Setting a orthographic projection matrix?

Question

I am having trouble setting a orthographical projection matrix of dimensions 4x2 with (0,0) being in the center, (-2, -1) being at the bottom left corner, and (2, 1) being at the top right corner.

I use glutInitWindowSize(600, 300); to initialize the window size. In my reshape function, I use glViewport(0, 0, w, h); to set the viewport. Also in my reshape function, I use gluOrtho2D(-(float)w/h, (float)w/h, -2.0, 2.0); to set the ortho.

However, when I move my mouse around to see the world coordinates, the bottom left corner is (-1, -1) and the top right corner is (1, 1). Is there something I am doing wrong here? I am assuming since I am setting bottom and top to -2 and 2 respectively in the gluOrtho2D call, it should give me the right coordinates.

Please help me out if you see anything that might be wrong.

Here is my code so far, please ignore the arm variables and drawing functions.

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <iostream>

#if 0 /*unix*/
#include <GL/glut.h>
#endif

#if 1 /*apple */
#include <GLUT/glut.h>
#include <OPENGL/gl.h>
#include <OPENGL/glext.h>
#endif

#if 0 /*windows*/
#include <io.h>
#include <fcntl.h>
#include <glut.h>
#endif

int GW, GH;
bool animfore, animupper;
float foreangle, upperangle;

using namespace std;

void drawShoulder();
void drawUpper();
void drawFore();

void display() {
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();

    //draw shoulder
    glPushMatrix();
        drawShoulder();
        //draw upper arm
        glPushMatrix();
            drawUpper();
            //draw forearm
            glPushMatrix();
                drawFore();
            glPopMatrix();
        glPopMatrix();
    glPopMatrix();

    glutPostRedisplay();
    glutSwapBuffers();
}

void drawShoulder() {
    //cout << "Enters" << endl;

    glBegin(GL_POLYGON);
        glColor3f((float)30/255, (float)0/255, (float)30/255);
        glVertex2f(-2.0, -0.4);
        glVertex2f(-2.0, -1.0);
        glVertex2f(-1.0, -1.0);
        glVertex2f(-1.0, -0.4);
    glEnd();
}

void drawUpper() {

}

void drawFore() {

}

void reshape(GLsizei w, GLsizei h) {
    GW = w;
    GW = h;
    glViewport(0, 0, w, h);
    glLoadIdentity();
    gluOrtho2D(-(float)w/h, (float)w/h, -1.0, 1.0);
    cout << "Enters" << endl;
}

void keyboard(unsigned char key, int x, int y) {
    switch(key) {
        case 'q' : case 'Q' :
            exit(EXIT_SUCCESS);
            break;
    }
}

//control arm animations
void idle() {
    if(animupper) {

    }
    if(animfore) {

    }
}
float p2w_x(int gx) {
    return (float)2.*GW/(GW*GH-GH)*gx-(float)GW/GH;
}

float p2w_y(int gy) {
    int py = GH-1-gy;
    return (float)2./(GH-1.)*py-1;
}

void mouseMove(int x, int y) {
    cout << "(" << p2w_x(x) << "," << p2w_y(y) << ")" << endl;
}

int main(int argc, char **argv) {
    // global variable intializations
    GW = 600;
    GH = 300;
    animfore, animupper = true;

    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE);

    // initialization
    glutInitWindowSize(600, 300);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("Robot Arm");
    glClearColor(1.0, 1.0, 1.0, 1.0);

    // callback functions
    glutDisplayFunc(display);
    glutKeyboardFunc(keyboard);
    glutReshapeFunc(reshape);
    glutIdleFunc(idle);
    glutMotionFunc(mouseMove);

    glutMainLoop();
}

Answer 1

Use glMatrixMode(GL_PROJECTION) before setting any projection matrix, since each matrix operation operates in the current matrix stack.

Then for drawing operations, use glMatrixMode(GL_MODELVIEW).

Answer 2

Is your problem that the values being printed don't match up with the coordinate system you want? The projection manrix (glOrtho and friends) only have effect on the coordinates you specify to openGL - and have approx 0 to do with how your windowing toolkit communicates mouse positions back to your app.

It it up to you to un-project the window-coords given by GLUT into the format/coord system you want. It sounds like your p2w_y and p2w_x functions aren't coded to do what you want them to?