crazy homeless guy

Can anyone help me decipher this statement? ...it was in the 3dsmax help files for Mental Ray. Specifically the statement of re-using the photon map independent of geometry and image size?

When I read this, it sounds to me that if I change the geometry, the Photon Map is still valid. Which doesn't make a lot of sense when I try to think about it.

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travis schmiesing

muchhal.sandeep@gmail.com

Hi Travis,

I shall say that we both are feeling the same.
as we change the geometry we shall recalculate the PMAP.

the image size has got no problems
once we save the photon map then we can render it at any required size need not recalculate.

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Sandeep (got a new machine)
(Intel 5299-ws chassis, 2 X e-5420 , 500-XVN MB, 4 GB FBECC Ram ) This is what sandeep is all about

crazy homeless guy

good to know. i was reading a another mental ray thread post today, and it referenced a different anomaly in the help files max mental ray help files.

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travis schmiesing

Brian Bradley

I assume that the help is making a distinction between possible
GI solutions that people may be using, such as Radiosity which
is very much Geometry dependant. Chabnge the geometry and
you will need to recalc your GI solution, which is not the case
with photon mapping. (as a rule)

regards
Bri

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crazy homeless guy

Why would you not need to recaculate the photon mapping if you change the geometry? ....this is what doesn't make sense to me. maybe i need to study more on how photon mapping works.

The majority of my GI experience is with Vray's irradiance map, so I am processing this in relation to those terms. Using irradiance passes, the map is representation of the 3d geometry in the scene. If I change the geometry, then the map created by the irradiance solution does not have the proper geometry to align to, and becomes invalid.

It is this thinking that is confusing me on photon mapping. I remember the tutorial saying something about the way it is calculated, but I can't remember exactly what it is now. I will look at it again in the morning, and try to decipher it.

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travis schmiesing

Brian Bradley

Nothing wrong with your understanding. If you make significant
alterations to your meshes in both VR and MR you would want
to recalc your IR or PM. But with radiosity, if you change say the
base number of segments in a box, then the solution becomes
invalid because radiosity IS geometry dependant, the lighting
info is stored at Mesh level! This is not so with photon or
irradiance mapping.

regards
Bri

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crazy homeless guy

So. What I am making out is that the photon map is a map of the light, or more precisely the transport of the light. This is why changing the geometry does not affect the map. The photon map already has the amount of light that will strike a newly introduced surface or geometry modification stored into the file. However, if I add additional, or change the light source, I will need to re-compute the photon map because the map of the light is no longer accurate to the actual lights in the scene?

Whereas, or at least I believe that, irradiance maps pretty much become invalid as soon as the geometry is changed. I say this because if you open the Vray irradiance map file in the irradiance map viewer, you can see that that the surfaces are defined by how the light hits them. If you move a surface, the saved irradiance map does not reflect that move without recalculating. Or at least it does not reflect it with any deal of accuracy.

Is there a viewer, or any way to look at a photon map to better understand how it works?

…also, I assume that placing geometry close to a light source will affect how the photon map should behave, therefore making the saved map un-accurate because the distribution of light in the scene would have changed(?).

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travis schmiesing

crazy homeless guy

after more research, i have to go with my initial inclination. ...when you change the geometry in the scene, you should recaculculate your photon map, because it is not accurate.

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travis schmiesing

iduna79

Well, photons bounces against the geometry. If the geometry is moving you have to recalculate the photon maps (the same goes for fg maps) or the calculations will be incorrect. Otherwise the saved photons bounces against something not there and so on.

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www.tenjin.se

Brian Bradley

At the risk of sounding picky it would be more
precise to say the photons interact with the shaders
assigned to the geometry. Without the photon info
contained in there you would get notihng from your
photons.

Regards
Bri

__________________
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