[Radiance-general] Re: physically-based landscapes

[Digital Architectural Illustration Services]

Dear Radiance users

Continuing the discussion on the use of global illumination, I'd like to
contribute three points to the discussion.

1)  Orientation of the HDRI background image on the global environment cube.

2)  Lighting ratios for correct exposure when using other light sources in
conjunction with a global illumination background.

3) Excluding materials and geometry with -ae argument in relation to -ar
argument and determination of scene bounding cube values.

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1)  When I was first experimenting with a global illumination background,
the background was rendering rotated through 90 degrees.  This issue was
mentioned by another user on this thread.  I corrected this problem by
simply transposing two values in the CAL file angmap.cal.  Different CAD
software sometimes re-orient the CAD model through 90 degrees, so
experimentation may be necessary.  Here is the first
version by Paul E. Debevec :

####################################

{
angmap.cal

Convert from directions in the world to coordinates on the angular sphere
image

-z is forward (outer edge of sphere)
+z is backward (center of sphere)
+y is up (toward top of sphere)
}

sb_u = 0.5 + DDx * r;
sb_v = 0.5 + DDy * r;

r = 0.159154943*acos(DDz)/sqrt(DDx*DDx + DDy*DDy);

DDy = Py*norm;
DDx = Px*norm;
DDz = Pz*norm;

norm = 1/sqrt(Py*Py + Px*Px + Pz*Pz);

####################################

Here is the corrected file for use with a cube with inward facing normals
created in CAD software:

####################################

{
angmap2.cal

Convert from directions in the world to coordinates on the angular sphere
image

-z is forward (outer edge of sphere)
+z is backward (center of sphere)
+y is up (toward top of sphere)

DDy and DDz were transposed for correct alignment.
}

sb_u = 0.5 + DDx * r;
sb_v = 0.5 + DDy * r;

r = 0.159154943*acos(DDz)/sqrt(DDx*DDx + DDy*DDy);

DDz = Py*norm;
DDx = Px*norm;
DDy = Pz*norm;

norm = 1/sqrt(Py*Py + Px*Px + Pz*Pz);

####################################

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2)  When using other light sources in conjunction with a global illumination
background, it is necessary to consider the lighting ratios between
different light sources and the global illumination background.  Radiance is
so similar to real world lighting that a Radiance user should try and think
like a photographer balancing lighting ratios on a set, for example.

When I first experimented with global illumination backgrounds, I found
that the HDRI image on the global environment cube was
emitting light several thousand times more intense than spotlights in the
scene, so when PFILT exposed for the background, illumination from the
spotlights disappeared.  Either the spotlight intensity could be increased
to an appropriate ratio, or in my case, I re-exposed the HDRI background
with PFILT, reducing its intensity by several thousand times before using
it as a global illumination background.  Experimentation is often necessary.

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3)  Please can the issue be clarified concerning excluding materials and
geometry with -ae argument in relation to -ar argument, ambient resolution
and determination of scene bounding cube values.

For example, if a small object is surrounded by a huge cube being used as
the global illumination background, it would be desirable for the ambient
resolution to be concerned only with the small object, otherwise a huge -ar
value would be required, adversely influencing rendering time.  If the -ae
argument is used to exclude a material and thus the associated geometry
(preferably of the global illumination cube), does the ambient resolution
determined by -ar now only consider the small object and divide the ambient
resolution across this small object, or does it still look at the bounding
cube for the whole scene, including the global illumination cube?

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Users interested in viewing a high quality render using global illumination
may be interested in viewing a current project (under construction) at the
following Internet address:

www.daiservices.btinternet.co.uk/Virtual_Sculpture/Virtual_Sculpture.htm


John Graham
DAI Services