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Chapter 2Graphics Programming

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Using OpenGL in Visual C++ – 1/3 Opengl32.dll and glu32.dll should be in the

system folder Opengl32.lib and glu32.lib should be in the

lib folder for VC++ gl.h and glu.h should be in a folder called

GL under the include folder for VC++ Get glut32.lib, glut32.dll and glut.h from th

e course homepage and put them in the same places as the other files

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Using OpenGL in Visual C++ – 2/3 Fire up visual studio Create a new project as the following:

File New Project (input your project name, then a directory (workspace) with the same name will be built) Win32 Console Application An Empty Application

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Using OpenGL in Visual C++ – 3/3 In the workspace click on “File View” to

access (expand) the source code tree In “Source Files”, add in the source code

(*.c files) Select Project Settings Link and

in “Object/library modules” add “Opengl32.lib glu32.lib glut32.lib”

Press “F7” to build “your_project.exe”

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Sierpinski Gasket – 1/21. Pick an initial point at random inside the triangle

2. Select one of the three vertices at random

3. Find the point halfway between the initial point and the randomly selected vertex

4. Display the new point

5. Replace the initial point with this new point

6. Return to step 2

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Sierpinski Gasket – 2/2main()

{

initialize_the_system();

for(some_number_of_points)

{

pt = generate_a_point();

display_the_point(pt);

}

cleanup();

}

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Programming 2D Applications A vertex is a location in space glVertex*

* is in the form of nt or ntv n is the number of dimensions t denotes the data type:

integer (i), float (f), or double (d); pointer to an array (v)

#define GLfloat float

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Examples glVertex2i(GLint xi, GLint yi) glVertex3f(GLfloat x, GLfloat y, GLfloat

z) GLfloat vertex[3]

glVertex3fv(vertex)

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OpenGL Object Examples glBegin(GL_LINES);

glVertex2f(x1, y1);glVertex2f(x2, y2);

glEnd(); glBegin(GL_POINTS);

glVertex2f(x1, y1);glVertex2f(x2, y2);

glEnd();

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Code of Sierpinski Gasket – 1/3Typedef GLfloat point2[2];

void display(void)

{

point2 vertices[3]={{0,0},{250,500},{500,0}};

/* an arbitrary triangle */

static point2 p = {75,50};

/* any desired initial point */

int j, k;

int rand(); /* standard random-number generator */

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Code of Sierpinski Gasket – 2/3for(k=0;k<5000;k++)

{

j=rand()%3; /* pick a random vertex from 0, 1, 2 */

p[0]=(p[0]+vertices[j][0])/2; /* compute new point */

p[1]=(p[1]+vertices[j][1])/2;

glBegin(GL_POINTS);

glVertex2fv(p); /* display new point */

glEnd();

}

glFlush();

}

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Code of Sierpinski Gasket – 3/3

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Questions1. In what colors are we drawing

2. Where on the screen does our image appear

3. How large will the image be

4. How do we create an area of the screen – a window – for our image?

5. How long will the image remain on the screen?

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Coordinate Systems

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OpenGL API

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Graphics Functions – 1/3

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Graphics Functions – 2/3 Primitive functions:

points, line segments, polygons, pixels, text, curves, surfaces

Attributes functions:color, pattern, typeface

Viewing functions:position, orientation, clipping

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Graphics Functions – 3/3 Transformation functions:

rotation, translation, scaling Input functions:

keyboards, mice, data tablets Control functions:

communicate with windows, initialization, error handling

Inquiry functions: number of colors, camera parameters/values

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Graphics Pipeline and State Machine Functionalities of graphics functions:

Define primitive: glvertex* Change opengl state (most of them!)

Present color, current matrix

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OpenGL Interface GL (OpenGL in Windows) GLU (graphics utility library)

uses only GL functions, creates common objects (such as spheres)

GLUT (GL Utility Toolkit)interfaces with the window system

GLX: glue between OpenGL and Xwindow, used by GLUT

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OpenGL Library Organization

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API Dilemma for Primitives Minimal or maximal? Convenience versus portability OpenGL’s intermediate approach:

GL contains a small set of basic primitives GLU contains richer set of objects

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OpenGL Primitives Geometric primitives and raster primitives

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Points and Line Segments

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Polygon Basics – 1/3

Filled objects Methods of displaying a polygon

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Polygon Basics – 2/3 Simple, convex, and flat

Simple

Nonsimple

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Polygon Basics – 3/3

Convexity

Convex objects

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Polygon Types in OpenGL – 1/2

In OpenGL, edges and interior must be drawn separately!

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Polygon Types in OpenGL – 2/2

Use triangles if possible because of efficiency!

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Drawing a Sphere – 1/5

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Drawing a Sphere – 2/5

x

y

z

sin),(

coscos),(

cossin),(

z

y

x

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Drawing a Sphere – 3/5c=M_PI/180.0; // degrees to radians, M_PI=3.14159…

for(phi=-80.0; phi<=80.0; phi+=20.0) {

glBegin(GL_QUAD_STRIP);

for(theta=-180.0; theta<=180.0; theta+=20.0) {

x=sin(c*theta)*cos(c*phi);

y=cos(c*theta)*cos(c*phi);

z=sin(c*phi);

glVertex3d(x, y, z);

x=sin(c*theta)*cos(c*(phi+20.0));

y=cos(c*theta)*sin(c*(phi+20.0));

z=sin(c*(phi+20.0));

glVertex3d(x, y, z);

}

glEnd();

}Drawing the portion of lower latitudes

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Drawing a Sphere – 4/5x=y=0;

z=1;

glBegin(GL_TRIANGLE_FAN);

glVertex3d(x, y, z);

c=M_PI/180.0;

z=sin(c*80.0);

for(theta=-180.0; theta<=180.0; theta+=20.0) {

x=sin(c*theta)*cos(c*80.0);

y=cos(c*theta)*sin(c*80.0);

glVertex3d(x, y, z);

}

glEnd();Drawing the portion around the north pole

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Drawing a Sphere – 5/5x=y=0;

z=-1;

glBegin(GL_TRIANGLE_FAN);

glVertex3d(x, y, z);

z=-sin(c*80.0);

for(theta=-180.0; theta<=180.0; theta+=20.0) {

x=sin(c*theta)*cos(c*80.0);

y=cos(c*theta)*sin(c*80.0);

glVertex3d(x, y, z);

}

glEnd();Drawing the portion around the south pole

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Text – Stroke or Raster

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Attributes for Lines, Texts …

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Color – 1/4

Additive color matching

C=T1R+T2G+T3B, T1, T2, T3 are the tristimulus values

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Color – 2/4 Basic tenet of three-color theory:

If two colors produce the same tristimulus values, then they are visually indistinguishable

The range of colors that we can produce on a given system is called that system’s color gamut

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Color – 3/4

Color Solid

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Color – 4/4

Additive Color Subtractive Color

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RGB Color glColor3f(1.0, 0.0, 0.0):

Can be used to specify 24 bits true colors Alpha channel:

Used in OpenGL as an opacity or transparency value glClearColor(1.0, 1.0,1.0,1.0)

glPointSize(2.0)

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Indexed Color

Color-lookup table

Example: k=m=8: pick 256 out of 16M colors

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Two-dimensional Viewing

Objects before clipping Image after clipping

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Viewing Volume

OpenGL default: 222 cube

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Orthographic View

void glOrtho(GLdouble left, GLdouble right, …)

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Matrix Mode There are two matrices in OpenGL:

Model-view: defines COP and orientation Projection: defines the projection matrix

glMatrixMode(GL_PROJECTION);glLoadIdentity();gluOrtho2D(0.0, 500.0, 0.0, 500.0);glMatrixMode(GL_MODELVIEW);

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Control Functions OpenGL assumes origin is bottom left glutInit(int *argcp, char **argv); glutCreateWindow(char *title); glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH

| GLUT_DOUBLE); glutInitWindowSize(480,640); glutInitWindowPosition(0,0); OpenGL default: RGB color, no hidden-surface rem

oval, single buffering

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Aspect Ratio

Viewing rectangleglOrtho(…)

Display windowglutInitWindowSize(…)

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Viewports

void glViewport(GLint x, GLint y, GLsizei w, GLsizei h)Viewport is part of the state.

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main, display, myinit functions …

void glutMainLoop(void)Why do we need this?

void glutDisplayFunc(void (*func)(void))When is the function invoked?

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Sample main programVoid main(int argc, char **argv)

{

glutInit(&argc,argv);

glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);glutInitWindowSize(500, 500);

glutInitWindowPosition(0, 0);glutCreateWindow(“Simple OpenGL example”);

glutDisplayFunc(display);

myinit();

glutMainLoop();

}For most non-interactive applications…

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Gasket Program – 1/3void myinit(void)

{

/* attributes */

glClearColor(1.0, 1.0, 1.0, 1.0); /* white background */

glColor3f(1.0, 0.0, 0.0); /* draw in red */

/* set up viewing */glMatrixMode(GL_PROJECTION);glLoadIdentity();gluOrtho2D(0.0, 500.0, 0.0, 500.0);glMatrixMode(GL_MODELVIEW);

}

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Gasket Program – 2/3void display(void)

{

typedef GLfloat point2[2]; /* define a point data type */point2 vertices[3]={{0.0,0.0},{250.0,500.0},{500.0,0.0}} /* triangle */

int i, j, k;int rand();

point2 p={75.0, 50.0}; /* arbitrary point inside triangle */

glClear(GL_COLOR_BUFFER_BIT); /* clear the window */

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Gasket Program – 3/3/* compute and output 5000 new points */

for(k=0; k<5000; k++) {

j=rand()%3;/* compute point halfway between vertex and old point */p[0]=(p[0]+vertex[j][0])/2.0;p[1]=(p[1]+vertex[j][1])/2.0;

glBegin(GL_POINTS); /* plot point */glVertex2fv(p);glEnd();

}

glFlush();

}

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Using Recursion – 1/3void triangle(point2 a, point2 b, point2 c)

{

glBegin(GL_TRIANGLES);

glVertex2fv(a);

glVertex2fv(b);

glVertex2fv(c);

glEnd();

}

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Using Recursion – 2/3void divide_triangle(point2 a, point2 b, point2 c, int k)

{

point2 ab, ac, bc;

int j;

if(k>0) {

for(j=0; j<2; j++) ab[j]=(a[j]+b[j])/2;

for(j=0; j<2; j++) ac[j]=(a[j]+c[j])/2;

for(j=0; j<2; j++) bc[j]=(b[j]+c[j])/2;

divide_triangle(a, ab, ac, k-1);

divide_triangle(c, ac, bc, k-1);

divide_triangle(b, bc, ab, k-1);

}

else triangle(a, b, c); /* draw triangle at end of of recursion */

}

a

b c

ab ac

bc

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Using Recursion – 3/3Void display(void)

{

glClear(GL_COLOR_BUFFER_BIT);

divide_triangle(v[0], v[1], v[2], n);

glFlush();

}

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Three-Dimensional Gasket – 1/2typedef Glfloat point3[3];

point3 vertices[4] = {{0,0,0,0},{250,500,100},{500,250,250},{250,100,250}};

Point3 new,old={250, 100, 250};

j=rand()%4;

new[0]=(old[0]+vertices[j][0])/2;

new[1]=(old[1]+vertices[j][1])/2;

new[2]=(old[2]+vertices[j][2])/2;tetrahedron

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Three-Dimensional Gasket – 2/2/* plot point */

glBegin(GL_POINTS);

glVertex3fv(new);

glEnd();

/* replace old point by new */

old[0]=new[0];

old[1]=new[1];

old[2]=new[2];

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3D Recursive Gasket – 1/3void triangle(point3 a, point3 b, point3 c)

{

glBegin(GL_POLYGON);

glVertex3fv(a);

glVertex3fv(b);

glVertex3fv(c);

glEnd();

}

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3D Recursive Gasket – 2/3void divide_triangle(point3 a, point3 b, point3 c, int k)

{

point3 ab, ac, bc;

int j;

if(k>0) {

for(j=0; j<3; j++) ab[j]=(a[j]+b[j])/2;

for(j=0; j<3; j++) ac[j]=(a[j]+c[j])/2;

for(j=0; j<3; j++) bc[j]=(b[j]+c[j])/2;

divide_triangle(a, ab, ac, k-1);

divide_triangle(c, ac, bc, k-1);

divide_triangle(b, ab, ab, k-1);

}

else triangle(a, b, c); /* draw triangle at end of recursion*/

}

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3D Recursive Gasket – 3/3void tetrahedron(int n)

{

glColor3f(1.0, 0.0, 0.0);

divide_triangle(v[0], v[1], v[2], k);

glColor3f(0.0, 1.0, 0.0);

divide_triangle(v[3], v[2], v[1], k);

glColor3f(0.0, 0.0, 1.0);

divide_triangle(v[0], v[3], v[1], k);

glColor3f(0.0, 0.0, 0.0);

divide_triangle(v[0], v[2], v[3], k);

}

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Hidden-Surface Removal – 1/3

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Hidden-Surface Removal – 2/3 Z-Buffer glutInitDisplayMode(GLUT_SINGLE | GL

UT_RGB | GLUT_DEPTH); glEnable(GL_DEPTH_TEST) glClear(GL_COLOR_BUFFER_BIT | GL_

DEPTH_BUFFER_BIT);

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Hidden-Surface Removal – 3/3void display()

{

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

tetrahedron(n);

glFlush();

}

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Summary and Notes Coding OpenGL in Visual C++ OpenGL API

Graphics (primitives, attributes), viewing, control, communications with windows …

Sample codes