Inside a pipe

So the level I'm currently working on has Juni running around on the outside of some pipes, which is all well and good ('cus drawing pipes is easy  ) except that eventually I have Juni running round inside the pipes, and so the inside will have to be drawn. I'm having trouble drawing the inside of the pipe, and making it look curved. I've been searching for photos that might show what I'm after, but not really finding anything; I get the impression this is one of those times when you have to fool the eye, rather than actually draw something as it would really appear in Real Life.

If anyone has any advice of any sort (words or pictures I can use for reference, or whatever) I'd be very grateful.

If it helps, I'm including a screenshot of my pipes in action (for some reason, when I made the tileset (which was a long time ago now, before I started working on 'My Loving Hadron Collider' or 'Petals of a Dying Flower') I decided to make the pipes only just big enough to fit Juni once she gets inside).

The picture above holds the clue. the pipes appear rounded for 2 reasons: we recognize them as pipes and know they're rounded, they are bright in the middle and dark at the edges. This second part is a trick of the eye. We interpret brighter colors as being closer. To draw the inside curve of the pipe, make the middle dark and the edges bright. In other words, invert the gradient.

Edit: Also, the brightest spot on your pipe exterior is slightly above center. This gives the illusion that the room is lit from above. For consistency, the darkest spot should be slightly above center on your pipe interiors.

With optical illusions, there is one simple rule to give you what you want.

If you invert your image one way or the other, it will look as if it is the opposite.

What you want to do is make the gradient from the bright color, to the dark color, to the bright color, and suddenly, it will look hollow. Thats what you want.

I tried inverting it... I dunno, maybe I should have then put the inverted image in context. I'll give it another go. Thanks.

Took a brief crack at it.

I'd say they could use a few more shades closer to the middle. They look a bit flat there.

If you have a 3D image with  top-left set as bright, and bottom-right as dark, and you turn that image 180 degrees around, the image will look from outset to inset.

Note that our brains are capable of understanding the image the right if we turn it while looking at it, so turn it around while looking at something else.