Re: Some question about "Lighting and Material of Halo3" and "Lightmap compression in Halo 3"

Hi,
I believe the source of confusion is that in this equation (5) things are expressed in the local coordinate frame, the fact that they show 3 coefficients implies they are using quadratic SH (so when rotated into the local frame and integrated against a clamped cosine function you only need the 3 ZH coefficients.)  The general case (where you don't know the coordinate frame, or you want to evaluate for any normal) requires all 9 coefficients.

I think the "5*3" comes from the compression mentioned in Hao's slides (linear SH + RGB for directional light in "optimal linear" direction.  Which is 5*3 scalars.)
The compression work is Wang et al, I think Yaohua's slides are more indicative of what was actually used...
Peter-Pike Sloan
Date: Fri, 29 Jan 2010 16:28:57 +0100
From: [email protected]
To: [email protected]
Subject: [Algorithms] Some question about "Lighting and Material of Halo3"	and "Lightmap compression in Halo 3"
















Hello all, 

 

I tried to contact the author of this two
(now old) paper, without success,:

"Lighting and Material of Halo 3"
published at siggraph 2008 (http://ati.amd.com/developer/SIGGRAPH08/Chapter01-Chen-Lighting_and_Material_of_Halo3.pdf)

and GDC 2008
conference "Lightmap compression in Halo 3" (http://toomuchlogic.com/Lightmap_Compression_2008_02_22.pdf)

so I will ask some question to the list, if
anyone is interested by the subject and has better understanding of math than
me :)

 

1. "Lighting and Material of Halo 3"
 About equation (5) the diffuse reflectance using SH basis

the diffuse reflectance using SH basis is :
k_d R_d Sum Lambda_i A_i

A_i is the projection of the cosine lobe in
SH, and as it is radially symetric.

all coefficient with m != 0 are 0.

After that the author give the first three
term of A_i.

I wondering how many band are use for this
calculation  ? (order 3, 4 or more ?)

 

I read from "Stupid spherical
harmonics tricks" from Perter Pike sloan (http://www.ppsloan.org/publications/StupidSH36.pdf)

that order 3 SH is sufficient for
approximate light source

but for HDR light sources he recommand
order 5.

As order 4 is 0 (From paper
http://www.eecs.berkeley.edu/~ravir/lighting.pdf I get the formula for A_i)

This mean 4 coefficient to store by color
channel.

As I am pretty sure the author store  HDR
data, can someone lighten me ?

 

2. About texture storage (which are deduced
from above statement)

 

I try to figure out how are encoded the
incident radiance in their SHLightmap.

>From GDC 2008 conference "Lightmap
compression in Halo 3" I can read that the author need to

store for each texel a vector of 5 * 3 float
values.

I don't figure what are the values exactly.

 

My assumption is that "3" is for
each channel color RGB,

But I can't figure what's the 5 is ?

Are they 5 first band of SH order like I suppose
above (but as I said, we only need 4 coefficient in this case)

or maybe order 6 ?

 

In this same paper later I found:

A. Two DXT5 texture for each SH coefficient
channel (HDR, positive/negative)

And

B. Each band of the SH coefficients (RGB)
are converted to Luvw space

 

I suppose that Luvw space is what is
describe in this paper "Rendering from Compressed High Dynamic Range
Textures on

Programmable Graphics Hardware" by
Peter Pike sloan and al.(ftp://ftp.research.microsoft.com/pub/tr/TR-2006-152.pdf)

 

What I don't understand is that A and B
seem different. I can't understand what is store.

Are they storing for each band the triplet
RGB of SH coeeficient, mean 3 float value for two DXT5 x order

or do they store store 5 SH coefficient for
channel color R in two DXT5 ?

 

So what are the total storage cost of all
the 5 * 3 float value in term of DXT5 texture ?

Cause It looks like to be pretty big.

 

Thanks for anyone interested by this post

 

Best regards

 

Lagarde Sébastien

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