[Radiance-general] vta 2 vth

[Greg Ward]

Hi Mehlika,

I think the output of this command was originally intended for the  
"total" command to add up illuminance in lux.  Thus, the pixel values  
get added up and divided by the total number of above-hemisphere  
pixels, which you don't want to do in this case.  I guess what you're  
trying to do instead is reweight your pixels according to the cosine,  
but it doesn't really make sense to me.  If you actually want to  
change the projection rather than creating a reweighted image, you  
should use pinterp, instead:

	pinterp -vf input.hdr -vth -x 1024 -y 1024 -ff input.hdr 1 > hemi.hdr

(Just saw Lars' response, which is along similar lines to this...)

If you still want to go ahead with the command, you can modify it  
thusly to get more what you say you're expecting:

pcomb -e 'sq(x):x*x' \
	 -e 'ar=190/2*PI/180*sqrt(sq(2/xmax*x-1)+sq(2/ymax*y-1))' \
	-e 'cf=WE/le(1)*if(ar-PI/2,0,if(ar-.01,cos(ar) *sin(ar)/ar,1))' \
	-e 'lo=cf*li(1)'  input.hdr > corrected.hdr

Best,
-Greg

> From: Mehlika Inanici <inanici at u.washington.edu>
> Date: February 18, 2009 10:32:07 AM PST
>
> Hi Greg (and everybody in the group),
>
> I have used the formula Greg provided for a previous discussion  
> (http://www.radiance-online.org/pipermail/radiance-general/2009- 
> January/005644.html) to convert an angular projection to  
> hemispherical fisheye projection.
>
> I am getting bizarre results. So, just to see what is going on, I  
> have generated an overcast sky with vta option (-vta -vp 1.000  
> 1.000 0.000 -vd 0.000 0.000 1.000 -vu -1 0 0 -vh 180 -vv 180 -vs 0 - 
> vl 0)
>
> I have used your formula:
>
> pcomb -e 'sq(x):x*x' \
> 	 -e 'ar=190/2*PI/180*sqrt(sq(2/xmax*x-1)+sq(2/ymax*y-1))' \
> 	-e 'cf=WE*sq(190*PI/180*2/(xmax+ymax))*if(ar-PI/2,0,if(ar-.01,cos 
> (ar) *sin(ar)/ar,1))' \
> 	-e 'lo=cf*li(1)'  input.hdr > corrected.hdr
>
> (I used 181 degrees instead of 190)
>
> and I was hoping to have an image similar to what I would have  
> generated using the vth option (-vth -vp 1.000 1.000 0.000 -vd  
> 0.000 0.000 1.000 -vu -1 0 0 -vh 180 -vv 180 -vs 0 -vl 0).
>
> After the correction, my luminance values dropped significantly  
> (2500 cd/m2 dropped to 4.3). You can find the original and  
> corrected images at:  http://faculty.washington.edu/inanici/vta/
>
> Any ideas why I am having difficulties with this? Thanks in advance...
>
> Cheers,
> Mehlika