You could use the "inside mode" and set the background alpha to 0, that way anything behind the Q3D object would be visible, and anything above would be visible. Inside mode however is buggy on some browsers (i can't fix the fact they dont support video to texture properly). The better option is to use the behind mode, set layer transparent to on so you can see the Q3D canvas below the layer, and make your "parallax background" a Q3D object or textured Q3D plane far in the background, which is how a 3D platformer like Trine is doing it in the first place. With Behind mode you can't show construct content behind the Q3D canvas, but you don't really have to, and this will work on all platforms without the overhead of video to texture.