[Peder]

Hello all!
With all the expertise on light propagation on this forum I thought I would pose a question to y'all.:

When measuring the ability of a lightsource to render colour a CRI value is used. This value is established by taking 8 colour samples and looking how well the colour is represented in comparison with a reference lightsource with a perfect colour rendering ability -typically an incandescent source. The problem is that the lightsource can be poor in rendering 4 colours and really good in four. This will achieve the same result as a lightsource which is halfway decent in reproducing all 8 colours.

Posing an opinion:
In my view the most important ability for a lightsource is to reproduce the human skin. We are social beings where the face of another person is the most important visual element in an enviroment. So the problem is that perhaps some of these lightsources are really poor at rendering human skin but get the same points for color rendering as one that is good at this task.

Also it is important for many reasons for humans to make assessments about the health of the person we are interacting with, and with poor colour rendering people tend to look sick - I suppose there is some similarities in the way light is propagated through sick tissue as with a lightsource with poor CRI.

Also often the impression of human skin is that it is a solid surface when in fact (at least in healthy skin) a lot of the colour comes from tissue below the surface.

My impression of some fluorescent tubes is that they change the relationship between the direct reflection of the surface and the subsurface scattering that occurs in comparison with an incandescent lightsource.

Can anyone point me toward research in this field. Does anyone have opinions on this subject? I would also be interested in how this is handled in CG.

Peder

[deadalvs]

healthy tissue...

http://www.highend3d.com/gallery/charac ... php?id=757




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interesting question...

for me personally, i think this is not a problem of special rendering or more accurate simulation of real world but just a problem of post work. in cg, You will never have the time to tweak all the parameters and depth properties of human skin if You want to render it. this is just all done in post production.

or... ?

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deadalvs

[Peder]

Will Maxwell in -it's strive for absolute realism- support noncontious spectrums from lightsources -like those emitted by fluorescents?

Peder

[deadalvs]

You mean an aequivalent for the IOR materials, just on the emitter side ?

hmm... it would already be great to define an emitter by the spectral graph one could draw over the whole visible spectral range. (maybe this could be cool...)


and there should be also an other way to tweak the sun intensity other than with multilight...

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deadalvs

[Peder]

That would be cool. A side effect would be the ability to define colours as they are produced in theatre gels:

http://www.leefilters.com/LPFD.asp?PageID=420

Peder

[Peder]

The reason for me ranting on about this is the fact that human vision is similar to a display in that we have receptors in the eye that respond to the three primary colours. The problem here is that if we take for instance an RGB LED lighting instrument we can indeed produce millions of nuances, but we will still fail to get light back from many nuances of the object we shine this light on. The colour of the light is not what we see. To get an accurate representation of the surfaces the eye needs to see what frequencies can get bounced back.

I suppose this "deadening" effect on skin is not present when you look at an image on a tv. But look the other way at the face of the person sitting next to you watching that screen and you will (hopefully) see my point...

Peder

[Peder]

Here is a link to an interesting article on the subject of the relationship between light and the rendering of object colour.

http://www.jimworthey.com/RndrAsk.pdf
http://www.jimworthey.com/RndrAskFigs.pdf

It would be really great to hear some input from NL on these issues and how Maxwell might handle them. As I understand it not only lightsources may have an nonlinear frequency. Materials may also have a nonlinear frequency response as expressed as "metamerism"
http://en.wikipedia.org/wiki/Metamerism_(color)

Peder