Question

I have written a simple OpenGL program in C++ that displays a line joining the center of the window to the current position of the mouse pointer.

My code is :

#ifdef __APPLE__
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif
#include<iostream>

using namespace std;

void passive(int,int);
void reshape(int,int);
void init(void);
void display(void);
void camera(void);

int x=3,y=3;

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(void) {
    glClearColor (0.0,0.0,0.0,1.0);
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();
    camera();

    glBegin(GL_LINES);
        glVertex3f(0,0,0);
        glVertex3f(x,y,0);
    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) {
    glViewport(0,0,(GLsizei)w,(GLsizei)h);
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity();
    gluPerspective(60,(GLfloat)w/(GLfloat)h,1.0,100.0);
    glMatrixMode(GL_MODELVIEW);
}

void passive(int x1,int y1) {
    x=x1; y=y1;
}

The problem I am facing is that the x and y values set in the passive() function is not correctly mapped into the screen which uses perspective projection. So the line drawn is joining the center to some other coordinate outside the screen. Any modifications to the code to get it working properly?

Answer 1

An easy way would be to create an orthographic projection matrix and then render all of your “2D” elements (including this line, using the screen coordinates provided by glutPassiveMotionFunc).

Something like this:

void display() {
    // clear
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective( ... ) // create 3D perspective projection matrix
    glMatrixMode(GL_MODELVIEW);

    // Render 3D content here

    // Render 2D content
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, width, height, 0); // create 2D orthographic projection matrix with coordinate system roughly equivalent to window position
    glMatrixMode(GL_MODELVIEW);

    glBegin(GL_LINES);
    glVertex2f( width / 2, height / 2 ); // now we can use "pixel coordinates"
    glVertex2f( cursorX, cursorY );
    glEnd();

    ...
}

Compare this to your modification of the perspective projection in your reshape method.

Obviously you’ll also want to disable states that don’t make sense for a “2D” rendering (like depth buffer checking, etc) but it should be pretty obvious. Take a look at this GDSE post for a discussion of how other people do this same task.