OpenGL Coordinate system confusion

Question

Maybe I set up GLUT wrong. Basically I want verticies to be reletive to their size in pixels. Ex:right now if I create a hexagon, it hakes up the whole screen even though the units are 6.

#include <iostream>
#include <stdlib.h> //Needed for "exit" function
#include <cmath>
//Include OpenGL header files, so that we can use OpenGL
#ifdef __APPLE__
#include <OpenGL/OpenGL.h>
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif

using namespace std;

//Called when a key is pressed
void handleKeypress(unsigned char key, //The key that was pressed
                    int x, int y) {    //The current mouse coordinates
    switch (key) {
        case 27: //Escape key
            exit(0); //Exit the program
    }
}

//Initializes 3D rendering
void initRendering() {
    //Makes 3D drawing work when something is in front of something else
    glEnable(GL_DEPTH_TEST);
}

//Called when the window is resized
void handleResize(int w, int h) {
    //Tell OpenGL how to convert from coordinates to pixel values
    glViewport(0, 0, w, h);

    glMatrixMode(GL_PROJECTION); //Switch to setting the camera perspective

    //Set the camera perspective
    glLoadIdentity(); //Reset the camera
    gluPerspective(45.0,                  //The camera angle
                   (double)w / (double)h, //The width-to-height ratio
                   1.0,                   //The near z clipping coordinate
                   200.0);                //The far z clipping coordinate
}

//Draws the 3D scene
void drawScene() {
    //Clear information from last draw
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);


    glLoadIdentity(); //Reset the drawing perspective
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

    glBegin(GL_POLYGON); //Begin quadrilateral coordinates

    //Trapezoid
    glColor3f(255,0,0);

    for(int i = 0; i < 6; ++i) {
        glVertex2d(sin(i/6.0*2* 3.1415),
            cos(i/6.0*2* 3.1415));
    }

    glEnd(); //End quadrilateral coordinates

    glutSwapBuffers(); //Send the 3D scene to the screen
}

int main(int argc, char** argv) {
    //Initialize GLUT
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH);
    glutInitWindowSize(400, 400); //Set the window size

    //Create the window
    glutCreateWindow("Basic Shapes - videotutorialsrock.com");
    initRendering(); //Initialize rendering

    //Set handler functions for drawing, keypresses, and window resizes
    glutDisplayFunc(drawScene);
    glutKeyboardFunc(handleKeypress);
    glutReshapeFunc(handleResize);

    glutMainLoop(); //Start the main loop.  glutMainLoop doesn't return.
    return 0; //This line is never reached
}

How can I make it so that a polygon of 0,0 10,0 10,10 0,10

defines a polygon starting at the top left of the screen and is a width and height of 10 pixels? Thanks

Answer 1

If you want the objects to be scaled that sort of way, you should use an orthographic projection.

Right now, with perspective, things are scaled not only by their size, but by their Z-axis position. So use this function instead of gluPerspective:

gluOrtho2D(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top);

That function basically defines the space you can see, which is like a big rectangular prism. That makes far things appear the same size as near things.

As for the exact scaling, it will also change relative to the viewport size. To get the pixels exactly right, you would have to constantly change the projection, or keep the viewport size fixed.

For it to work out as 1:1, if your viewport is x pixels wide, the orthographic projection should be x pixels wide as well.

Answer 2

If you're drawing in 2D, you don't want to use perspective projection. If you set up your camera with gluOrtho2D(0, window_width, window_height, 0); then you should get what you're looking for.