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

[Jia Hu]

Hi all:

In the Klems_init.cal file, for south window (using kbinS), the Klems
coordinate is defined as:

z+: pointing to inside (that is, (0,1, 0)  )
x+: pointing to West ( that is (-1, 0, 0) )
y+: pointing to Up (0,0,1)

This coordinate is consistent with the Klems coordinate system used in
WINDOW 6 (see this: https://dl.dropbox.com/u/15562376/coordinate.jpg). In
the figure, Klems coordinate system defines: x or xr points to (-1, 0, 0)
and  y or yr points to (0, 0, 1).

However, if using this command to calculate view matrix:

$ 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

where, kbinS defines z axis in the Klems coordinate system as (0 1 0), that
is, pointing to room interior.

Thus, the bin number calculated by Klems_init.cal is actually based on z
hemisphere <https://dl.dropbox.com/u/15562376/coordinate.jpg> (the
hemisphere protrudes towards room). the bin number corresponds with
"Transmittance Front" in BSDF xml file. For example, ray direction (-1, -1,
0) (backward, ray travels from inside to outside room) hits the bin
58<https://dl.dropbox.com/u/15562376/bin.jpg>
defined in z hemisphere because the hemisphere protrudes towards (0 1 0).
 But ray direction (-1 -1 0) hits bin
46<https://dl.dropbox.com/u/15562376/bin.jpg> defined in
z reverse (i.e., zr) coordinate because the hemisphere protrudes towards (0
-1 0) (towards outside).

When calculating Daylighting Matrix, it still uses Klems_init.cal and
kbinS, that is, z hemisphere coordinate system, therefore, Radiance seems
needing to use "Transmission Front", which is defined in z hemisphere.  But
currently Radiance uses "Transmission Back" (from inside to outside), which
defined in zr hemisphere,

So, should we need to

 (a) use "*kbinN*" (defining z axis of Klems coordinate towards (0 -1 0) )
 and "Transmission Back" for south window,  or
 (b) use "Transmission Front" and *kbinS*?

 I am very confused about this. Does anyone know the possible reasons?

Thank you !

Jia


On Sat, Oct 20, 2012 at 8:11 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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