3D object in OpenGL

Question

I made a 3D object in OpenGL. But I think my code is extremely bad and now, I want to make my code better.

Here is my code:

#include <GL/glut.h>

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glBegin(GL_POLYGON);
    glColor3f(0.5, 0.5, 0.5);
    glVertex2f(0.0, 0.0);
    glVertex2f(0.5, 0.0);
    glVertex2f(0.5, 0.5);
    glVertex2f(0.0, 0.5);
    glVertex2f(0.75, 0.75);
    glVertex2f(0.25, 0.75);
    glVertex2f(0.0, 0.5);
    glVertex2f(0.5, 0.0);
    glVertex2f(0.75, 0.25);
    glVertex2f(0.75, 0.75);
    glEnd();
    glBegin(GL_LINES);
    glColor3f(1.0, 1.0, 1.0);
    glVertex2f(0.0, 0.0);
    glVertex2f(0.5, 0.0);
    glVertex2f(0.5, 0.0);
    glVertex2f(0.5, 0.5);
    glVertex2f(0.5, 0.5);
    glVertex2f(0.0, 0.5);
    glVertex2f(0.0, 0.5);
    glVertex2f(0.0, 0.0);
    glVertex2f(0.5, 0.0);
    glVertex2f(0.75, 0.25);
    glVertex2f(0.75, 0.25);
    glVertex2f(0.75, 0.75);
    glVertex2f(0.75, 0.75);
    glVertex2f(0.5, 0.5);
    glVertex2f(0.75, 0.75);
    glVertex2f(0.25, 0.75);
    glVertex2f(0.25, 0.75);
    glVertex2f(0.0, 0.5);
    glEnd();
    glFlush();
}

int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE);
    glutCreateWindow("OpenAdventrue");
    glutDisplayFunc(display);
    glutFullScreen();
    glutMainLoop();
}

(Compile with gcc file.c -o file -lGL -lglut)

user673679 · Accepted Answer · 2021-03-20 08:21:57Z

There is not much code to review.

GLUT is deprecated. The original GLUT ceased support more than 20 years ago. FreeGLUT is an open source alternative, but I'm not sure what the status is, since the last release was 18 months ago. Use something modern like GLFW or SDL2.
glBegin() and glEnd() are deprecated. The immediate-mode API was deprecated in version 3.0 (we are now at version 4.6). You should use Modern OpenGL (vertex arrays, buffer objects). Any performant OpenGL code should not use the immediate mode API. See this SO answer why.
glVertex2f essentially sets the z-coordinate of the vertex to 0.0. So the function is drawing a 2D point, not a 3D point. So it is not 3D object.

score 2 · Accepted Answer · 2021-03-20 09:00:05Z

I somehow like the code - very minimal (except the glutFullScreen()), and it is working.

It also raises a fundamental question: what is a 3D-Object? Can we look at one without special 3D equipment?

You have hardcoded the perspective. And it is no coincidence you picked the classical cube view - nice demonstration of what you can do with outlines and lines with only 0, 0.25, 0.5, 0.75 XY-coordinates.

But what if you want to change the viewpoint? Or add a cube in front, or want to draw a different shape? Your program is bad (as you say yourelf) because it is a dead-end street.

A cube is more easily defined with XYZ coordinates. The corner points (vertices) are:

0,0,0
0,1,0
1,0,0
1,1,0
0,0,1
0,1,1
1,0,1
1,1,1

You can make a wire cube from lines, or a box from triangles from these points.

For basic 3D view it, you also have to define an angle and distance. Through geometrical rules you can figure out the final XY coords. You let the user change the point of view and maybe add a light direction - that is my idea of a 3D Object in OpenGL.

Problem is only: with "modern" OpenGL (i.e. not fixed-pipeline, but "dynamic") you end up working with projection matrices in GL Shading Language. Either pen-and-paper with sine and cosine etc., or graphic card with vertex buffers, textures and shaders.

glut itself does not help in that direction, unless icosahedrons and teapots is all that you need.

Where did you start from?

Do you have any idea what you want your program to do? At least you have proven that you know bad code when you see it.

On webglfundamentals.org I found an article stating that WebGL actually is a rasterization engine, and not a 3D library.

You have to provide it with clip space coordinates that represent what you want drawn. Sure you provide a x,y,z,w and it divides by W before rendering but that's hardly enough to qualify WebGL as a 3D library. In the 3D libraries you supply 3D data, the libraries take care of calculating clip space points from 3D.

But modern OpenGL is the same: it gives you access to shaders etc. so you can fill the window with pixels in an efficient way.

Well here is a good answer (6 years ago):

You should be aware that modern desktop OpenGL does not use glLight, glRotate etc. these are considered bad practice for a very long time now (10 years or more), and so modern OpenGL has this same 'problem' of not solving a lot of geometry and algorithmic problems for you.

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