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

[Sam M. <sam...@ge...>]

Hi Peter,

 

Do you have a link to a paper/slides explaining why they store an "optimal linear" direction separately? I'm not sure why you wouldn't want to just use the L1 coeffs as a vector?

 

I couldn't find an explanation in the links below. 

 

Thanks,

Sam

 

From: Peter-Pike Sloan [mailto:pet...@ho...] 
Sent: 01 February 2010 21:45
To: gda...@li...
Subject: Re: [Algorithms] 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: seb...@do...
To: gda...@li...
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