PDA

View Full Version : cancelling out reflections?



sammael
03-13-2009, 02:27 AM
Is there a way to cancel out unwanted reflections on a surface but keep other reflections from elements of the scene?
Does that even make sense? :)

colkai
03-13-2009, 02:48 AM
I think you should be able to set the objects 'unseen by rays' flag in the properties ('p' when object selected).

sammael
03-13-2009, 03:59 AM
Ahh yes I know that method, I should clarify. I need the object in question to be reflected on some surfaces but not others so turning it off altogether is no good.

Kirdian
03-13-2009, 05:10 AM
It is impossible for single pass, but can be done in composition.

sammael
03-13-2009, 05:56 AM
Damn, I had feared as much, I guess I;m going to have to learn some compositing... any pointers for starting out? :)

Kirdian
03-13-2009, 06:31 AM
Quick tutorial about basic compositing in Fusion (http://www.cgsutra.com/eyeon_fusion_tutorials/render_passes/compositing_render_passes_fusion.php)
Also, search this forum for multipass rendering techniques in LW.

sammael
03-13-2009, 06:48 AM
Thak you.

Weepul
03-13-2009, 04:26 PM
If you just need one set of custom excluded reflections, you could use a Refraction node with IOR set to 0, making it just like a reflection node, and then use the Incoming Eta output of the Spot Info node, fed into a scalar math Logic node, to detect when a surface is being seen by the special "reflection" or not (probably set its mode to A is less than B and set B to something low like 0.1, since I've had some problems detecting an exact IOR using nodes.)

toby
03-13-2009, 11:54 PM
*schoom!* right over the head!

I'm with you right up until "special reflection"... how do you determine that the reflection is special - (and shouldn't the ior be set to 1?)

Weepul
03-14-2009, 04:32 AM
(and shouldn't the ior be set to 1?)
Not for this - you might already know about total internal reflection, when light passes from a material of higher refractive index to lower refractive index.

An index of refraction of 0 isn't based on anything physical but it will make LightWave treat the entire surface with "total internal reflection", except it's not internal to anything so it's just like normal reflection...except LightWave will tag those reflection rays with an IOR of 0.


I'm with you right up until "special reflection"... how do you determine that the reflection is special
See above. :) Use the Incoming Eta output of the Spot Info node - it'll be 0 for the rays coming from the tweaked refraction node, and 1 or more for other rays. That info can be used to change a surface to be invisible only when seen by the reflections coming from the 0 IOR refraction node.

sammael
03-14-2009, 07:16 AM
Thanks weepul I will try this. You seem to have a fairly good understanding of nodes, do you know if it is possible to convert the % based values in a weight map to a distance in mm? Or at least how a % value is converted to a distance when plugged into a distance based input?

toby
03-14-2009, 12:52 PM
Not for this - you might already know about total internal reflection, when light passes from a material of higher refractive index to lower refractive index.

An index of refraction of 0 isn't based on anything physical but it will make LightWave treat the entire surface with "total internal reflection", except it's not internal to anything so it's just like normal reflection...except LightWave will tag those reflection rays with an IOR of 0.


See above. :) Use the Incoming Eta output of the Spot Info node - it'll be 0 for the rays coming from the tweaked refraction node, and 1 or more for other rays. That info can be used to change a surface to be invisible only when seen by the reflections coming from the 0 IOR refraction node.
Ok I gotcha, the ior of 0 *is* the tag to distinguish the reflection. Very sneaky!