Luminance variation with distance

I am a na�ve user of Radiance involved in modeling various lighting
conditions encountered during spacecraft docking/berthing operations.
Recently I encountered some unexpected luminance values when modeling
distant views of docking lights.

As the separation between the viewpoint and a light source is increased, the
luminance of the source should remain constant. Sources I have modeled
using Radiance behave in this way until they occupy a solid angle relative
to the observer of less than 4.9e-06 steradian or so (for example, a
two-inch radius disk source at 134 feet). Beyond this distance, however,
the modeled luminance of the source decreases in inverse proportion to the
square of the separation distance. This behavior is only exhibited by light
sources. In contrast, models of small reflective surfaces illuminated by a
fixed local source exhibit constant luminance as the surface viewing
distance is increased, finally disappearing from view altogether when they
occupy a solid viewing angle of approximately 2.2e-06 steradian. Can anyone
explain the light sources'departure from ideal constancy of luminance with
distance?

anyone interested.

Charles K. Bowen, Ph.D.
Graphics Research and Analysis Facility (GRAF)
Johnson Space Center, NASA

we know didly squat!

···

-----Original Message-----
From: BOWEN, CHARLES K. (JSC-SF5) (JEC)
[SMTP:[email protected]]
Sent: 10 December 2002 20:55
To: '[email protected]'
Subject: [Radiance-general] Luminance variation with distance

I am a na�ve user of Radiance involved in modeling various lighting
conditions encountered during spacecraft docking/berthing operations.
Recently I encountered some unexpected luminance values when modeling
distant views of docking lights.

As the separation between the viewpoint and a light source is increased,
the
luminance of the source should remain constant. Sources I have modeled
using Radiance behave in this way until they occupy a solid angle relative
to the observer of less than 4.9e-06 steradian or so (for example, a
two-inch radius disk source at 134 feet). Beyond this distance, however,
the modeled luminance of the source decreases in inverse proportion to the
square of the separation distance. This behavior is only exhibited by
light
sources. In contrast, models of small reflective surfaces illuminated by
a
fixed local source exhibit constant luminance as the surface viewing
distance is increased, finally disappearing from view altogether when they
occupy a solid viewing angle of approximately 2.2e-06 steradian. Can
anyone
explain the light sources'departure from ideal constancy of luminance with
distance?

anyone interested.

Charles K. Bowen, Ph.D.
Graphics Research and Analysis Facility (GRAF)
Johnson Space Center, NASA

_______________________________________________
[email protected]

I appologise for the last e-mail - it was not intended to trouble the group.

Regards

Andrew

···

-----Original Message-----
From: Andrew Bissell [SMTP:[email protected]]
Sent: 12 December 2002 16:42
To: '[email protected]'
Subject: RE: [Radiance-general] Luminance variation with distance

we know didly squat!

> -----Original Message-----
> From: BOWEN, CHARLES K. (JSC-SF5) (JEC)
> [SMTP:[email protected]]
> Sent: 10 December 2002 20:55
> To: '[email protected]'
> Subject: [Radiance-general] Luminance variation with distance
>
> I am a na�ve user of Radiance involved in modeling various lighting
> conditions encountered during spacecraft docking/berthing operations.
> Recently I encountered some unexpected luminance values when modeling
> distant views of docking lights.
>
> As the separation between the viewpoint and a light source is increased,
> the
> luminance of the source should remain constant. Sources I have modeled
> using Radiance behave in this way until they occupy a solid angle
relative
> to the observer of less than 4.9e-06 steradian or so (for example, a
> two-inch radius disk source at 134 feet). Beyond this distance,
however,
> the modeled luminance of the source decreases in inverse proportion to
the
> square of the separation distance. This behavior is only exhibited by
> light
> sources. In contrast, models of small reflective surfaces illuminated
by
> a
> fixed local source exhibit constant luminance as the surface viewing
> distance is increased, finally disappearing from view altogether when
they
> occupy a solid viewing angle of approximately 2.2e-06 steradian. Can
> anyone
> explain the light sources'departure from ideal constancy of luminance
with
> distance?
>
> anyone interested.
>
> Charles K. Bowen, Ph.D.
> Graphics Research and Analysis Facility (GRAF)
> Johnson Space Center, NASA
>
>
> _______________________________________________
> [email protected]

_______________________________________________
[email protected]

Hi Charles !

I haven't examined the matter in full detail, but I think I may give a
first hint:

In rpict, an extra "source-drawing" function is called for tiny sources,
(resp. those appearing tiny) and this is done (most probably) for the
following reason: If an object is very small and far away, it may not be
hit by a view ray at all. That wouldn't matter much for normal stuff,
but for sources, this might be confusing. So this "drawsources" function
assures that at least a bright spot appears.
I've tested it the other way, if the routine is disabled, a tiny source
becomes invisible when the distance is too big or the solid angle too
small, (ressembling the disappearing of your illuminated objects). I
noticed the effect already at solid angles of 1e-5 steradian, of course
it depends strongly on the absolute pixel resolution of the image.

As said, I haven't examined this "drawsources" routine, but probably it
doesn't perform an exact calculation as its purpose is just to create an
impression of the source being there far away as tiny spot. This could
explain the deviations observed by you. Although, I am wondering about
this, too, as just displaying the original luminance would be no
problem. Hmm?!?

-Carsten

Hi again,

just one more short contemplation on the matter: In order for something
to appear in an image, obviously the minimum drawing size is a pixel.
This means if far away tiny sources are to be drawn in the image to
generate a vivid impression, they have to be spread out over a larger
area, say, e.g from a quarter of a pixel to a full pixel. In order not
to exaggerate, it makes sense to reduce the luminance accordingly. Now,
the necessary amout of "spreading out" increases proportional to the
square of the distance, so the luminance has to decrease accordingly,

-Carsten

Hi Charles,

Carsten is probably right -- the src/rt/srcdraw.c routines are likely responsible for the observations you are making in your renderings. These routines were included to prevent annoying aliasing artifacts that result when sources are too small to be reliably sampled by rpict view rays.

I can explain the reasoning behind the routines thus. Normally, rpict is followed by a pfilt command to perform anti-aliasing, and ideally you would send such a large number of initial samples (oversampling) that you were guaranteed to see everything there is to see in each pixel. These pixels would then be filtered down (averaged together) to compute the final output pixel, which would then be equivalent to the photon sampling that goes on in a physical camera. (Equivalent -- not identical, as rpict simulates an ideal pinhole camera with zero extent.) If we were able to perform this calculation in our lifetimes, what we would see for light sources (and for all objects) is a constant pixel luminance while the object completely covered the pixel, then a proportionally decreasing value as the object covered less than a full pixel as it moved farther away. As Carsten pointed out, an object's diminishing size with distance is proportional to 1/r^2 since there are two dimensions to an image, and that is what you should see and would see with a real camera.

In short, if you apply pfilt following rpict, you should see an approximation of this behavior for all objects, not only light sources. Since the straight output of rpict has only one sample (at most) per pixel, the sample either hits an objec or it doesn't. The reason you get effective oversampling for light sources is because of the special routines in srcdraw.c. The approximation in these routines is not exact (nothing is exact in a simulation), but it's fairly close to being correct -- certainly a lot closer than you could get with Monte Carlo sampling.

If this behavior is a problem, you can couple the vwrays program with rtrace to avoid all of rpict's sampling optimizations.

-Greg

···

From: "BOWEN, CHARLES K. (JSC-SF5) (JEC)" <[email protected]>
To: "'[email protected]'"
<[email protected]>
Date: Tue, 10 Dec 2002 14:55:12 -0600
Subject: [Radiance-general] Luminance variation with distance

I am a na�ve user of Radiance involved in modeling various lighting
conditions encountered during spacecraft docking/berthing operations.
Recently I encountered some unexpected luminance values when modeling
distant views of docking lights.

As the separation between the viewpoint and a light source is increased, the
luminance of the source should remain constant. Sources I have modeled
using Radiance behave in this way until they occupy a solid angle relative
to the observer of less than 4.9e-06 steradian or so (for example, a
two-inch radius disk source at 134 feet). Beyond this distance, however,
the modeled luminance of the source decreases in inverse proportion to the
square of the separation distance. This behavior is only exhibited by light
sources. In contrast, models of small reflective surfaces illuminated by a
fixed local source exhibit constant luminance as the surface viewing
distance is increased, finally disappearing from view altogether when they
occupy a solid viewing angle of approximately 2.2e-06 steradian. Can anyone
explain the light sources'departure from ideal constancy of luminance with
distance?