OpenGL 2d rectangle not being rendered

Question

I am trying to render a rectangle onto the screen. When the program is run, only the clear color shows up, and no rectangle.

Here's the code:

glClearColor(0.0, 0.0, 0.0, 0.0);
glViewport(0, 0, 1280, 720);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 1280, 720, 0, -10, 10);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT || GL_DEPTH_BUFFER_BIT); //Clear the screen and depth buffer

int x = 100;
int y = 100;
while (!glfwWindowShouldClose(window)) {
    glfwPollEvents();

    glBegin(GL_QUADS);
        glVertex2f(x, y);
        glVertex2f(x + 10, y);
        glVertex2f(x + 10, y + 10);
        glVertex2f(x, y + 10);
    glEnd();

    gsm->update();
    gsm->render();

    glfwSwapBuffers(window);
}

`GL_COLOR_BUFFER_BIT` & `GL_DEPTH_BUFFER_BIT` are bitflags, you need to bitwise-OR them (`|`) together, not logical-OR (`||`). — genpfault, Dec 22 '16 at 15:36

score 1 · Accepted Answer · edited May 23 '17 at 10:30

It got culled. You had inverted Y axis with your projection, by supplying bottom =720 larger than top 0. Your quad is counterclockwise in your local coordinates, but in normalized coordinates it is clockwise. Remember, projection matrix is a part of global transform matrix! Now, if that's default state, then out of those two winding directions

the GL_CCW is the actual one, it is considered "Front". By default OpenGL culls triangles with mode glCullFace(GL_BACK), and quad internally is considered as pair of triangles).

Either change order of vertices

glBegin(GL_QUADS);
    glVertex2f(x, y);
    glVertex2f(x, y + 10);
    glVertex2f(x + 10, y + 10);
    glVertex2f(x + 10, y);
glEnd();

or change culling mode to match left-handedness of your coordinate system or disable culling.

See also: 1. https://www.khronos.org/opengl/wiki/Viewing_and_Transformations 2. The answer to Is OpenGL coordinate system left-handed or right-handed?

OpenGL 2d rectangle not being rendered

1 Answers1