Introduction to OpenGL 1. 2 OpenGL A Graphics rendering API introduced in 1992 by Silicon Graphics...

Introduction to OpenGL

1

OpenGL

• A Graphics rendering API introduced in 1992 by Silicon Graphics Inc

• Provide the low-level functions to access graphics hardware directly

• Cross-platform

3

Related API

• GLU (OpenGL Utility Library)– Part of OpenGL– Use the prefix of glu (ex: gluLookAt())

• GLUT (OpenGL Utility Toolkit)– Not officially part of OpenGL– hide the complexities of differing window system

APIs. – Use the prefix of glut (ex: glutDisplayFunc())

4

OpenGL Utility Toolkit (GLUT)

• Visual C++– http://www.xmission.com/~nate/glut.html– glut32.dll to %WinDir%\System, – glut32.lib to $(MSDevDir)\..\..\VC\lib– glut.h to $(MSDevDir)\..\..\VC\include\GL.

5

A Simple Example

#include <GL/glut.h>

void GL_display();

void GL_reshape(GLsizei w, GLsizei h);

void main(void)

{

glutCreateWindow("Sample");

glutDisplayFunc(GL_display);

glutReshapeFunc(GL_reshape);

glutMainLoop();

}

6

A Simple Example

void GL_display()

{

glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

glClear(GL_COLOR_BUFFER_BIT);

glBegin( GL_LINES );

glColor3f(0.0f, 1.0f, 0.0f);

glVertex3f(1.0, 1.0, 0.0);

glVertex3f(-1.0, -1.0, 0.0);

glColor3f(1.0f, 0.0f, 0.0f);

glVertex3f(-1.0, 1.0, 0.0);

glVertex3f(1.0, -1.0, 0.0);

glEnd();

glFlush();

}7

A Simple Example

void GL_display()

{




glColor3f(0.0f, 1.0f, 0.0f);

glVertex3f(1.0, 1.0, 0.0);

glVertex3f(-1.0, -1.0, 0.0);

glColor3f(1.0f, 0.0f, 0.0f);

glVertex3f(-1.0, 1.0, 0.0);

glVertex3f(1.0, -1.0, 0.0);

glEnd();

glFlush();

}Color Buffer Depth Buffer 8

A Simple Example

void GL_display()

{




glColor3f(0.0f, 1.0f, 0.0f);

glVertex3f(1.0, 1.0, 0.0);

glVertex3f(-1.0, -1.0, 0.0);

glColor3f(1.0f, 0.0f, 0.0f);

glVertex3f(-1.0, 1.0, 0.0);

glVertex3f(1.0, -1.0, 0.0);

glEnd();

glFlush();

}9

Primitives

10

A Simple Example

void GL_display()

{




glColor3f(0.0f, 1.0f, 0.0f);

glVertex3f(1.0, 1.0, 0.0);

glVertex3f(-1.0, -1.0, 0.0);

glColor3f(1.0f, 0.0f, 0.0f);

glVertex3f(-1.0, 1.0, 0.0);

glVertex3f(1.0, -1.0, 0.0);

glEnd();

glFlush();

}11

OpenGL as a State Machine

• Put a value into various states, then it will remain in effect until being changed.– e.g. glColor*()

• Many state variables are enabled or disabled with glEnable(), glDisable()– e.g. glEnable(GL_LIGHT0)

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A Simple Examplevoid GL_display()

{




glColor3f(0.0f, 1.0f, 0.0f);

glVertex3f(1.0, 1.0, 0.0);

glVertex3f(-1.0, -1.0, 0.0);

glColor3f(1.0f, 0.0f, 0.0f);

glVertex3f(-1.0, 1.0, 0.0);

glVertex3f(1.0, -1.0, 0.0);

glEnd();

glFlush();

}13

Command Syntax

glVertex3f( 1.0, 1.0, 0.0 );

OpenGL API Name The number and types of arguments

Data Type C-Language Type OpenGL Type Definition

b 8-bit integer signed char GLbyte

s 16-bit integer short GLshort

i 32-bit integer long GLint, GLsizei

f 32-bit floating-point float GLfloat, GLclampf

d 64-bit floating-point double GLdouble, GLclampd

ub 8-bit unsigned integer unsigned char GLubyte, GLboolean

us 16-bit unsigned integer unsigned short GLushort

ui 32-bit unsigned integer unsigned long GLuint, GLenum, GLbitfield14

A Simple Examplevoid GL_display()

{




glColor3f(0.0f, 1.0f, 0.0f);

glVertex3f(1.0, 1.0, 0.0);

glVertex3f(-1.0, -1.0, 0.0);

glColor3f(1.0f, 0.0f, 0.0f);

glVertex3f(-1.0, 1.0, 0.0);

glVertex3f(1.0, -1.0, 0.0);

glEnd();

glFlush();

}15

A Simple Example

void GL_reshape(GLsizei w, GLsizei h)

{

glViewport(0, 0, w, h);

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f);

glMatrixMode(GL_MODELVIEW);

glLoadIdentity();

}

16

A Simple Example


{



glLoadIdentity();

glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f);


glLoadIdentity();

}

17

A Simple Example


{



glLoadIdentity();

glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f);


glLoadIdentity();

}

18

Matrix in OpenGL

• There are two matrix stacks.– ModelView matrix (GL_MODELVIEW)– Projection matrix (GL_PROJECTION)

• When we call functions of transformation, we should change to the appropriate matrix stack first.

glMatrixMode(GL_MODELVIEW);//now we are in modelview matrix stack!//do modelview transformation here…..

glMatrixMode(GL_PROJECTION);//now we are in projection matrix stack!//do projection transformation here….

19

ModelView Matrix

• Modeling Transformation• Perform rotate, translate, scale and

combinations of these transformations to the object.

• Viewing Transformation• To positioning and aiming the camera

20

Modeling Transformations

• glTranslate{fd}(x, y, z)– Multiplies current matrix by a matrix that moves

an object by x,y,z

glTranslatef( 0, 0, -1 )

21


• glRotate{fd}(angle, x, y, z )– Multiplies current matrix by a matrix that rotates

an object in a counterclockwise direction about the ray from origin to (x,y,z) with angle as the degrees

glRotatef( 45.0, 0, 0, 1)

22


• glScale{fd} (x, y, z)– Multiplies current matrix by a matrix that scales

an object along axes.

glScalef( 2.0, -0.5, 1.0 )

23

Viewing Transformations

• gluLookAt (eyex, eyey, eyez, atx, aty, atz, upx, upy, upz );

24

Matrix in OpenGL

• Matrix multiplications always apply to the top of matrix stack.

Top matrixIn the stack

Translation matrix(glTranslatef)

X

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Matrix in OpenGL

• The order of transformations is critical.

26

glTranslatef( 1,0,0 );

glRotatef(45.0, 0,0,1 );

drawObject();

glRotatef(45.0, 0,0,1 );

glTranslatef( 1,0,0 );

drawObject();

Projection Transformations

• Orthographic Projection• glOrtho( GLdouble left, GLdouble right, GLdouble

bottom, GLdouble top, GLdouble near, GLdouble far )• gluOrtho2D( GLdouble left, GLdouble right, GLdouble

bottom, GLdouble top)

27


• Perspective Projection– gluPerspective( GLdouble fovy, GLdouble

aspect, GLdouble near, GLdouble far );

28


• Perspective Projection– glFrustum( GLdouble left, GLdouble right,

GLdouble bottom, GLdouble top, GLdouble near, GLdouble far );

29

A Simple Example


{



glLoadIdentity();

glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f);


glLoadIdentity();

}

30

OpenGL Transformations

31

ModelviewMatrix

ModelviewMatrix

ViewportTransformation

ViewportTransformation

ProjectionMatrix

ProjectionMatrix

PerspectiveDivision

eye coordina

tes

clip coordina

tes

normalized device

coordinates

window coordina

tes

XYZW

VertexVertex

XY

glTranslatefglRotatefglScalfgluLookAt

gluPerspectivegluOrtho2DglFrustumglOrtho glViewport()

Matrix in OpenGL

• Mantain matrix stack– glPushMatrix() : used to save current stack– glPopMatrix() : used to restore previous

stack

glPushMatirx()

x

glScalef glPopMatrix()

32

Matrix in OpenGLglPushMatrix();

glTranslatef (-1.0, 0.0, 0.0); glRotatef ((GLfloat) shoulder, 0.0, 0.0, 1.0); glTranslatef (1.0, 0.0, 0.0); glPushMatrix();

glScalef (2.0, 0.4, 1.0); glutWireCube (1.0);

glPopMatrix();

glPushMatrix(); glTranslatef (1.0, 0.0, 0.0); glRotatef ((GLfloat) elbow, 0.0, 0.0, 1.0); glTranslatef (1.0, 0.0, 0.0); glPushMatrix();

glScalef (2.0, 0.4, 1.0); glutWireCube (1.0);

glPopMatrix(); glPopMatrix();

glPopMatrix();

33

Matrix in OpenGL

• Reference: Object Hierarchy– http://www.gamedev.net/reference/articles/

article1267.asp

34

References

• OpenGL officially website:– http:/www.opengl.org

• NeHe (useful installation guides included)– http://nehe.gamedev.net/

• Nate Robins (for demonstration)– http://www.xmission.com/~nate/tutors.html

• The Red Book (OpenGL Programming Guide)

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Introduction to OpenGL 1. 2 OpenGL A Graphics rendering API introduced in 1992 by Silicon Graphics...

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