Re: Pick dominant light from sh coeffs

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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 <
[email protected]> 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: [email protected]
>
> To: [email protected]
> 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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<div>So in=A0layman&#39;s terms I am using these three=A0<a href=3D"http://=
imgur.com/oOddh">http://imgur.com/oOddh</a>=A0coefficients to derive the di=
rection of the strongest light. Makes sense now.<div><br></div><div>That is=
 much easier than I expected, thank you for taking the time to explain it.<=
/div>
<br><div class=3D"gmail_quote">On Fri, Feb 3, 2012 at 10:21 AM, Peter-Pike =
Sloan <span dir=3D"ltr">&lt;<a href=3D"mailto:[email protected]"=
>[email protected]</a>&gt;</span> wrote:<br><blockquote class=3D=
"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding=
-left:1ex">



<div><div dir=3D"ltr">
This is actually quite straightforward.<div><br></div><div>The optimal line=
ar direction is -L[3],-L[1],L[3] - assuming the usual sign conventions, sto=
rage, etc.</div><div><br></div><div>where in your case L could be the lumin=
ance of the RGB SH vectors.</div>
<div><br></div><div>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.</div><div><br></div><div>That give y=
ou the direction.=A0</div>
<div><br></div><div>To generate the color of the light, you can simply eval=
uate the outgoing radiance using the above direction (as in the shader code=
 in the appendix), and that&#39;s the color for the light (if illuminating =
a white material that is the light in the given direction that would give y=
ou the same diffuse response as the SH probe.)</div>
<div><br></div><div>See the section of the paper titled: &quot;Extracting C=
onventional Lights from SH&quot; on a technique to solve for both a colored=
 directional light and an ambient light given SH coefficients, but if you a=
re using the storage in appendix 10 it&#39;s a bit trickier since the DC te=
rm has been &quot;polluted&quot; by part of the quadratic ZH function to ma=
ke evaluation faster. You would have to reconstruct the original vector to =
do the math...</div>
<div><br></div><div>Peter-Pike Sloan</div><div><br></div><div><br><div><div=
></div><hr>Date: Fri, 3 Feb 2012 09:22:21 +1000<br>From: <a href=3D"mailto:=
[email protected]" target=3D"_blank">[email protected]</a><div =
class=3D"im">
<br>To: <a href=3D"mailto:[email protected]" target=
=3D"_blank">[email protected]</a><br>Subject: [Algori=
thms] Pick dominant light from sh coeffs<br><br></div><div><div></div><div =
class=3D"h5">
<div>I have=A09 red, 9 green and 9 blue sh coefficients (packed using the m=
ethod in appendix 10 of=A0<a href=3D"http://www.ppsloan.org/publications/St=
upidSH36.pdf" target=3D"_blank">http://www.ppsloan.org/publications/StupidS=
H36.pdf</a>).</div>
<div>
<br></div>I want to pick a single=A0dominant=A0light to use for specular. H=
ow would I go about efficiently extracting the direction and color of that =
light from the coefficients?<div><br></div><div>Looks like I need to calcul=
ate the &quot;optimal linear direction&quot; which is supposedly in this pa=
per=A0<a href=3D"http://research.microsoft.com/en-us/um/people/johnsny/pape=
rs/ldprt.pdf" target=3D"_blank">http://research.microsoft.com/en-us/um/peop=
le/johnsny/papers/ldprt.pdf</a>,=A0however I can&#39;t see it.=A0Worse stil=
l, if it is in there, it is probably an integral that I will struggle to tu=
rn into code!</div>

<div><br></div><div>Thanks</div><div><br></div><div>Dan</div><div><div><br>=
</div><div><br></div></div>
<br></div></div>-----------------------------------------------------------=
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