Reversing or Mimicking Transformations

The geometric processing pipeline is very good at using viewing and projection matrices and a viewport for clipping to transform the world (or object) coordinates of a vertex into window (or screen) coordinates. However, there are situations in which you want to reverse that process. A common situation is when an application user utilizes the mouse to choose a location in three dimensions. The mouse returns only a two-dimensional value, which is the screen location of the cursor. Therefore, the application will have to reverse the transformation process to determine from where in three-dimensional space this screen location originated.

The Utility Library routine gluUnProject() performs this reversal of the transformations. Given the three-dimensional window coordinates for a location and all the transformations that affected them, gluUnProject() returns the world coordinates from where it originated.

int gluUnProject(GLdouble winx, GLdouble winy, GLdouble winz, const GLdouble modelMatrix[16], const GLdouble projMatrix[16], const GLint viewport[4], GLdouble *objx, GLdouble *objy, GLdouble *objz);

Map the specified window coordinates (winx, winy, winz) into object coordinates, using transformations defined by a modelview matrix (modelMatrix), projection matrix (projMatrix), and viewport (viewport). The resulting object coordinates are returned in objx, objy, and objz. The function returns GL_TRUE, indicating success, or GL_FALSE, indicating failure (such as an noninvertible matrix). This operation does not attempt to clip the coordinates to the viewport or eliminate depth values that fall outside of glDepthRange().

There are inherent difficulties in trying to reverse the transformation process. A two-dimensional screen location could have originated from anywhere on an entire line in three-dimensional space. To disambiguate the result, gluUnProject() requires that a window depth coordinate (winz) be provided and that winz be specified in terms of glDepthRange(). For the default values of glDepthRange(), winz at 0.0 will request the world coordinates of the transformed point at the near clipping plane, while winz at 1.0 will request the point at the far clipping plane.

Example 3-8 demonstrates gluUnProject() by reading the mouse position and determining the three-dimensional points at the near and far clipping planes from which it was transformed. The computed world coordinates are printed to standard output, but the rendered window itself is just black.

Example 3-8 : Reversing the Geometric Processing Pipeline: unproject.c

#include <GL/gl.h>

#include <GL/glu.h>

#include <GL/glut.h>

#include <stdlib.h>

#include <stdio.h>

void display(void)

{

   glClear(GL_COLOR_BUFFER_BIT);

   glFlush();

}

void reshape(int w, int h)

{

   glViewport (0, 0, (GLsizei) w, (GLsizei) h);

   glMatrixMode(GL_PROJECTION);

   glLoadIdentity();

   gluPerspective (45.0, (GLfloat) w/(GLfloat) h, 1.0, 100.0);

   glMatrixMode(GL_MODELVIEW);

   glLoadIdentity();

}

void mouse(int button, int state, int x, int y)

{

   GLint viewport[4];

   GLdouble mvmatrix[16], projmatrix[16];

   GLint realy;  /*  OpenGL y coordinate position  */

   GLdouble wx, wy, wz;  /*  returned world x, y, z coords  */

   switch (button) {

      case GLUT_LEFT_BUTTON:

         if (state == GLUT_DOWN) {

            glGetIntegerv (GL_VIEWPORT, viewport);

            glGetDoublev (GL_MODELVIEW_MATRIX, mvmatrix);

            glGetDoublev (GL_PROJECTION_MATRIX, projmatrix);

/*  note viewport[3] is height of window in pixels  */

            realy = viewport[3] - (GLint) y - 1;

            printf ("Coordinates at cursor are (%4d, %4d)\n",

               x, realy);

            gluUnProject ((GLdouble) x, (GLdouble) realy, 0.0,

               mvmatrix, projmatrix, viewport, &wx, &wy, &wz);

            printf ("World coords at z=0.0 are (%f, %f, %f)\n",

               wx, wy, wz);

            gluUnProject ((GLdouble) x, (GLdouble) realy, 1.0,

               mvmatrix, projmatrix, viewport, &wx, &wy, &wz);

            printf ("World coords at z=1.0 are (%f, %f, %f)\n",    

               wx, wy, wz);

         }

         break;

      case GLUT_RIGHT_BUTTON:

         if (state == GLUT_DOWN)

            exit(0);

         break;

      default:

         break;

   }

}

int main(int argc, char** argv)

{

   glutInit(&argc, argv);

   glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);

   glutInitWindowSize (500, 500);

   glutInitWindowPosition (100, 100);

   glutCreateWindow (argv[0]);

   glutDisplayFunc(display);

   glutReshapeFunc(reshape);

   glutMouseFunc(mouse);

   glutMainLoop();

   return 0;

}

gluProject() is another Utility Library routine, which is related to gluUnProject().gluProject() mimics the actions of the transformation pipeline. Given three-dimensional world coordinates and all the transformations that affect them, gluProject() returns the transformed window coordinates.

int gluProject(GLdouble objx, GLdouble objy, GLdouble objz, const GLdouble modelMatrix[16], const GLdouble projMatrix[16], const GLint viewport[4], GLdouble *winx, GLdouble *winy, GLdouble *winz);

Map the specified object coordinates (objx, objy, objz) into window coordinates, using transformations defined by a modelview matrix (modelMatrix), projection matrix (projMatrix), and viewport (viewport). The resulting window coordinates are returned in winx, winy, and winz. The function returns GL_TRUE, indicating success, or GL_FALSE, indicating failure.