that I'm trying to simulate a 3D Symbols on a wheel.

Hard to understand...Are you trying to control which symbols are the last to pass by the screen before the reels stop?

Basically, There are 12 unique symbols, but they are repeated 3 times all over the cylinder, so a total of 36 symbols. but I have 36 meshes that hold all those 36 symbols. I want to know what is the mesh index at the giving angle :/

Yes, or in other words, at any time, when the reel is stopped, of course there will be 3 meshes that will appear to the player.

Pick some mesh to be mesh 0 corresponding to angle = 0. The meshes are presented at intervals of 360/36 = 10 degrees. Then, mesh index = angle/10. No?

I would like to change the 3 symbols, that are behind the three symbols which will be appeared

That's the part I didn't understand.

When the reel is stopped, at any angle. There will be three symbols that are shown to the user, other (meshes) or symbols, are not shown due to clipping by the screen. Only three symbols are shown to the users but those are not desirable for the moment, I I would like to change those three hidden meshes ( behind or down the viewed symbols), or know their indices, at any stopping angle.

Well, I assume you would calculate the index of the mesh in the center row (or some other reference row), then add or subtract (with wraparound in mind) to get the index of any nearby offscreen meshes.

Still curious (for no good reason) about your intent -- are you trying to compose certain "near misses" that the player would observe as the reel slows down to a stop?

yea I'm trying to do something like you said. I will try it tomorrow and I hope that you keep up following with me. Thanks

An aside: a game manufacturer did that a few years ago after the video slots first came into the casinos. It seemed to work as the game did fairly well...but the regulators caught on eventually (a competitor found out and told them) and the practice was made illegal in at least one US jurisdiction as a result. But that's ground-based real money gambling, of course, maybe not what you are doing.

it worked.

Is it possible to define a critical angle ( red line ), where If the symbol before 1 in the picture, is going to pass it ? How would I calculate it ?

My cylinder is just a 3DS Model.

In the video slot games I've been involved with, the final position of each reel is obtained by getting a random integer [0, #stops). The motion is entirely scripted, occasionally with some staged mechanical backlash as an embellishment. Do you really want to simulate a mechanical slot? In that case, you'll have forces to contend with, and those will determine this critical angle you describe. Otherwise, it is arbitrary -- you decide what it should be.

It looks like you get a random stopping angle [0,360) and want to go from there. I guess then this critical angle would be some fraction of the solid angle taken by each mesh. You would account for this in how you convert angle to mesh, you basically truncate after adding an offset. index = (angle + (10 * (1-fraction)))/10.

Yeah, i'm in a hurry so that might not be clear cuz I haven't defined anything but I think you'll get the point.