/////////////////////////////////////////////////////////////
// shear.cpp
// This program animates a box sheared on impact by a ball.
// The shear matrix is explicitly computed.
// 
// Interaction: 
// Press the left and right arrow key to move the ball.
// Press r to reset.
//
// Sumanta Guha
/////////////////////////////////////////////////////////////

#include <iostream>

#ifdef __APPLE__
#  include <GLUT/glut.h>
#else
#  include <GL/glut.h>
#endif

using namespace std;

/* Globals */
static float dist = 0.0; // Distance traveled by ball.
static float boxShearLength = 0.0; // Length of shear at top of box.
static float matrixdata[16] = // Shear matrix.
{
   1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0
};

void setup(void) 
{
   glClearColor(1.0, 1.0, 1.0, 0.0); 
}

void drawScene(void)
{
   glClear (GL_COLOR_BUFFER_BIT);
   glLoadIdentity ();          
   gluLookAt (0.0, 0.0, 30.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);

   glColor3f(0.0, 0.0, 0.0);

   // Moving ball.
   glPushMatrix();
   glTranslatef(-20.0 + dist, 8.0, 0.0);
   glutWireSphere(1.0, 10, 8);
   glPopMatrix();

   // Box.
   // Translate box (after contact with ball) to compensate for the shear -
   // so that the base of the box remains stationary.
   if (dist >= 17.0) glTranslatef(boxShearLength, 0.0, 0.0); 

   glMultMatrixf(matrixdata); // Apply the shear matrix.
   glScalef(4.0, 16.0, 4.0);
   glutWireCube (1.0);

   glutSwapBuffers();
}

void resize(int w, int h)
{
   glViewport(0, 0, (GLsizei) w, (GLsizei) h); 
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   glFrustum(-5.0, 5.0, -5.0, 5.0, 5.0, 100.0);

   glMatrixMode(GL_MODELVIEW);
}

void keyInput(unsigned char key, int x, int y)
{
   switch (key) 
   {
	  case 'r':
		 dist = 0.0;
		 boxShearLength = 0.0;
		 matrixdata[4] = 0.0;
		 glutPostRedisplay();
		 break;
      case 27:
         exit(0);
         break;
      default:
         break;
   }
}

void specialKeyInput(int key, int x, int y)
{
   if(key == GLUT_KEY_RIGHT) dist += 1.0;
   if(key == GLUT_KEY_LEFT) dist -= 1.0;
   if (dist >= 17.0)
   {
      boxShearLength = (dist - 17.0)/2; 
	  matrixdata[4] = boxShearLength/8.0; // Tan of the angle of shear = 
	                                      // boxShearLength/y-value of the ball.
   }
   glutPostRedisplay();
}

void printInteraction(void)
{
   cout << "Interaction:" << endl;
   cout << "Press the left and right arrow key to move the ball." << endl
        << "Press r to reset." << endl;
}

int main(int argc, char **argv) 
{
   printInteraction();
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB); 
   glutInitWindowSize(500, 500);
   glutInitWindowPosition(100, 100); 
   glutCreateWindow("shear.cpp");
   setup(); 
   glutDisplayFunc(drawScene); 
   glutReshapeFunc(resize);  
   glutKeyboardFunc(keyInput);
   glutSpecialFunc(specialKeyInput);
   glutMainLoop(); 

   return 0;  
}