mimicing 3d cylinders help

AlanPS · Post by **AlanPS** » Mon Nov 13, 2006 10:49 pm

I was attempting to mimic 3d cylinders. I had a rectangle shape set to face camera. Above it and below it were circles (one made to look like a top and another to be the bottom of the cylinder) rotated to 90 degrees so that at any angle it looked like a cylinder.

This effect is pretty cool except when your camera goes below the object. Instead of the rectangle shape hiding the top face the top face shows and looks like some bizarro Picasso perspective.

I was playing around with masks but then realized the masks would have to be "smart" to know when the cylinder top was above or below camera/eye level to hide it.

Anybody got any ideas how to achieve this?

slowtiger · Post by **slowtiger** » Tue Nov 14, 2006 12:09 am

Keep those two ellipses as reference, draw a rectangle on top of it, add more points in in top and bottom line, bend those two lines according to the ellipses.

Erase the ellipses. Duplicate the layer and flip it vertically. Now you have front and back of a hollow cylinder.

Rasheed · Post by **Rasheed** » Tue Nov 14, 2006 1:44 pm

You could also create a 3D block as a reference image (Scripts -> 3D -> Cube), add a vector layer on top of it and draw your 2D image in there. The center of the top and bottom circle is where the two diagonals of either the top or bottom side of the block cross. You can also see that the distorted circles aren't ellipses at all. They are wider in the front and narrower in the back, due to foreshortening.

slowtiger · Post by **slowtiger** » Tue Nov 14, 2006 5:53 pm

uhm, no. Circles in perspective are ellipses and stay that. Only very wide angle lenses show the effect you mentioned.

You can easily test this yourself. Take two identical plates. Mark one of it with its center and two axis. Do some photographs with a normal lens (about 50mm). Open the pics in Photoshop and draw some ellipses on top.