2D Rotation Without Using OpenGL Function For Rotation

Program to perform 2D rotation without using OpenGL function for rotation

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

int x, y, p, q, xa, ya, ra, i, a[10], b[10], da, db;
float dx, dy, theta;

void line(int x, int y, int a, int b)
{
  glVertex2d(x, y);
  glVertex2d(a, b);
}

void display_rectangle()
{
  for(i = 0; i < 4; i++)
  {
    if(i != 3)
      line(a[i], b[i], a[i+1], b[i+1]);
    else
      line(a[i], b[i], a[0], b[0]);
  }
}

void Init()
{
  /* Set clear color to white */
  glClearColor(1.0, 1.0, 1.0, 0);
  /* Set fill color to black */
  glColor3f(0.0, 0.0, 0.0);
  gluOrtho2D(0 , 640 , 0 , 480);
}

void rotate()
{
  // Find the vertices of the rotated rectangle
  theta = (float)(ra*(3.14/180));
  for(i = 0;i<4;i++)
  {
    a[i] = (xa + ((a[i] - xa)*cos(theta) - (b[i] - ya)*sin(theta)));
    b[i] = (ya + ((a[i] - xa)*sin(theta) + (b[i] - ya)*cos(theta)));
  }
}

void transform()
{
  glClear(GL_COLOR_BUFFER_BIT);

  glBegin(GL_LINES);
  // Find the original rectangle vertices
  da = p - x; db = q - y;
  a[0] = x; b[0] = y;
  a[1] = x + da; b[1] = y;
  a[2] = x + da; b[2] = y + db;
  a[3] = x; b[3] = y + db;

  // Draw original rectangle
  display_rectangle();

  rotate();
  
  // Draw rotated rectangle
  display_rectangle();
  
  glEnd();

  glFlush();

}

void main(int argc, char **argv)
{
  printf("\n**********************************************\n");
  printf("Enter the upper left corner of the rectangle:\n");
  scanf("%d%d", &x, &y);
  printf("\n**********************************************\n");
  printf("Enter the lower right corner of the rectangle:\n");
  scanf("%d%d", &p, &q);

  printf("\n********************Rotation********************\n");
  printf("Enter the value of fixed point and angle of rotation:\n");
  scanf("%d%d%d", &xa, &ya, &ra);

  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  glutInitWindowPosition(0, 0);
  glutInitWindowSize(640, 480);
  glutCreateWindow("Rotation");
  Init();
  glutDisplayFunc(transform);
  glutMainLoop();
}

Written by Munia Balayil