Mkillum usage? with stained glass....

I'm recreating a medieval chapter house from ruins in radiance for my
thesis at uni. Im using the mkillum lighting technique on the windows,
but I have some questions which I need to ask to be sure I'm approaching
the problem correctly..

Firstly - Ive read a lot about the mkillum approach, so for stained
glass I'm guessing I create an object that is textured glass material,
and have the mkillum secondary source behind it? Shining through. This
presumably will project the colours of the glass into the room?

Secondly - You can see mkillum sources from the rear side, I'm guessing
you turn them off for renders of the outside of buildings looking at the
windows?

Secondly - I'm really interested in the internal lighting, how bright
the inside will be at different times of day etc. When I render
internally with no av settings (rview), and set e to 1 is it bringing
the environment into a range that we can see? What im saying is, how do
I know what settings to use to get the most realistic representation of
what it would have been like to the human eye? I know this is a tricky
problem considering that the human eye adjusts to light. How can I
approach this problem?

Many thanks

(im new to this mailing list so go easy on me)

John Sutherland

Firstly – Ive read a lot about the mkillum approach, so for stained glass I’m guessing I create an object that is textured glass material, and have the mkillum secondary source behind it? Shining through. This presumably will project the colours of the glass into the room?

I still struggle with the correct usage of illum, but let's have a go at it. Generally, for most accurate results, the glass panes themselves would be converted to illums. But in this case, I'm imagining that your window is comprised of lots of irregularly shaped pieces of glass. These will never resolve properly when mkillum tries to convert these polygons into illums. I'd guess your best bet is to create some impostor geometry just on the *inside* of the stained glass window geometry.

Secondly – You can see mkillum sources from the rear side, I’m guessing you turn them off for renders of the outside of buildings looking at the windows?

Hmm. For an exterior rendering, I'd turn them off, but not for the reason you mention. Illums are invisible when visible. That is to say, in views where the illum geometry is directly visible, it is not rendered. So, don't worry about excluding illums from your scenes if visibility is ever a concern.

But in this example case, a rendering outside the building, the illums aren't contributing anything to the image so why even waste time adding them to the scene and calculating them? So, for your exterior views you should use a different set of parameters, and leave out the stained window illum geometry (the stained glass itself will get illuminated by your sky & sun and thus will still look correct.)

Secondly – I’m really interested in the internal lighting, how bright the inside will be at different times of day etc. When I render internally with no av settings (rview), and set e to 1 is it bringing the environment into a range that we can see? What im saying is, how do I know what settings to use to get the most realistic representation of what it would have been like to the human eye? I know this is a tricky problem considering that the human eye adjusts to light. How can I approach this problem?

Whoo-whee. Well, that is a monster question. Simple answer is that there is no simple answer to this. Every model you do in Radiance will present new challenges, but Radiance by and large can handle whatever you throw at it. Have you used the rad program yet? Rad will certainly get you going in the right direction quickly. You can also save the various parameters that rad feeds to rpict and inspect them later. It's quite educational. You're on the right track with illum there, that will improve your results and accuracy immensely. Careful application of mkillum, a handful of ambient bounces and say Q=M V=H D=[M or H] for your rad settings and you'll be in good shape, in general.

Your question about eye adaptation leads me to one of my favorite tools in the Radiance suite, pcond. Pcond is a tonemapping program which already has a very useful commandline parameter (-h) built into it to simulate human visibility in a Radiance image. So, after you generate your radiance images, you simply run them through pcond like so:

pcond -h image.pic image-human.pic

Pcond takes the radiance image and performs a tonemapping operation. The results are fantastic, and it's really one of the best ways to demonstrate the power of Radiance to people.

Have fun, and welcome aboard!

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On Mar 15, 2004, at 8:05 PM, John Sutherland wrote:

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Rob Guglielmetti
www.rumblestrip.org