Question

I’m looking for a way to create a second view from the top of my current 3D scene. I would like to do this as easy as possible. The basic idea is that you have a subwindow that will display a top view of the setting.

I’ve looked into subwindows in openGL but the problem is you have to redraw everything (basically ending up with 2 scene’s with different angle = not good). Also because this will be used in a 3D game called “tower box stacking” (you have to place boxes on top of each other and make a high tower) its impossible to use the subwindows way to do it (since you would get 2 scenes with different blocks/locations/actions/…)

So how can I add a “second camera” to my current scene and then position it on top.

Answer 1

I’ve looked into subwindows in openGL but the problem is you have to redraw everything (basically ending up with 2 scene’s with different angle = not good)

This is actually the one and only way to do this with OpenGL.

So how can I add a “second camera” to my current scene and then position it on top.

OpenGL doesn’t have cameras. It doesn’t even have a scene. OpenGL merely draws very simple shapes: Points, Lines and Triangles. Above that OpenGL has no understanding of geometry or complex scenes.

Scene management is up to you and drawing multiple views of a scene is up to be implemented by you.

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Update: Pseudocode

draw_scene:
    for o in objects:
        glPushMatrix()
        glMultMatrix(o.transform)
        o.draw()
        glPopMatrix()

render_main_view:
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    glFrustum(...)

    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity()

    glMultMatrix(main_camera_transform)

render_secondary_view:
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    glFrustum(...)

    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity()

    glMultMatrix(secondary_camera_transform)

scissor_viewport(x,y,w,h)
    glScissor(x,y,w,h)
    glViewport(x,y,w,h)

render:
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)

    glEnable(GL_SCISSOR_TEST)
    scissor_viewport(main_viewport.x,main_viewport.y,main_viewport.w,main_viewport.h)
    render_main_view()

    glClear(GL_DEPTH_BUFFER_BIT)
    scissor_viewport(secondary_viewport.x,secondary_viewport.y,secondary_viewport.w,secondary_viewport.h)
    render_secondary_view()