real-world lighting guide?

[jeremybirn]

If we _were_ using Photometric light definitions, there are some sites with tables of data for different bulb types. Here's one with some data (scroll down to LIGHTING PHOTOMETRICS):
http://www.mts.net/~william5/library1.htm

Renderers that use Photometric light definitions (I remember LightScape was like that) generally also include menus of light bulb types, so an architect could pick a particular type of hallogen bulb, and approximate the appearance of that kind of bulb lighting a room.


But, we're not using Photometric light models. The intensity for a normal 100 watt light bulb in Mentalray might be perfectly reproduced with an intensity of 1000 in one photoreal scene, and an intensity of .1 in another perfectly matched photoreal scene - for the exact same lightbulb. And they could both be be dead-on perfect matches to live-action plates lit with that bulb.

-jeremy

[sean_G]

thank you for your prompt and informative response Jeremy. although like most good answers, yours inspired a few more questions. . . regaurding an established equivalency model between CGI lighting and real-world counterparts.

if 'we' arent using photometric light definitions, then what kind of light calculations are we using? --by 'we' i take you to mean users of Mental ray and Renderman.--

why is it possible to have such a vast descrepency in reletive lighting properties for the same scene in Mentalray, such as you described?-- doesnt this make it MORE problematic for shading artists to create materials that will have precisely appropriate properties in any given scene, despite the reletive scale of the environment and artist whose going to light it?

that Lightscape you were talking about sounds great, every render-engine should incorporate that kind of user-independant functionality. once again thank you for your reply~and your work

[jeremybirn]

I didn't mean to make Photometric techniques sound too desirable - tweakable and controllable lights are better for most professional work than complex presets based on hard-to-edit data files.

The brightness you use in lights in Mental Ray is very close to the final brightness of the rendered image - there is no simulation of a camera with different exposure settings, nor of a human eye adjusting itself to different environments. This makes it easy to precisely control your lights, develop all your shaders and maps in a standard lighting setting with the Key at 1 (with no decay), and adjust your lights without editing huge complex files full of hard-to-tweak data.

Sure,you need to compensate for things: apply a dirt mapping technique to the diffuse on your shaders, you may have to crank the light brighter. Set the Decay to 2 (Quadratic) and you may need a brightness of 1000 instead of 1, depending on scene size. Render Las Vegas at night, and you might need each light very dim to avoid overexposing the street. But, basically, it's a simpler, more direct, controllable approach than having gobs of presets.

-jeremy

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