1. ModelView Matrix:
   • From object coordinates to eye coordinates
   • glMatrixMode(GL_MODELVIEW);
2. Projection Matrix
   • From eye coordinates to clip coordinates
   • glMatrixMode(GL_PROJECTION);

• The Viewing Transform:
  • Positions the viewing volume in the world
  • Usually set using gluLookAt()
• The Modeling Transform
  • "Positions" the model in the world
  • Usually set using glTranslate*(), glScale*(), glRotate*()

• Orthographic:
  • glOrtho(left, right, bottom, top, near, far)
• Perspective:
  • glFrustum(fovy, aspect, zNear, zFar)
  • where fovy is the field of view (in degrees) in the \(y\) direction, aspect is the aspect ration that determines the FOV in the \(x\) direction, zNear is the positive distance from the viewer to the near clipping plane, and zFar is the positive distance from the viewer to the far clipping plane

• Two Observations:
  • Thus far we have only worked with one object at a time
  • Working with multiple objects requires multiple transformations
• A Warning:
  • The order of the transformations is important and requires careful thought
  • Management of the stack of transformations (using glPushMatrix() and glPopMatrix()) is important and requires careful thought

openglexamples/transforms/solarsystem.c (Fragment: display)

/**
 * The display callback
 */
void display() {
  glClear(GL_COLOR_BUFFER_BIT);

  glPushMatrix();

  // Draw the Sun at the origin with a radius of 2, 16 lines of latitude,
  // and 16 lines of longitude
  //
  // Use a wireframe so we can see the orientation
  glColor3f(1.0, 1.0, 0.0);
  glutWireSphere(1.0, 16, 16);

  // Rotate the local coordinate system
  // around the z-axis based on the day of the year
  //
  // Note: After drawing the sphere we are at the origin. So, we
  // are rotating around a ray from [0 0 0] to [0 0 1].
  glRotatef(((GLfloat) day) / 365.0f * 360.0f, 0.0, 0.0, 1.0);

  // Translate the local coordinate system along the x-axis the
  // radius of the Earth's orbit
  //
  // Note: The x-axis has already been rotated.
  glTranslatef(4.0, 0.0, 0.0);

  // To rotate the Moon around the center of the Earth we need
  // a different local coordinate system. Save the current
  // local coordinate system first
  glPushMatrix();

  // Rotate the new local coordinate system around the z-axis
  // based on the day of month
  glRotatef(((GLfloat) dom) / 28.0f * 360.0f, 0.0, 0.0, 1.0);

  // Translate the new local coordinate along the x-axis the
  // radius of the Moon's orbit
  glTranslatef(1.0, 0.0, 0.0);

  // Draw the Moon
  glColor3f(1.0, 1.0, 1.0);
  glutWireSphere(0.1, 4, 4);

  // Pop the previous local coordinate system
  glPopMatrix();

  // Rotate the local coordinate system around the z-axis based on
  // the hour of the day
  //
  // Note: The origin is now at the appropriate point on the orbit.
  glRotatef(((GLfloat) hour) / 24.0f * 360.0f, 0.0, 0.0, 1.0);

  // Draw the Earth
  //
  // Use a wireframe so we can see it revolve
  glColor3f(0.0, 0.0, 1.0);
  glutWireSphere(0.5, 16, 16);

  glPopMatrix();

  glFlush();
  glutSwapBuffers();
}
        

openglexamples/transforms/hinges.c (Fragment: display)

/**
 * The display callback
 */
void display() {
  glClear(GL_COLOR_BUFFER_BIT);

  glPushMatrix();

  // Translate the local coordinate system to the hinge
  // of the first block
  glTranslatef(-1.0, 0.0, 0.0);

  // Rotate the local coordinate system around the z-axis
  //
  // Note: We are rotating around a ray to [0 0 1].
  glRotatef((GLfloat) alpha, 0.0, 0.0, 1.0);

  // Translate the local coordinate system back to the center
  glTranslatef(1.0, 0.0, 0.0);

  // Draw the first block
  glutWireCube(2.0);

  // Translate the local coordinate system to the hinge
  // of the second block
  glTranslatef(1.0, 0.0, 0.0);

  // Rotate the local coordinate system around the z-axis
  //
  // Note: We are rotating around a ray to [0 0 1].
  glRotatef((GLfloat) beta, 0.0, 0.0, 1.0);

  // Translate the local coordinate system to the center of the
  // second block
  glTranslatef(1.0, 0.0, 0.0);

  // Draw the second block
  glutWireCube(2.0);

  glPopMatrix();

  glFlush();
  glutSwapBuffers();
}
        

openglexamples/transforms/firstperson.c

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

/**
 * \file
 * An application that displays a first person view of a simple world.
 *
 * On Linux, compile with:
 *   g++ -o firstperson -Wall firstperson.c -lglut -lGL -lGLU
 *
 * NOTE: The camera/viewer starts in the middle of the world. So, you should
 * probably move back several times to see everything.
 * 
 * @author  Prof. David Bernstein, James Madison University
 */

static GLfloat x = 0.0, z = 0.0, angle = 0.0;

/**
 * Convert degrees to radians.
 *
 * @param degrees   The angle in degrees
 * @return          The angle in radians
 */
inline GLfloat deg2rad(GLfloat degrees) {
  return degrees * M_PI / 180.0;
}

/**
 * Convert radians to degrees
 *
 * @param radians   The angle in radians
 * @return          The angle in degrees
 */
inline GLfloat rad2deg(GLfloat radians) {
  return radians * (180.0 / M_PI);
}

/**
 * Draw the objects in the world.
 */
void drawWorld() {
  glColor3f(1.0f, 1.0f, 1.0f);

  // The "near" side of the world
  glPushMatrix();
  glTranslatef(0.0, 0.0, 5.0);
  glRotatef(-90.0, 1.0, 0.0, 0.0);
  glutSolidCone(1.0, 1.0, 10, 10);
  glPopMatrix();

  // The "right" side of the world
  glPushMatrix();
  glTranslatef(5.0, 0.0, 0.0);
  glutSolidTorus(0.5, 1.0, 10, 10);
  glPopMatrix();

  // The "left" side of the world
  glPushMatrix();
  glTranslatef(-5.0, 0.0, 0.0);
  glutSolidSphere(1.0, 10, 10);
  glPopMatrix();

  // The "far" side of the world
  glPushMatrix();
  glTranslatef(0.0, 0.0, -5.0);
  glutSolidTeapot(1.0);
  glPopMatrix();
}

/**
 * The display callback.
 */
void display() {
  glMatrixMode(GL_MODELVIEW);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  // Reset the local coordinate system
  glLoadIdentity();

  // Rotate around the y-axis
  glRotatef(angle, 0.0, 1.0, 0.0);

  // Translate the local coordinate system "backwards" an
  // appropriate amount (based on the rotation angle)
  glTranslatef(-x, 0.0, -z);

  // Draw the world
  drawWorld();

  glFlush();
  glutSwapBuffers();
}

/**
 * Initialize GLUT and OpenGL.
 */
void init() {
  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
  glutInitWindowSize(500, 500);
  glutCreateWindow("First Person");
  glClearColor(0.0, 0.0, 0.0, 1.0);

  glShadeModel(GL_SMOOTH);
  glEnable(GL_DEPTH_TEST);
  glEnable(GL_LIGHTING);
  glEnable(GL_LIGHT0);  // Use the default light
}

/**
 * Handle keyboard events.
 */
void keyboard(unsigned char key, int xMouse, int yMouse) {
  GLfloat deltaX, deltaZ;

  // Move forward or backward
  if ((key == 'f') || (key == 'b')) {
    // Calculate the change in x and z necessary for a 1-unit
    // move in the direction I am facing
    deltaX = sin(deg2rad(angle));
    deltaZ = cos(deg2rad(angle));

    if (key == 'f') {
      x += deltaX;
      z -= deltaZ;
    } else if (key == 'b') {
      x -= deltaX;
      z += deltaZ;
    }
  }
  // Rotate right (clockwise) or left (counter-clockwise)
  else if (key == 'r') {
    angle += 10.0;
    if (angle > 360)
      angle -= 360;
  } else if (key == 'l') {
    angle -= 10.0;
    if (angle < 0)
      angle += 360;
  }
  // Reset
  else if (key == ' ') {
    angle = 0.0;
    x = 0.0;
    z = 0.0;
  }

  glutPostRedisplay();
}

/**
 * The reshape callback (i.e., the function that is called
 * each time the window is re-sized).
 *
 * @param width   The new width
 * @param height  The new height
 */
void reshape(int width, int height) {
  GLfloat aspect;

  glViewport(0, 0, (GLsizei) width, (GLsizei) height);

  // Setup the projection matrix
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  aspect = (GLfloat) width / (GLfloat) height;
  gluPerspective(60.0, aspect, 1.0, 1000.0);
}

/**
 * The entry point of the application.
 *
 * @param argc  The number of command line arguments
 * @param argv  The array of command line arguments
 * @return      A status code
 */
int main(int argc, char **argv) {
  // Initialize
  glutInit(&argc, argv);
  init();

  // Callbacks
  glutReshapeFunc(reshape);
  glutDisplayFunc(display);
  glutKeyboardFunc(keyboard);

  // Start the event loop
  glutMainLoop();

  return 0;
}