Question

I am trying to find the screen coordinates from the opengl coordinates(projected in 3D space) I have used glproject call for this purpose,i have used rotation and translation to in my code

At certain point after performing some transformation, i called glproject api to get the screen coordinate of a particular projected point P(x,y,z)

glproject(x,y,z,modelMatrix,projectionmatrix,viewport,*x_s,*y_s,*z_s);

I am able to get x screen coordinate correctly in x_s , but y coordinates are different

The only change in y which is not in x is when initially i called the glperspective to set fovy(The field of view angle, in degrees, in the y-direction). gluPerspective(60.0f, Width/Height,0.0001f,1000.0f);

Let me Rephrase the question I have created a 3D point on screen and now i am getting the 2d(x,y) coordinates of that point through GLproject they come different from the mouse coordinates What could be the possible solution to get correct coordinates.

Here is the code snippet

#include<GL/glut.h>     /* Header File For The GLUT Library */
GLint Window;              /* The number of our GLUT window */
float tmp_x,tmp_y,tmp_z;
GLfloat w = 1200;         /* Window size. Global for use in rotation routine */
GLfloat h = 1200;
GLint prevx, prevy;        /* Remember previous x and y positions */
GLfloat xt=1.0,yt=1.0,zt=1.0;   /* translate */
int width = 1600;
int height = 1200;

//This function will set windowing transformation
void transform(GLfloat Width , GLfloat Height )
{
    glViewport(0,0,  (GLfloat)Width,  (GLfloat)Height);
    glPushMatrix();
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(60.0f, Width/Height,0.0001f,1000.0f); 
    glTranslatef(0.0, 0.0, -15.0f);     /* Centre and away the viewer */
    glMatrixMode(GL_MODELVIEW);
    glPopMatrix();
}


GLvoid draw_room() 
{   
int i;
glPushMatrix();
glShadeModel(GL_SMOOTH); 
glLineWidth(1.0);                             
glPointSize(4.0);                             /* Add point size, to make it clear */
glBegin(GL_POINTS);                /* start drawing the cube.*/
glColor3f(0.0f,0.0f,1.0f);            /* Set The Color To Orange*/
glColor3f(1.0f,0.0f,0.0f);            /* Set The Color To Orange*/
glVertex3f(3.1f,2.1f,2.1f); 
glEnd();                              /* Done Drawing The Cube*/
glEnable(GL_DEPTH_TEST); 
}
//OpenGl Display  callback Function It call init room
void DrawGLScene()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 
    printf("%f %f %f\n",xt,yt,zt);
    glPushMatrix();
    glMatrixMode(GL_PROJECTION);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(xt,yt, zt);
    draw_room();
    glPopMatrix();
    glutSwapBuffers();               /* Swap buffers */
    glFlush();
}

GLvoid Mouse( int b , int s, int xx, int yy)
{

    double a1,a2,a3;    
    GLint viewport[4];
    GLdouble modelview[16];
    GLdouble projection[16]; 
    glGetIntegerv(GL_VIEWPORT, viewport);  
    glGetDoublev(GL_MODELVIEW_MATRIX, modelview);   
    glGetDoublev(GL_PROJECTION_MATRIX, projection);  
    //gluProject(xt+3.1f,yt+2.1f,zt+2.1f, modelview, projection, viewport, &a1, &a2, &a3);
    gluProject(3.1f,2.1f,2.1f, modelview, projection, viewport, &a1, &a2, &a3);

    printf("Mouse: %d %d\n",xx,yy);                // Both Print are giving different coordinaes.
    printf("Unproject %f %f %f\n",a1,a2,a3);    
    switch (b) {
    case GLUT_LEFT_BUTTON:  /* only stash away for left mouse */
        prevx = xx - w/2;
        prevy = h/2 - yy;
        break;
    case GLUT_MIDDLE_BUTTON:
        break;
    case GLUT_RIGHT_BUTTON:
        break;
    }
}

What could be the possible solution for this?

Answer 1

OpenGL uses lower left corner of screen as coordinate system orign. Window system usually uses upper left corner as coordinate system orign. You need to handle this difference. Example:

printf("Mouse: %d %d\n",xx,yy);                
printf("Unproject %f %f %f\n",a1,viewport[1]-a2,a3);