Re: [Algorithms] Pick dominant light from sh coeffs

[Dan T. <dan...@gm...>]

So in layman's terms I am using these three http://imgur.com/oOddh coefficients
to derive the direction of the strongest light. Makes sense now.

That is much easier than I expected, thank you for taking the time to
explain it.

On Fri, Feb 3, 2012 at 10:21 AM, Peter-Pike Sloan <
pet...@ho...> wrote:

>  This is actually quite straightforward.
>
> The optimal linear direction is -L[3],-L[1],L[3] - assuming the usual sign
> conventions, storage, etc.
>
> where in your case L could be the luminance of the RGB SH vectors.
>
> If you are using the coefficients from appendix 10, it turns out to be
> even simpler - it is just the .xyz coefficients after doing a luminance
> weighting of cAr/cAg/cAb.
>
> That give you the direction.
>
> To generate the color of the light, you can simply evaluate the outgoing
> radiance using the above direction (as in the shader code in the appendix),
> and that's the color for the light (if illuminating a white material that
> is the light in the given direction that would give you the same diffuse
> response as the SH probe.)
>
> See the section of the paper titled: "Extracting Conventional Lights from
> SH" on a technique to solve for both a colored directional light and an
> ambient light given SH coefficients, but if you are using the storage in
> appendix 10 it's a bit trickier since the DC term has been "polluted" by
> part of the quadratic ZH function to make evaluation faster. You would have
> to reconstruct the original vector to do the math...
>
> Peter-Pike Sloan
>
>
> ------------------------------
> Date: Fri, 3 Feb 2012 09:22:21 +1000
> From: dan...@gm...
>
> To: gda...@li...
> Subject: [Algorithms] Pick dominant light from sh coeffs
>
> I have 9 red, 9 green and 9 blue sh coefficients (packed using the method
> in appendix 10 of http://www.ppsloan.org/publications/StupidSH36.pdf).
>
> I want to pick a single dominant light to use for specular. How would I go
> about efficiently extracting the direction and color of that light from the
> coefficients?
>
> Looks like I need to calculate the "optimal linear direction" which is
> supposedly in this paper
> http://research.microsoft.com/en-us/um/people/johnsny/papers/ldprt.pdf, however
> I can't see it. Worse still, if it is in there, it is probably an integral
> that I will struggle to turn into code!
>
> Thanks
>
> Dan
>
>
>
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