Pondering Reflections

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Old 11 November 2013   #1
Pondering Reflections

So I'm working on a fly through of a medical screw and I'm mystified by what the reflective material is doing. It's basically a full 100% reflective channel with Metal specularity and some blurring and that's it. No color, no other channel. for speed, and since it's the only object in the scene, I have the option to only reflect the environment and not itself. The screw is full of holes as you run down the shaft. The only light is an HDRI sky object, which is only necessary for the first half of the animation where there is another object. You would expect flying down the the center of the shaft it would be mostly dark except for the holes where you would see extra reflection. Wrong. It's completely lit up. I don't get it. How is this possible? Normals are all correct.
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Old 11 November 2013   #2
Im fairly certain this is because you are hitting the ray trace reflection depth limit on the inside of your screw (100% reflective walls reflecting each other - creating that infinite mirrors effect). It seems that C4D defaults to showing the environment when you hit the reflection limit. Im not sure if there is a way to change this setting (for example in Max, you could set the reflection to show a color when it hits the ray trace reflection limit or show the environment.)

you might have to resort to some trickery. for example, using a black object with a composite tag set to not visible to camera placed to block the environment reflection once you look down into the screw. or use a material that is not very reflective for the inside shaft of your screw - since there is nothing in there to reflect anyway except itself.
 
Old 11 November 2013   #3
I think it's just a weird optical property of a mirror tube. I think it's physically accurate, other than the absence of Fresnel effect. If there's a bend in the tube and you add Fresnel to the reflection then it will look more "real".
 
Old 11 November 2013   #4
Originally Posted by travismetcalf: Im fairly certain this is because you are hitting the ray trace reflection depth limit on the inside of your screw (100% reflective walls reflecting each other - creating that infinite mirrors effect).


The screw is not reflecting itself. The only thing that is reflected is the reflection map on the sky object which should be hidden to the inner surfaces of the screw. In the Render Settings in the Options tab I have "Limit reflections to floor and sky" which it will only calculate the sky object that has a reflection map applied to it. So it's not that, unfortunately.
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Old 11 November 2013   #5
Originally Posted by Troyan: The screw is not reflecting itself. The only thing that is reflected is the reflection map on the sky object which should be hidden to the inner surfaces of the screw. In the Render Settings in the Options tab I have "Limit reflections to floor and sky" which it will only calculate the sky object that has a reflection map applied to it. So it's not that, unfortunately.


That's part of your problem. The object in effect can't see itself, so it's reflecting the environment without occlusion. In other words, one side of the pipe can't "see" the other side, so it's reflecting the environment that it properly should not be able to see.
 
Old 11 November 2013   #6
Originally Posted by AdamT: That's part of your problem. The object in effect can't see itself, so it's reflecting the environment without occlusion. In other words, one side of the pipe can't "see" the other side, so it's reflecting the environment that it properly should not be able to see.


But that's just it. Even though it won't reflect the other wall, it still shouldn't be able to see the environment since it should be blocked by the tube.
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Old 11 November 2013   #7
Originally Posted by travismetcalf: Im fairly certain this is because you are hitting the ray trace reflection depth limit on the inside of your screw (100% reflective walls reflecting each other - creating that infinite mirrors effect). It seems that C4D defaults to showing the environment when you hit the reflection limit. Im not sure if there is a way to change this setting (for example in Max, you could set the reflection to show a color when it hits the ray trace reflection limit or show the environment.)

you might have to resort to some trickery. for example, using a black object with a composite tag set to not visible to camera placed to block the environment reflection once you look down into the screw. or use a material that is not very reflective for the inside shaft of your screw - since there is nothing in there to reflect anyway except itself.


And you're right, this is the workaround, but logically I'm not understanding how it's reflecting anything.
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Old 11 November 2013   #8
Originally Posted by Troyan: But that's just it. Even though it won't reflect the other wall, it still shouldn't be able to see the environment since it should be blocked by the tube.


I don't think it works that way. The way it actually works is that reflective surfaces can ONLY see the sky and floor, and they can see them even if the reflections rays would otherwise be blocked. In other words, the sky and floor are always reflected.
 
Old 11 November 2013   #9
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