Converting 3D coordinate to screen coordinate in OpenGL - c++

Question

I am facing a problem with converting 3D coordinate to screen coordinate. I have to convert the center of sphere to the screen coordinate. My code is here:
// OpenGL problem
#ifdef __APPLE__
# include <GLUT/glut.h>
#else
# include <GL/glut.h>
#endif
void passive(int,int);
void reshape(int,int);
void init(void);
void display(void);
void camera(void);
int cursorX,cursorY,width,height;
double centerX,centerY,centerZ;
//GLfloat modelviewMatrix[16],projectionMatrix[16];
GLdouble modelviewMatrix[16],projectionMatrix[16];
GLint viewport[4];
int main (int argc,char **argv) {
glutInit (&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_DEPTH | GLUT_RGBA);
glutInitWindowSize(1364,689);
glutInitWindowPosition(0,0);
glutCreateWindow("Sample");
init();
glutDisplayFunc(display);
glutIdleFunc(display);
glutPassiveMotionFunc(passive);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}
void display() {
glClearColor (0.0,0.0,0.0,1.0);
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// Render 3D content
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60,(GLfloat)width/(GLfloat)height,1.0,100.0); // create 3D perspective projection matrix
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
camera();
glTranslatef(-6,-2,0);
glColor3f(1,0,0);
glutSolidSphere(5,50,50);
glPopMatrix();
glGetDoublev(GL_MODELVIEW_MATRIX, modelviewMatrix);
glGetDoublev(GL_PROJECTION_MATRIX, projectionMatrix);
glGetIntegerv(GL_VIEWPORT, viewport);
// get 3D coordinates based on window coordinates
gluProject(-6, -2, 0, modelviewMatrix, projectionMatrix, viewport, &centerX, &centerY, &centerZ);
// Render 2D content
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, width,height, 0); // create 2D orthographic projection matrix
glMatrixMode(GL_MODELVIEW);
glColor3f(1,1,1);
glBegin(GL_LINES);
glVertex2f( centerX,centerY ); // coordinate of center of the sphere in orthographic projection
glVertex2f( cursorX,cursorY );
glEnd();
glutSwapBuffers();
}
void camera(void) {
glRotatef(0.0,1.0,0.0,0.0);
glRotatef(0.0,0.0,1.0,0.0);
glTranslated(0,0,-20);
}
void init(void) {
glEnable (GL_DEPTH_TEST);
glEnable (GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_COLOR_MATERIAL);
}
void reshape(int w, int h) {
width=w; height=h;
}
void passive(int x1,int y1) {
cursorX=x1; cursorY=y1;
}
In my code the centerX and centerY coordinates are going out of the screen. I have to draw a line between the center of the sphere and the mouse pointer. Any modification to the code to get it working properly?

Answer 1

Usually the function I use to switch between 3D / 2D coordinate. See if it works for you.
void toggle2DMode(bool flag, GLint Width, GLint Height) const
{
if(flag)
{
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0.0, Width, Height, 0.0, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glDisable(GL_DEPTH_TEST);
glEnable(GL_BLEND);
}
else
{
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
glEnable(GL_DEPTH_TEST);
glDisable(GL_BLEND);
}
}

Answer 2

my function for checking resolution of a quad
void WINDOW_TEXT::calcResolution( GLshort & x , GLshort & y )
{
GLdouble tx, ty, ttx, tty, z; // target 2d coords (z is 'unused')
GLdouble model_view[16];
glGetDoublev(GL_MODELVIEW_MATRIX, model_view);
GLdouble projection[16];
glGetDoublev(GL_PROJECTION_MATRIX, projection);
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
// get window coords based on 3D coordinates
gluProject( up_right.x, up_right.y, up_right.z,
model_view, projection, viewport,
&tx, &ty, &z);
gluProject( down_left.x, down_left.y, down_left.z,
model_view, projection, viewport,
&ttx, &tty, &z);
x = (GLshort)(tx-ttx);
y = (GLshort)(tty-ty);
};
"z" is ignored and you get pure resolution stored in x, and y