The same six-faced camera as renderCubemap3d(), but showing the volume's own colour rather than a lit surface. No lighting, no gamma, no surface threshold.
Instead of stopping at a surface, each ray travels all the way through and adds up what it passes. At every step the volume both blocks a little light and gives off a little colour of its own, and what it gives off is dimmed by everything already in front of it.
This is how you render smoke, cloud, dye in water, or a nebula: things with no surface anywhere, that you see into rather than at. Trying to find a surface in a cloud gives you a lumpy solid, which is not what a cloud is.
Density and absorption both control how quickly the volume blocks light, and emission controls how much it gives off, so a thin bright volume and a thick dim one are separately reachable.