shininess;

uniform
vec4
vPosition,
light_position
diffuse_light,

specular_light;

uniform
mat4
ModelViewMatrix,
ModelViewProjectionMatrix,

NormalMatrix;

void
main()
{

vec4

v
=
vPosition;

vec4

t
=
sin(0.001*time
+
5.0*v);

v.y
=
0.1*t.x*t.z;

gl_Position
=
ModelViewProjectionMatrix
*
v;

vec4
diffuse,
specular;

vec4
eyePosition
=
ModelViewMatrix
*
vPosition;

vec4
eyeLightPos
=
light_position;

Mesh Shader

vec3
N
=
normalize(NormalMatrix
*
Normal);

vec3
L
=
normalize(eyeLightPos.xyz
-‐
eyePosition.xyz);

vec3
E
=
-‐normalize(eyePosition.xyz);

vec3
H
=
normalize(L
+
E);

float
Kd
=
max(dot(L,
N),
0.0);

float
Ks
=
pow(max(dot(N,
H),
0.0),
shininess);

diffuse

=
Kd*diffuse_light;

specular
=
Ks*specular_light;

color

=
diffuse
+
specular;

}

Mesh Shader (cont’d)

Texture Mapping

Texture Mapping
s
t
x
y
z
image
geometry screen

Texture Mapping in OpenGL
 Images and geometry flow through separate
pipelines that join at the rasterizer
 “complex” textures do not affect geometric
complexity
Geometry
Pipeline
Pixel
Pipeline
Rasterizer
Vertices

Pixels

Fragment
Shader

Applying Textures
 Three basic steps to applying a texture
1. specify the texture
 read or generate image
 assign to texture
 enable texturing
2. assign texture coordinates to vertices
3. specify texture parameters
 wrapping, filtering

1. specify textures in texture objects
2. set texture filter
3. set texture function
4. set texture wrap mode
5. set optional perspective correction hint
6. bind texture object
7. enable texturing
8. supply texture coordinates for vertex
Applying Textures

Texture Objects
 Have OpenGL store your images
 one image per texture object
 may be shared by several graphics contexts
 Generate texture names
glGenTextures(n,
*texIds);

Texture Objects (cont'd.)
 Create texture objects with texture data and
state
 glBindTexture(target,
id);

 Bind textures before using
 glBindTexture(target,
id);

 Define a texture image from an array of
texels in CPU memory
glTexImage2D(target,
level,
components,

w,
h,
border,
format,
type,
*texels);

 Texel colors are processed by pixel pipeline
 pixel scales, biases and lookups can be
done
Specifying a Texture Image

 Based on parametric texture coordinates
 Coordinates need to be specified at each vertex
Mapping a Texture
s
t 1, 1
0, 1
0, 0 1, 0
(s, t) = (0.2, 0.8)
(0.4, 0.2)
(0.8, 0.4)
A
B C
a
b
c
Texture Space Object Space

Applying the Texture in the Shader
// Declare the sampler
uniform sampler2D diffuse_mat;
// GLSL 3.30 has overloaded texture();
// Apply the material color
vec3 diffuse = intensity *
texture2D(diffuse_mat, coord).rgb;

Texturing the Cube
// add texture coordinate attribute to quad
function
quad(int a, int b, int c, int d) {
quad_colors[Index] = vertex_colors[a];
points[Index] = vertex_positions[a];
tex_coords[Index] = vec2(0.0, 0.0);
Index++;
… // rest of vertices
}

Creating a Texture Image
// Create a checkerboard pattern
for (int i = 0; i < 64; i++) {
for (int j = 0; j < 64; j++) {
GLubyte c;
c = (((i & 0x8) == 0) ^ ((j & 0x8) == 0)) * 255;
image[i][j][0] = c;
image[i][j][1] = c;
image[i][j][2] = c;
image2[i][j][0] = c;
image2[i][j][1] = 0;
image2[i][j][2] = c;
}
}

Texture Object
GLuint
textures[1];

glGenTextures(1,
textures);

glBindTexture(GL_TEXTURE_2D,
textures[0]);

glTexImage2D(GL_TEXTURE_2D,
0,
GL_RGB,
TextureSize,

TextureSize,
GL_RGB,
GL_UNSIGNED_BYTE,
image);

glTexParameterf(GL_TEXTURE_2D,
GL_TEXTURE_WRAP_S,
GL_REPEAT);

GL_TEXTURE_WRAP_T,
GL_REPEAT);

GL_TEXTURE_MAG_FILTER,
GL_NEAREST);

GL_TEXTURE_MIN_FILTER,
GL_NEAREST);

glActiveTexture(GL_TEXTURE0);

Vertex Shader
in vec4 vPosition;
in vec4 vColor;
in vec2 vTexCoord;
out vec4 color;
out vec2 texCoord;
void main() {
color = vColor;
texCoord = vTexCoord;
gl_Position = vPosition;
}

Fragment Shader
in vec4 color;
in vec2 texCoord;
out vec4 FragColor;
uniform sampler texture;
void main() {
FragColor = color * texture(texture, texCoord);
}

University of Texas at Austin CS384G - Computer Graphics Fall 2010 Don Fussell 106
Next class: Visual Perception
" Topic:
How does the human visual system?
How do humans perceive color?
How do we represent color in computations?
" Read:
• Glassner, Principles of Digital Image Synthesis,
pp. 5-32. [Course reader pp.1-28]
• Watt , Chapter 15.
• Brian Wandell. Foundations of Vision. Sinauer
Associates, Sunderland, MA, pp. 45-50 and
69-97, 1995.
[Course reader pp. 29-34 and pp. 35-63]

lectureAll-OpenGL-complete-Guide-Tutorial.pdf

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