Reflection ONLY

[Elm]

Hi all!

 

I always was wondering if objects can be set to cast reflections ONLY (and not be visible to the camera in renderings).

Is that possible somehow?

 

I'm talking about a sphere around the scene that I want to just cause reflections but keep the renderings alpha channel "clean"...

 

 

Greetz,

Elm.

[agep]

I dont think it is possible. I wish it was. One of my "workaround" are to cut a hole in the sphere where the camera looks through. The reflections provided by the rest of the sphere is usually sufficient for most of my projects

 

[Fuchur]

I dont think it is possible. I wish it was. One of my "workaround" are to cut a hole in the sphere where the camera looks through. The reflections provided by the rest of the sphere is usually sufficient for most of my projects

 

 

There is more than one feature-request for that... the newest can be found here:

A:M Feature-Request: Reflection only

 

An earlier one is this one asking for something equal but a little bit more:

A:M Feature-Request: Second Rays only

 

If you think one or both a useful, please give your comment in the comments section of A:M Reports.

 

See you

*Fuchur*

[robcat2075]

I always was wondering if objects can be set to cast reflections ONLY (and not be visible to the camera in renderings).

Is that possible somehow?

 

I'm talking about a sphere around the scene that I want to just cause reflections but keep the renderings alpha channel "clean"...

 

If you need this now, I'd suggest doing one normal render to get the reflections and then another with the object gone and swap the alpha channel from the second render into the first render.

 

The second render could be done quickly with lights off and other things to speed up the render since you only want the alpha channel.

[Rodney]

Okay... after a couple failed attempts at a better solution here is an approach that should work.

 

You'll need:

- One or more objects in your scene (with some surface reflectivity)

 

- One half of a sphere (decaled) - This will give you the reflection on the object(s)

The missing half of the sphere will allow for your clean Alpha Channel.

(For some interesting effects you should be able to just make that half of the sphere partially transparent!)

 

Important (I think): Makes sure under the object properties Options you have 'Flat Shaded' and 'Cast Reflections' set to on. You may get some reflections without these settings but not the image applied to the half sphere.

 

- One camera with Alpha Channel, reflections and all other appropriate settings turned on placed between the half sphere and the objects (pointed toward the open half of the sphere)

 

 

The camera will record the scene with the Objects reflecting the half dome behind the camera while preserving the Alpha Channel.

 

I can try to clean up my test project and share that if my instructions aren't clear. I tried so many things it's a mess!

[Rodney]

Elm,

I have to thank you for making me a whole lot smarter today!

 

At the last minute while I was typing that last post my brain locked in on something I hadn't yet tested so I added it:

(For some interesting effects you should be able to just make that half of the sphere partially transparent!)

 

This not only allows for some great effects but should also remove the need for using a half sphere altogether. Perhaps more importantly it allows us to add reflectivity to the backs of the objects. Personally I like the half dome because it guarantees I will have my Alpha Channel. Anything that isn't Alpha I know I have put there myself so I can find it and take it back out.

 

Thanks Elm!

[Fuchur]

I always was wondering if objects can be set to cast reflections ONLY (and not be visible to the camera in renderings).

Is that possible somehow?

 

I'm talking about a sphere around the scene that I want to just cause reflections but keep the renderings alpha channel "clean"...

 

If you need this now, I'd suggest doing one normal render to get the reflections and then another with the object gone and swap the alpha channel from the second render into the first render.

 

The second render could be done quickly with lights off and other things to speed up the render since you only want the alpha channel.

 

That is the way I did it when I had to create such an effect till now...

 

See you

*Fuchur*

[Rodney]

I should have also mentioned that you'll want to render out to EXR to get the reflections separately.

 

If anyone has a test project that they'd like to see resolved post it and I'll be glad to look into it.

[Xtaz]

my two cents .... you must use environment map on the object ....

[Fuchur]

my two cents .... you must use environment map on the object ....

 

If you have only one object it is a very good solution to use the Environment Map or the Environment Shader for that... but if you want several object to show different reflections based on there position in the 3d room (different viewing angles, etc.) it can be much harder to use env-maps, etc. For that you will need such a solution.

 

But yes, in many situations you can get away with Env-Maps.

 

See you

*Fuchur*

[Elm]

Hi again!

 

I aleays use the workarounds you described. The half dome doesn't work for me now, as my camera does wild things in space (like whole turns, etc..)

So it really seems to be a feature request

[agep]

I think there is still some hope by using my method, with the small hole/window in the sphere. You can constraint the environment sphere to the camera and set the Projection Map material to Global Axis ON. this will make the hole/window follow the camera while the environment map is static. I hope the attached video explains it a bit better

 

screen_001.mp4

[Rodney]

I haven't viewed Stian's video yet (I will!) but after posting I read back and saw he had a similar approach as me.

 

I aleays use the workarounds you described. The half dome doesn't work for me now, as my camera does wild things in space (like whole turns, etc..)

 

What I would do in your situation with a moving camera is (perhaps) to constrain the sphere to your camera so that the open half always is in front of the camera (In Stian's case the hole he's created would remain in front of the camera). In the real world that huge half sphere would be a problem but in 3D space it's not gonna knock anything over as you move the camera around all over the place. If a static reflection is required then the camera could punch a hole in the sphere wherever it is looking via a number of methods (A Boolean cutter would be a fairly easy setup I would think. You could have that shape rotate like the camera and cut through the sphere in front of the camera.

 

Usually a feature request just formalizes or optimizes something we can already do in A:M.

[robcat2075]

I think a "Matte Object" woudl fulfill the function that Elm is looking for. A:M doesn't have matte Objects yet but perhaps if we specified it well in an AMReport Steffen could add it.

 

 

Let me guess what you are doing... you have a 3D model that you want to put in real footage and it has reflective surfaces that need to reflect that real environment, which of course it can't because it isn't really there so you simulate that by putting a dome around it with a map of the real environment on it, but with the dome there in the background the object isn't cut out with an alpha channel anymore.

 

Is that what you are trying to do?