[Tinusch]

I have an odd question... Maybe this is something extremely simple that just went over my head, or maybe it's complicated. I don't know. My question is: Which direction do reflections go in? Take the following picture for example:



I whipped that up in about 5 minutes, but hopefully it'll get the point across. Assuming that the big gray thing is the main object, a pipe, and all the little things around it are puddles, in which puddle(s) would the reflection of the pipe appear in? I know that sometimes reflections point straight downward, but I also know that they reflect things from other directions too. So how do I know which directions the reflections should go in?

[Frost]

Hey Tinusch.

Reflections are rather simple to understand, but often DIFFICULT to render or draw, because it requires a lot of thinking and estimating of angles and distance in 3d space. But I'll try to have a go at it as simply as I can.

First, a reflection is based on 2 things: the viewer's angle, and the surface orientation. Basically, you can find what is being reflected at a certain point on a surface by tracing a ray (line) from the viewer (I would guess, the middle of the image) to the surface of the object that reflects, and deflecting that ray off the surface to show where that ray is pointing... what is being reflected.

Here's a 2d example of how you would trace reflections in 2d.



It's much more complicated in 3D because you have to take into account the curves of a surface on 3 axis, not only 2.

Puddles have volume, and are rounded at the edges, bulging, so you have to take that into account. Since this bulge is also small and the angle falls off very tightly and quickly, the pipe will be a very very short speck on the edge of it. Try making a top view of your scene and trace those rays reflecting on the surface as if the puddles were cylinders in top view. Then draw one from a head-on front view, and trace the vertical reflection rays in this one. Then merge both and find the ray intersections it will give you a good idea where the region of the puddle actually reflects the pipe.

I hope this was more or less clear. =)

cheers
frost.

BTW, specular light is just a reflection of the light source onto a surface, and is calculated exactly the same way.

[This message has been edited by Frost (edited June 10, 2000).]

[Nex]

this is what 3dsmax did when I reconstructed your scene. I hope it helps a bit.

[Tinusch]

Thanks a lot Frost and Nex! That really helped me understand what reflections are and how they work. THe only thing I'm not too clear on is how you know at what angle the ray hits the reflecting object...

[Frost]

Tinusch,

You can experiment with a simple mirror. Move the mirror and see how your sight is propelled off it to see the walls and ceiling around you...

If you look straight onto a reflective plane, your vision will intersect this plane at a perfect 90 degrees, and reflect you right back. If you look at a mirror at 45 degrees, your vision will be propelled 45 degrees on the other side.

Basically, the angle at which the ray is reflected at is the absolute inverse as the one you project... (sigh) this is not clear... here:



Like I said, applying this in a 3d scene is DIFFICULT, and requires a great deal of good angle estimating -- you could try tracing rays on a top-view plan of your scene to help, etc.

Good luck. =)

[Frost]

I had time to waste to I made this useless thing.

[Tinusch]

Wooo! Thanks man!

[Digital Genesis]

You should really check out the FARP section at Elfwood.com
There is a great couple of articles on reflection and refraction. Both real and 'faked' for the sense of artistry

They're worth it.
(although Frost is doing a great job here!)

[Frost]

Digital: VERY interesting link indeed! The refracion stuff is pretty cool - although I would've liked to understand how much the rays are bent and what kind of calculation is used to get it. But nonetheless extremely interesting! Thanks. =)

[Cos]

I have no idea what u guys are on about but I thought I'd come in and smile anyway ;D

[Frost]

hehe =)

Frost hugs Cos.
=))

[Cos]

haha