1. Drawing in the plane
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2. Content Getting started OpenGL primitives Line Drawings in OpenGL
Interaction in OpenGL
Design and use of menus

3. Making pictures Picture created in screen window
Window coordinates – integer values of pixel’s position
Open GL device independent graphical library
OpenGL is a State machine

4. Windows based programming
Event driven approach
Event queue – press key, click mouse, move window etc.
Program is running as an event loop waiting for until receives a trigger
Events are associated with callback functions

5. Callback functions Each callback function should be first registered
Registering (callback) functions in main loop
glutDisplayFunc(MyDisplay);
Function MyDisplay (user function) is registered by glut - utility

6. Libraries of OpenGL
The basic library with header gl.h contains low-level graphical commands and predefined constants and graphical types
The OpenGL Utility library glu.h contains several routines that use lower-level OpenGL commands
The OpenGL Utility Toolkit glut.h window system-independent toolkit for working with window system.

7. Main function int main() {
glutInitWindowSize(480,480);//size of initial window
glutCreateWindow("My first attemt");//crete default window
glutDisplayFunc(MyDisplay);//register user function
MyInit();//initialization function
glutMainLoop();// looping
return 0; }

8. Initial function void MyInit(void) {
glClearColor(1.0,1.0,1.0,.0);//color of background
glColor3f(1.0,0.0,0.0);//color for drawing
glMatrixMode(GL_PROJECTION);//matrix state
glLoadIdentity(); //initialization of projection matrix
gluOrtho2D(0,480.0,0,480);//Size of world window }

9. Opening window for drawing
glutInitWindowSize(480,480) – size of the first occurrence of the window
glutCreateWindow("My first attemt") – creating a window with given title
glutDisplayFunc(MyDisplay) – registering of a drawing function
glutMainLoop(); - looping the function

10. Basic primitives
Each graphical object defined in OpenGL is composed by graphical primitives.
Each primitive is described by its type, position and properties.
Properties are given by the state of OpenGL

11. Basic primitives - example1
glBegin(GL_LINES);//line
glVertex2d(0,240);//coordinates
glVertex2d(480,240);
glVertex2d(240,0);
glVertex2d(240,480);
glEnd();

12. Basic primitives - example 2
glColor4f(0.0,0.0,1.0,0.0);//new color
glBegin(GL_TRIANGLES);//first triangle
glVertex2d(100,100);
glVertex2d(100,300);
glVertex2d(300,100);
glEnd();

13. Graphical primitives GL_POINTS individual points
GL_LINES pairs of vertices as individual line segments
GL_LINE_STRIP series of connected line segments
GL_LINE_LOOP closed series of connected line segments
GL_TRIANGLES series of triangles
GL_TRIANGLE_STRIP linked strip of triangles
GL_TRIANGLE_FAN linked fan of triangles
GL_QUADS quadruples interpreted as 4-gons
GL_QUAD_STRIP linked list of 4-gons
GL_POLYGON simple convex polygon

14. Including graphical libraries
#include <GL/glut.h>
Prefixes gl – basic graphical functions glu, glut – graphical utilities
(glut – graphical utilities toolbox)

15. First program in OpenGL
We draw too line segments in a square window
Line segments are of different color and different thickness.

16. First program in OpenGL
void MyInit(void) {
glClearColor(1.,1.,1.,.0);//white color of background
glMatrixMode(GL_PROJECTION);//initialization of projection matrix
glLineWidth(1.0);//initial width of lines
glLoadIdentity();
gluOrtho2D(0,480,0,480);//size of world window }

17. First program in OpenGL
void MyDisplay(void) {
glClear(GL_COLOR_BUFFER_BIT);//clear background
glColor4f(0.0,0.0,1.0,0.0);//blue color of first line
glBegin(GL_LINES);//first line
glVertex2d(480,0);//coordinates
glVertex2d(0,240);
glEnd();
glLineWidth(2.0);//width of second line
glColor4f(1.0,0.0,0.0,0.0);//red color of second line
glBegin(GL_LINES);
glVertex2d(0,0);
glVertex2d(480,480);
glFlush();//draw all primitives }

18. First program in OpenGL
int main() {
glutInitWindowSize(480,480);// size o initial window
glutCreateWindow("My first attemt");//crete default window
glutDisplayFunc(MyDisplay);//register user display function
MyInit();
glutMainLoop();
return 0; }

19. Drawing triangles The first triangle is filled with a default color
The second triangle has different colors of vertices. The fill color is an interpolation of vertex-colors.

20. Definition of primitives-triangles
glColor4f(0.0,0.0,1.0,0.0);//blue color of a triangle
glBegin(GL_TRIANGLES);//first triangle
glVertex2d(100,100);//coordinates of 1.triangle
glVertex2d(100,300);
glVertex2d(300,100);
glColor4f(1.0,1.0,0.0,0.0);//changing color
glVertex2d(300,300); //2.triangle
glColor4f(0.0,1.0,0.0,0.0);//changing color
glVertex2d(200,400);
glColor4f(1.0,.0,0.0,0.0);//changing color
glVertex2d(50,50);
glEnd(); }

21. Drawing set of points Sierpinski gasket
Choose 3 points T0,T1,T2 in the plane
Choose one of points p0 in random
Choose one of points T0,T1,T2 , T in random
Construct point pk as midpoint of pk-1 and T
Draw point pk.

22. Implementation of Sierpinski gasket
class GLintPoint //definition of a class point {
public:
int x, y; };
void drawDot(GLintPoint p) //drawing a point
glBegin(GL_POINTS);
glVertex2i(p.x,p.y);
glEnd(); }

23. Implementation of Sierpinski gasket
void Sierpinski(void) {
int index=1;
GLintPoint T[3]={{100,100},{500,100},{300,400}};
GLintPoint point=T[index];
glClear(GL_COLOR_BUFFER_BIT);
drawDot(point);
for (int i=0; i<10000; i++)
index=rand()%3;
point.x=(point.x+T[index].x)/2;
point.y=(point.y+T[index].y)/2; }
glFlush();

24. Drawing from the file void DrawDino(void) {
inStream.open("dino1.dat");//data of dinosaurus
if(inStream.fail())
return;
while(inStream)
inStream>>numLines;
glBegin(GL_LINE_STRIP);//different graphical primitives
for(int i=0; i<numLines; i++)
inStream>>x>>y;
glVertex2i(x,y); }
glEnd();

25. Drawing from the file Exaple of data file: 21 29 32 435 10 439 4 438
2 433
4 428
6 425
10 420
15 416
21 413
30 408
42 406
47 403
56 398
63 391
71 383
…………..
………………

26. Graph of a function float func(float t) { float value;
value=exp(-t)* cos(2* *t);
return value;
} // y=e-t.cos(2πt)

27. Graph of a function
Real values of coordinates does not fit to integer coordinates of the window
Necessary transformation of coordinates
point.x=A*x+B ;
point.y=C*func(x) +D;
glVertex2d(point.x,point.y);
Problem : how to find parameters A,B,C,D ?

28. Graph of a function Possible solution: A=screenWidth/4.0;
B=screenWidth/2.0;
C=screenHeight/6.0;
D=screenHeight/2.0