CG 2 - ROTATE A TRIANGLE

2. Create and rotate a triangle about the origin and a fixed point.

#include <stdio.h>
#include <GL/glut.h>

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glTranslatef( -0.14 , 0.62 , 0.0);
    glRotatef(90 , 0 , 0 , 1);
    glBegin( GL_TRIANGLES );
    glColor3f( 0 , 1 , 0);
    glVertex3f( -0.3 , -0.2 , 0 );
    glVertex3f(  0.3 , -0.2 , 0 );
    glVertex3f(  0 ,  0.3 , 0 );
    glEnd();
    glLoadIdentity();
    glTranslatef(0 , -0.3 , 0 );
    glBegin( GL_TRIANGLES );
    glColor3f( 1 , 0 , 0);
    glVertex3f( -0.6 , -0.5 , 0 );
    glVertex3f(  0.6 , -0.5 , 0 );
    glVertex3f(  0 , 0.5 , 0 );
    glEnd();
    glutSwapBuffers();
}

void intitOpenGL()
{
    glClearColor( 4, 5, 6, 7 );
}

void reshape (int w , int h)
{
    glViewport( 0 , 0 , w , h );
}

int main (int argc, char ** argv)
{
    glutInit (&argc, argv);
    glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
    glutInitWindowPosition (100, 100);
    glutInitWindowSize (500, 500);
    glutCreateWindow ("My First Program");
    intitOpenGL ( );
    glutDisplayFunc (display);
    glutReshapeFunc (reshape);
    glutMainLoop ( );
    return 0;

}

OUTPUT :

( click on image to zoom )

ALTERNATIVE SOLUTION :

#include<stdio.h>
#include<GL/glut.h>
int x,y;
int where_to_rotate=0;
float translate_x=0.0,translate_y=0.0,rotate_angle=0.0;
void draw_pixel(float x1,float y1)
{
glPointSize(5.0);
glBegin(GL_POINTS);
glVertex2f(x1,y1);
glEnd();
}
void triangle(int x,int y)
{
glColor3f(0.0,1.0,0.0); // set interior color of triangle to green
glBegin(GL_POLYGON);
glVertex2f(x,y);
glVertex2f(x+400,y+400);
glVertex2f(x+300,y+0);
glEnd();
glFlush();
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glColor3f(1.0,0.0,0.0); //color of point
draw_pixel(0.0,0.0);
if(where_to_rotate==1)
{
translate_x=0.0;
translate_y=0.0;
rotate_angle+=0.9;
}
if(where_to_rotate==2)
{
translate_x=x;
translate_y=y;
rotate_angle+=0.9;
glColor3f(0.0,0.0,1.0);
draw_pixel(x,y);
}
glTranslatef(translate_x,translate_y,0.0);
glRotatef(rotate_angle,0.0,0.0,1.0);
glTranslatef(-translate_x,-translate_y,0.0);
triangle(translate_x,translate_y);
glutPostRedisplay();
glutSwapBuffers();
}
void myInit()
{
glClearColor(1.0,1.0,1.0,1.0); //background color to white
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-800.0,800.0,-800.0,800.0);
glMatrixMode(GL_MODELVIEW);
}
void rotate_menu(int option)
{
if(option==1)
where_to_rotate=1;
if(option==2)
where_to_rotate=2;
if(option==3)
where_to_rotate=3;
display();
}
int main(int argc,char **argv)
{
printf("\nEnter fixed points for rotation (x,y) : ");
scanf("%d%d",&x,&y);
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB);
glutInitWindowSize(800,800);
glutInitWindowPosition(0,0);
glutCreateWindow("Rotate Created Triangle");
myInit();
glutDisplayFunc(display);
glutCreateMenu(rotate_menu);
glutAddMenuEntry("Rotate Around Origin",1);
glutAddMenuEntry("Rotate Around Fixed Points",2);
glutAddMenuEntry("Stop Rotation",3);
glutAttachMenu(GLUT_RIGHT_BUTTON);
glutMainLoop();

}

OUTPUT :
( click on image to zoom )