[Radiance-general] mkillum usage

[Marija Velickovic]

Hi Greg,

After years of work with Radiance, I can always learn something new :)

Related to FAQ, is Radiance Wiki still active, or  is there some other
knowledge database related to Radiance (beside list members :)


>
> *Question:* If outside environment is heavily obstructed, as it is
> usually the case in urban areas, how should we determine -av value for
> mkillum calculation? I think that simple usage of 'ground ambient value'
> for -av would overestimate luminance inside of the building.
>
> You are probably right that it should be set lower.  You can always set it
> to zero (the default) and accept that you will underestimate values
> slightly.  Otherwise, you can render a fisheye image just outside the
> window setting "-ab 0 -dv-" and compute the average pixel value with:
>
> pvalue -h -H -o -df  fisheye.hdr | total -if3 -m
>
> That's pretty close to what compamb does.
>
>  Interesting method, I'll surely try it.
So ambient value i get from this calculation I use -av parameters for
mkillum?

 Rad parameters:
> Beside setting of ambient parameters it is good idea to experiment a
> little and find nice set of -d* parameters.
> Example of parameters: -ab 3 -av 0 0 0 -ad 1024 -as 256 -aa 0.1 -ar 600
> -ps 1 -ds 0.05 -dj 1 -dt 0.03 -dc 0.9 -af tmp/floor1_illum.amb
>
>
> In daylighting situations, the calculation shouldn't be very sensitive to
> the -d* settings.  You can set -dt 0 if you're worried about it, though.
>

For nice luminance images of daylight spaces, I usually experiment with
these -d* parameters.
Combination of -d* parameters in example above (although too high and
computationally expensive) gives really nice images in most cases.

*Question:* Why it is not recommended to use mkillum for illuminance
> calculations?
>
> It's just that mkillum doesn't pay off if you are only calculating a small
> number of points.  The only reason to use it is to reduce the error when
> you cannot afford a difficult indirect calculation over a million or so
> pixels.  If you are only computing a few hundred illuminance values to
> begin with, you are better off simply cranking up the indirect calculation
> parameters and letting rtrace deal with the window complexity.
>

We usually calculate illuminance images over complete workplane and set
rather high ambient parameters ab>=5, and calculations of an image take
several hours.
Maybe mkillum in such process can make calculation time shorter, but again
we should determine correct ambient value previous to mkillum running. Also
if project has big number of windows, calculation of mkillum would take
additional time - so no real time saving in this approach.

As a conclusion: For illuminance calculations better use high ambient
parameters and avoid mkillum.

Thanks again,
Marija.
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