Question

I would like to reposition a circle, triangles, lines and an arc. How do I go about it? I’ve searched the web for solutions but nothing that address the issue specifically.

Any input that will lead me in the right direction will be helpful.

I’m using C++ with opengl.

Answer 1

I would like to reposition a circle, triangles, lines and an arc.

So you already did draw the scene and now want to change the position of the objects?

Well then, all you need to understand is, that OpenGL doesn’t maintain a scene graph of sorts. After submitting a draw call, things get only updates on the target framebuffer and that’s it. You want to change something? Clear the screen, redraw everything, but now with the adjustments applied.

Really, OpenGL just draws things, it’s nothing more than sophisticated brushes (textures) and pencils (primitives) for some paper offered by the operating system (viewport/window).

EDIT due to comment

The usual interactive OpenGL graphics program with animations, programmed in a imperative programming language, is structured like this (pseudocode)

float T = 0
float alpha = 0, beta = 0
float red = 0, green = 0, blue = 0

paused = false

on_mousemove(mousemove):
    alpha += mousemove.rel_x
    beta  += mousemove.rel_y

on_keypress(keypress):
    switch keypress.key:
        case Key_ESCAPE:
            queue_event(Quit)
            return

        case Key_PAUSE:
            paused = not paused

        case Key_R:
            red += 0.1
        case Key_r:
            red -= 0.1

        case Key_G:
            green += 0.1
        case Key_g:
            green -= 0.1

        case Key_B:
            blue += 0.1
        case Key_b:
            blue -= 0.1

    queue_event(Redraw)

render()
    glViewport(0, 0, window.width, window.height)

    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    apply_projection()

    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity()
    apply_modelview()

    glColor3f(red, green, blue)
    glRotatef(T * x_revolutions_per_second_factor, 1, 0, 0)
    glRotatef(alpha, 0, 1, 0)
    glRotate(beta, 0, 0, 1)

    draw_object()

    SwapBuffers()

main:
    initialize_libraries()
    load_resources()
    create_window_and_OpenGL_context()

    previous_loop = get_time()

    loop 'eventloop':
        event = peek_event()
        switch event.type:
            case MouseMove:
                on_mousemove(event.mousemove)

            case KeyPress:
                on_keypress(event.keypress)

            case Quit:
                break 'eventloop'

            case Redraw:
                render()
                break

            case NoEvents:
                fallthrough
            default:
                if not paused
                    render()

        current_loop = get_time()
        if not paused:
            T += current_loop - previous_loop
        previous_loop = current_loop