Rendering shadow problem with final gather

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  10 October 2012
Rendering shadow problem with final gather

hey everyone,

Any idea why there is a lack of shadow being cast onto my floor from the tires?... The wheel is sitting flush with teh floor but whatever I do it just seems to look as though is it floating?!



Kind regards, William.
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  10 October 2012
It looks like the tire object in not in the same partition, as if the tire is being ignored. Or, perhaps the black of the tire is the reason. Final gathering is first a color bleeding effect, then occlusion, but not great occlusion. Or, you may consider adding an occlusion pass, or a duplicate tire and extra light that only sees the duplicate tire, with shadows only.
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  10 October 2012
Ah interesting you say that, I am wondering how XSI handles polygons when rendering... because the tire closest to the floor is upside down effectively , does that perhaps mean that light is passing right through them? and is there a setting for double sided polygons if so?
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  10 October 2012
Do you mean the polygons normals are inverted? That would be a problem, and you can enable double sided polygons. Or invert the normals.
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  10 October 2012
Sorry I didn't describe that well at all, been a long stressfull day to say the least.

My basic question was - will light travel through a polygon if its normals are facing away from the light source, or will it still cast a shadow as if it was a solid object?
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  10 October 2012
Final gathering works backwards fron what you seem to be thinking it does. It's not a matter of light going through polygons because they are inverted, it's a matter of the floor not encountering those polygons when sampling available energy and reaching the ambient/dome instead of stopping at the tyres.
Final gathering is an irradiance caching mechanism sampling from the surface out, light emissions being obstructed would be the photons approach.
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  10 October 2012
In other words, black, light-absorbing tires have no energy to emit, and are not "seen" in the FG map.

So, add an occlusion pass.
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  10 October 2012
Originally Posted by Stoehr: In other words, black, light-absorbing tires have no energy to emit, and are not "seen" in the FG map.

Nope, not quite.

Black, light absorbing tyres would emit very little energy, but that's fine it WOULD add the contact patch he's after.

His problem is that the tyres are reversed, so when the floor looks up available energy (towards the environment) it DOES NOT find the tyres to block that energy (provided by dome or ambient), and comes out this non descript ambient colour like the rest of the ground plane.

If the tyres WERE found instead, it would NOT accumulate the energy from the environment and come out darker.

Capitalisation added for emphasis, not for condescending

Simplifying and generalising a bit:

Photon based illumination: Energy emitted from lights and the type of objects occluding irradiance he thinks about. Direct path tracing style.

Final gathering: Reversed tracing where object surfaces, when being shaded, query their surroundings to know how much energy (light) to infuse the shading with.

Edit:
Note that the choice of words like path tracing, direct, indirect, irradiance and so on are chosen for the sake of explaining something in English. Do not take my post for a literal explanation of things such as forward path tracing, irradiance caching, energy conservation in BRDF and so on, those tend to be very confusing unless you have an understanding of the mechanics involved.
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Last edited by ThE_JacO : 10 October 2012 at 03:07 AM.
 
  10 October 2012
Right on, thank you, Jaco.
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  10 October 2012
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