r

#include<GL/glut.h>
float a=0;
float angle=0;
float angleOld=0;
float angleNew=90;
void display()
{
 glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 glPushMatrix();
  glRotatef(angle,0,0,1);
  glutSolidTeapot(0.3);
 glPopMatrix();
 glutSwapBuffers();
}

void keyboard(unsigned char key, int x, int y)
{
 if (key=='r'){
  a+=0.1;
  angle = angleOld*(1-a) + angleNew*a;
 }
 glutPostRedisplay();
}

int main()
{
 glutInitDisplayMode(GLUT_DOUBLE | GLUT_DEPTH);
 glutCreateWindow("week15");

 glutKeyboardFunc(keyboard);
 glutDisplayFunc(display);
 glutMainLoop();
}