CODEWITHSUNDEEP
c++opengl3d2dglu
J.Baum
J.Baum

Reputation: 81

Weird Result with gluProject()

I have a problem with the gluProject Function (OpenGL). I would like to transform a simple 3D Point from the object space into the screen space. My code is the following:

int main(){
    GLdouble mvmatrix[16];
    GLdouble projmatrix[16];
    GLint viewport[4];
    GLdouble winx;
    GLdouble winy;
    GLdouble winz;
    glGetDoublev(GL_MODELVIEW_MATRIX, mvmatrix);
    glGetDoublev(GL_PROJECTION_MATRIX, projmatrix);
    glGetIntegerv(GL_VIEWPORT, viewport);
    gluProject(0.0, 0.0, 0.0, mvmatrix, projmatrix, viewport, &winx, &winy, &winz);
    std::cout << winx << winy;
    getchar();
    getchar();
    return 0;
}

The output is:

-1.71799e+009 -1.71799e+009

This is a weird result and does not make sense to me. Does anyone know what went wrong? I did not found anything online.

Upvotes: 1

Views: 144

Answers (1)

genpfault
genpfault

Reputation: 52082

Does anyone know what went wrong?

You haven't created a GL context and made it current before using glGetDoublev() and glGetIntegerv():

In order for any OpenGL commands to work, a context must be current; all OpenGL commands affect the state of whichever context is current. The current context is a thread-local variable, so a single process can have several threads, each of which has its own current context. However, a single context cannot be current in multiple threads at the same time.

You can use (Free)GLUT (among other things (not exhaustive)) to create a window & associated GL context:

#include <GL/glut.h>
#include <iostream>

int main(int argc, char **argv)
{
    // create context & make it current
    glutInit( &argc, argv );
    glutInitWindowSize( 200, 200 );
    glutInitDisplayMode( GLUT_RGBA );
    glutCreateWindow( "GLUT" );

    GLdouble mvmatrix[16];
    GLdouble projmatrix[16];
    GLint viewport[4];
    GLdouble winx;
    GLdouble winy;
    GLdouble winz;
    glGetDoublev( GL_MODELVIEW_MATRIX, mvmatrix );
    glGetDoublev( GL_PROJECTION_MATRIX, projmatrix );
    glGetIntegerv( GL_VIEWPORT, viewport );
    gluProject
        (
        0.0, 0.0, 0.0, 
        mvmatrix, projmatrix, viewport,
        &winx, &winy, &winz
        );
    std::cout << winx << " " << winy << std::endl;
    return 0;
}

Output:

100 100

Upvotes: 1

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