[Radiance-general] Re: Cubical sky maps?

[R Fritz]

Oh, cool.

Thanks!

Randolph

On 2010-06-13 09:13:34 -0700, Greg Ward said:

> Using a cube map and actually mapping a cube are separable issues. You 
> don't need local geometry to do this in other words.  The mapping,
> which you can translate pretty easily to a .cal file, is given in Paul
> Debevec's image-based lighting chapter in the HDR Imaging book,Section 
> 9.4.4.  Quoting:
> 
> World to Image:
> if Dz<0 and Dz<Dx and Dz<Dy
> u = 1.5 - 0.5 * Dx / Dz
> v = 2.5 - 0.5 * Dy / Dz
> else if Dz>0 and Dz>Dx and Dz>Dy
> u = 1.5 + 0.5 * Dx / Dz
> v = 0.5 + 0.5 * Dy / Dz
> else if Dy<0 and Dy<Dx and Dy<Dz
> u = 1.5 + 0.5 * Dx / Dy
> v = 1.5 - 0.5 * Dz / Dy
> else if Dy>0 and Dy>Dx and Dy>Dz
> u = 1.5 + 0.5 * Dx / Dy
> v = 3.5 + 0.5 * Dz / Dy
> else if Dx<0 and Dx<Dy and Dx<Dz
> u = 0.5 - 0.5 * Dz / Dx
> v = 2.5 + 0.5 * Dy / Dx
> else if Dx>0 and Dx>Dy and Dx>Dz
> u = 2.5 + 0.5 * Dz / Dx
> v = 2.5 + 0.5 * Dy / Dx
> 
> Image to World:
> if u>=1 and u <=2 and v>=0 and v<=1 // up
> Vx = (u - 0.5) * 6
> Vy = 1.0
> Vz = (v - 0.125) * -8
> else if u>=0 and u <=1 and v>=1 and v<=2 // left
> Vx = -1.0;
> Vy = (v - 0.375) * -8;
> Vz = (u – 1/6) * -6;
> else if u>=1 and u <=2 and v>=1 and v<=2 // forward
> Vx = (u - 0.5) * 6;
> Vy = (v - 0.375) * -8;
> Vz = -1.0;
> else if u>=2 and u <=3 and v>=1 and v<=2 // right
> Vx = 1.0;
> Vy = (v - 0.375) * -8;
> Vz = (u – 5/6) * 6;
> else if u>=1 and u <=2 and v>=2 and v<=4 // down
> Vx = (u - 0.5) * 6;
> Vy = -1.0;
> Vz = (v - 0.625) * 8;
> else if u>=1 and u <=2 and v>=3 and v<=4 // backward
> Vx = (u - 0.5) * 6;
> Vy = (v - 0.875) * 8;
> Vz = 1.0;
> normalize = 1 / sqrt(Vx * Vx + Vy * Vy + Vz * Vz)
> Dx = normalize* Vx
> Dy = normalize* Vy
> Dz = normalize* Vz
> 
> Best,
> -Greg
> 
>> From: R Fritz <RFritz at lbl.gov>
>> Date: June 13, 2010 7:56:16 AM PDT
>> 
>> On 2010-06-12 02:41:27 -0700, Lars O. Grobe said:
>> 
>>> On 11.06.2010 18:57, R Fritz wrote:
>>>> Anyone using these? Anyone think these might be possible?
>>> Should be possible, but why give up the advantages of the>> 
>>> dimensionless source-object? With cubical maps you must think about
> 
>>> where in you cube the scene is to be located, how much related>> error 
>>> to accept, then, how to get the increased scene bounding box
> 
>>> reflected in the ambient setting...
>> 
>> Thanks, Lars. The big reason for doing this is to make it possible> to 
>> use a captured sky, and it's probably easier to keep the exterior
> 
>> housing containing cameras clean if it's got flat surfaces, than if
> 
>> it's a sphere.


-- 
Randolph Fritz • RFritz at lbl.gov
Environmental Energy Technologies Division • Lawrence Berkeley Labs