[Radiance-general] Stencil method to reduce light leaks from adjacent spaces

[John Mardaljevic]

Scenario:  Creation of images showing the distribution of illuminance  
across a workplane from a high viewpoint and using the fore clipping  
plane to remove foreground objects.

Problem: With arbitrary shaped/orientated workplanes it is generally  
not possible to avoid including adjacent spaces and wall voids etc.  
in the final image. I usually create a stencil rendering for the same  
view parameters showing just the workplane (now set to "glow") and  
use those pixels to pick-out the pixels I want in the illuminance  
rendering (say, to profile the illuminance distribution across the  
workplane).  See the top two images in this PDF:
http://www.iesd.dmu.ac.uk/~jm/pickup/stencil.pdf

Now, this method works fine most of the time provided that you can  
crank-up the ambient parameters to avoid light leaks.  These can be  
fairly nasty when they do occur -- the pixels affected by a leak can  
have an illuminance much higher than nearby unaffected pixels.  In  
the example shown, the leaky pixels have a value >1000x (!) that of  
their non-leaky neighbours.  Needless to say, these affected pixels  
play havoc with any numerical measure of illuminance that you might  
want to get from your image.  The computational cost of leak  
prevention can be expensive [1].

I've long had this hunch that the propensity for leaks might be  
reduced if we could prevent rpict from doing any initial sampling   
off the workplane, and so avoid populating the ambient file with  
outside and wall-void values at the zero level.  It turns out that it  
can't be done directly with rpict.  However there is a way using  
rtrace and potpourri of other Radiance programs.  I couldn't figure  
out how to do it.  Fortunately, Greg could -- he came up with this  
suggestion:

------------------------------------------------------------------------ 
----

Rpict isn't very good about working on subimages, unless they're  
perfectly rectangular.  However, you can use rtrace to accomplish  
this, with a little effort.  Basically, you need to zero out the ray  
directions on all pixels you don't want computed.  Something like:

	vwrays -ff -x 512 -y 512 -vf best.vf \
		| rlam -if6 - -if1 '\!pvalue -h -H -df -b stencil.pic' \
		| rcalc -if7 -of -e '$1=$1;$2=$2;$3=$3' -e 'out(v)=if($7-.5,v,0)' \
			-e '$4=out($4);$5=out($5);$6=out($6)' \
		| rtrace -ffc `vwrays -d -x 512 -y 512 -vf best.vf` -i scene.oct \
		> stencilled_ill.pic

If I didn't mess up, irradiances should be calculated wherever the  
stencil pixels are greater than 0.5, and zeros will be written to the  
output picture everywhere else.  You can tweak the definition of the  
out(v) function to get a range of values, instead.

------------------------------------------------------------------------ 
----

Which worked first time!  Indeed, the approach does seem to be very  
effective.  The lower two images in the PDF show the "usual rpict  
method" and the "stencil method" renderings set to the same  
falsecolor scale (in Photosphere).  In addition to preventing the  
leak, the stencil method was four times faster (I expected it to be  
quicker, but not by this much).  Both renderings were, of course,  
generated using the same ambient parameters -- in this case:

-ab 2 -ad 2048 -ar 128 -aa 0.2 -as 512

These need to be included in the rtrace command for the stencil  
method.  The effectiveness in terms of leak prevention and increased  
speed will be highly dependent on the scene, the view and the ambient  
settings.  As ever, some testing is advised.

For me at least, this is a very useful technique.  And thanks to Greg  
for figuring out the command.

A (slightly belated) Happy (Chinese) New Year to you all.

-John

[1] Much like watery leaks as anyone who has recently had dealings  
with a plumber will know.  Sadly, my plumber will not accept payment  
in CPU cycles.

-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

jm at dmu.ac.uk
http://www.iesd.dmu.ac.uk/~jm