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n00body

[SOLVED] For Physical BRDFs, why use such high specular powers (eg. 8192)?

3 posts in this topic

Background

So I have been looking into physically-based BRDFs for some time now, and have observed a perplexing trend. The companies that post the papers always seem to use a ridiculously high specular power (eg. Treyarch for COD BO uses 8192 for lights, 2048 for environment maps) when using a Normalized Blinn Phong NDF. This seems strange to me, especially since I have read that these powers are for extremely rare, super-polished mirror surfaces. Also, it means that they have to store a high-res mip level in their prefiltered environment maps that will take up a lot of space but almost never get used.

 

Up until now, I have usually settled for a maximum spec power of 1024 for my analytical lights, and 256 for my environment maps. For my bloom, I have been using a threshold of 1.0 and additive blending (what Unity provides). 

 

Problem

Whenever I test these specular powers myself, it causes the resulting specular highlight intensity to shoot up so high that my bloom goes crazy. In particular, as my camera moves I get very distracting flashes of bright spots on sharp model edges that appear and disappear in a very annoying way. 

 

Questions

1.) What is the incentive for using such high powers?

2.) Am I doing my bloom theshold wrong, and should I move it up?

3.) Is this a consequence of the Bloom blending mode?

 

Any help would be most appreciated.

Edited by n00body
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For phong/Blinn style specular, where high powers == smooth surfaces, a mirror has a power value of infinity. A power value of 10k is actually still fairly blurry compared to a mirror.
They use this as their maximum power value so they can have polished materials. Stuff like (undamaged) chrome or glass are likely candidates for those values.

As for your bloom filter, what does 1.0 mean for starters? What units is that in, and what's the significance?
More to the point -- this type of bloom filter is not based on physics/reality and is just a made up effect, so there's no right answer other than 'fiddle with it'.
A realistic bloom shouldn't have a threshold at all -- a threshold is saying that the lens is perfect for low-intensity objects but imperfect for high intensity ones!!

Lastly, what kind of tonemapper are you using, and what range of intensity values do you use?
I found that when using an intensity range similar to human perception (~12 orders of magnitude or 0 to 2^12) and a logarithmic tone-mapper, then thresholdless bloom filters suddenl looked ok, whereas before they looked overpowering...
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@Hodgman

Well one thing I discovered was that I made the faulty assumption that my team's artist added a tonemapper at all. Needless to say, adding that helped a lot. I am just using the Bloom and tonemappers that ship with Unity. The 1.0 cutoff just means anything above 1.0 in intensity contributes to bloom. And we are using the "Photographic" setting for the tonemapper. 

 

@MJP

Yeah, I figured it was probably mostly because of aliasing, but I was reluctant to go down that path so soon since Unity has absolutely awful support for custom mipmaps. I did manage to get it to work, and it helped a lot. 

 

@all

So yeah, I now understand the point of such high spec powers for up close smoothness. At a distance, the precomputed specular AA makes it appropriately more rough. 

 

Thanks. 

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