Order of GenDayMtx values

radiance-general
1

Hi all,

I would like to check that my assumptions are correct regarding how the GenDayMtx rows are ordered.

The docs say:
“By default, gendaymtx assumes the positive Y-axis points north such that the first sky patch is in the Y-axis direction on the horizon, the second patch is just west of that, and so on spiraling around to the final patch near the zenith.”

I assume this means ant-clockwise ordering.

So for “gendaymtx -m 1 -A -c 1 1 1 singapore.wea > singapore.mtx”

We get 146 rows, if we count from zero

  • 0: ground
  • 1-30: altitude 6 degrees, patches start with north in Y direction, and are ordered anti clockwise
  • 31-60: altitude 18 degrees, patches start with north in Y direction, and are ordered anti clockwise
  • 61-74: altitude 30 degrees, patches start with north in Y direction, and are ordered anti clockwise
    etc
  • 145: the zenith patch

Also, may I check that the first patch for each layer, in the north direction, will have the center of the patch directly north, i.e. for the bottom layer the patch will be from azimuth -6 degrees to azimuth 6 degrees (and not from azimuth 0 to azimuth 12).

Patrick

2

Yes, I believe your description is correct for the location of the center of each patch, except that the counts aren’t quite right for each row. See the attached diagram, which is the sky dome as seen looking upwards with north at the top.

Cheers,
-Greg

TregenzaDiagram
TregenzaDiagram1442×1442 289 KB

3

Thanks Greg for the quick and clear response. It is kind of what I expected when I looked at the data, but I was thrown by the documentation.

Am I right that this is an error in the documentation. It says “the second patch is just west of that”. It should say “the second patch is just east of that”?

4

This array is ordered as looking up from the ground, not down from the sky, so patch 2 is just west of due north (patch 1). You have to mirror the normal compass configuration.

5

Sorry, I think I did not read your previous response carefully. I am afraid my brain is having some difficulty with the ‘look up and mirror compass’ concept, but never mind.

My understanding now is that when looking down at the sky dome from above, the rows are in fact ordered anti-clockwise.

6

Correct, clockwise looking up, counter-clockwise looking down.