[Radiance-general] possible inconsistency of bin numbers between BSDF and kelms_int.cal

[Jia Hu]

Hi Andy:

"the patch numbers will be different for incident and exiting hemispheres.
 one side uses left handed numbering the other uses right handed numbering."
------
Yes, from this figure  https://dl.dropbox.com/u/15562376/coordinate.jpg, Z
hemisphere coordinate uses right handed and Zr uses left handed, but for
forward rays (both incoming and outgoing rays whose direction is from
inside room to outside), these rays will only use Z hemisphere, Zr
hemisphere does not exist for those forward rays, and thus no left handed
coordinate in this sense. Thus outgoing rays still uses Z hemisphere patch
number.

To make it clear, you may read this report
http://windows.lbl.gov/software/window/6/Venetian%20Technical%20Document.pdf ,
*Page 6, 7, 10 and 47 -50*, the report shows that both incoming and exiting
rays' patch numbers are based on only one Hemisphere, either Z or Zr
hemisphere but not both, that is why, *if you see the page 49 in the report
*( or https://dl.dropbox.com/u/15562376/PDF_page_49.jpg*)*, the incoming
patch number is 31 and the outgoing patch is still 31 because they are
using the same hemisphere. In the outgoing side, there is no hemisphere and
outgoing ray still uses the hemisphere in the incoming ray side to
determine the patch number. That is why they have the same patch numbers if
their directions are the same.

I think you figure is correct if you stand inside the room and look at the
window. Klems_int.cal uses Z hemisphere to determine the patch number.

My concern is that Radiance loads "Transmission Back" data which uses Zr
hemisphere but VewMatrix and DaylightMatrix uses Z hemisphere. In other
words, when Radiance loads "Transmission Back" data, it assumes all the
patch number is determined based on Zr hemisphere (both incoming and
outgoing rays patch number is based on this Zr hemisphere). But V and D,
the patch numbers of both incoming and outgoing rays  determine patch
numbers based on Z hemisphere (right hemisphere in the figure).

Thanks,

Jia

On Mon, Oct 22, 2012 at 12:45 PM, Andrew McNeil <amcneil at lbl.gov> wrote:

> Jia,
> This diagram: https://dl.dropbox.com/u/15562376/bin.jpg is wrong.
> You should look at klems_int.cal to determine what the patch numbers are.
>  My bad.
> Andy
>
> On Sat, Oct 20, 2012 at 5:11 AM, Jia Hu <hujia06 at gmail.com> wrote:
>
>> Hi all:
>>
>> I have a question about the bin number.
>>
>> To calculate view matrix:  (south-facing window)
>> $ rtcontrib < test.pts -f klems_int.cal -bn Nkbins -b kbinS -bn Nkbins -m
>> window_mat -I+ -ab 3 -ad 2000 -lw 1e-4 interior.oct > south.vmx
>>
>> kbinS uses the (0, 1, 0) as normal (pointing to inside room).
>>
>> For a ray direction (-1, -1, 0), using klems_int.cal to calculate the bin
>> number, the bin number is about #57 (I did it manually).
>>
>> Klems_int.cal (download Klems_init.cal<https://dl.dropbox.com/u/15562376/klems_int.cal>)
>> seems use "z" hemisphere (see this figure:
>> https://dl.dropbox.com/u/15562376/coordinate.jpg). From the figure, we
>> can see "z" hemisphere points to the room (that is, the hemisphere is
>> located in the room) "zr" (z reverse) hemisphere is towards outside (the
>> hemisphere is outside the room).
>>
>> The possible inconsistency is:
>>
>> (1) Klems_int.cal uses "z" hemisphere to calculate the bin number, for
>> example,  for ray direction (-1, -1, 0), the bin number is #57 for "z"
>> hemisphere coordinate. (see this figure:
>> https://dl.dropbox.com/u/15562376/bin.jpg)
>>
>> (2) The "Transmission Back" data in the BSDF .XML is used in the three
>> phase DC method (see Greg's explanation<http://permalink.gmane.org/gmane.comp.graphics.radiance.general/2729>),
>> and thus the calculation is based on "zr" (z reverse) hemisphere , for ray
>> direction (-1, -1, 0), the bin number is about #69 according to the "z
>> reverse" hemisphere coordinate (see this figure:
>> https://dl.dropbox.com/u/15562376/bin.jpg).
>>
>> So my questions are:
>>
>> (a) Given the ray direction, which hemisphere ( "z" or "zr" ) does
>>  "Klems_init.cal" use?
>>
>> (b) whether the bin numbers (for the same ray direction) between
>> "Klems_int.cal" and BSDF .xml  are inconsistent or not?
>>
>> (c) if they are using different bin numbers, how to handle ViewMatrix *
>> BSDF in the calculation?
>>
>> Thank you in advance,
>>
>> Jia
>>
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>>
>
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