Hi Jan,
How are you calculating the solid angles in your table? I assume these are
per-pixel, since the total should equal the actual solid angle of a 0.5°
source. I use the following to calculate total solid angle as understood by
pcomb:
pcomb -e 'lo=if(gi(1)-10,S(1),0)' input.hdr | pvalue -pG -h -H -d |
total
This simply adds up all the solid angles corresponding to pixels greater than
10, which will just be the sun in your case.
I've noticed there's actually about a +/-5% error in the disk size using a
5000x5000 pixel rendering. If you increase this to 15000x15000, you can get
this error down below 1% or so. This error stems from the on/off nature of the
boundary and random sampling. To get more consistent results, I also recommend
adding "-pj 0" to your rpict line.
What kind of differences are you actually expecting between these two
calculations? Since you are using rpict to render the sky, the "uncorrected"
fisheye is the one that will add up to the correct solar solid angle, which is
about 6e-5 for a 0.5° source. The above pcomb command should give you this
value for all of your sun positions in the uncorrected versions. I confirmed
this from the 15000x15000 runs.
The "corrected" images will still be interpreted by pcomb as having "-vta"
projections, so will yield incorrect estimates of the sun's solid angle. Had
you fed them actual fisheye captures using an equi-solid-angle lens, then
presumably they would end up agreeing again. Off-hand, I don't know how to
generate such test images, but when I run the 15000x15000 images through the
above pcomb command, I get the following ratios for the new over the original
solid angles:
Angle modified/original
5° 1.00
15° 0.97
25° 0.96
45° 0.93
75° 0.91
I don't know what to do with this, however.
Cheers,
-Greg
> From: Jan Wienold <[email protected]>
> Date: June 27, 2017 10:00:50 AM PDT
>
> Hi Greg,
> the image is 5000x5000 pixels.
> The sun is 225 pixels for the original and the corrected for the 5° position.
> I also tested other positions - the results are in the table below.
>
> angle [°] Pix-Sun-org Pix-Sun-corr Pixel-Ratio solid angle vta
> solid angle equi-dist. Solid-Angle-Ratio
> 5 225 225 1.00 2.654E-07 3.20E-07 0.83
> 15 204 204 1.00 2.915E-07 3.20E-07 0.91
> 25 189 187 1.01 3.157E-07 3.20E-07 0.99
> 45 167 159 1.05 3.548E-07 3.20E-07 1.11
> 75 151 138 1.09 3.902E-07 3.20E-07 1.22
> According to this, the ratio of the "sun" pixels change after 25°, which is
> the angle when the solid-angle for vta gets larger than the solid-angle of
> the equi-solid-angle projection.
>
> In the table the column two and three are the amount of pixels for the sun
> for the two images, column 4 is a ratio for those two, the last column is the
> ratio of the two solid angles.
> FYI, these are the scripts for producing the files:
>
> for i in 5 15 25 45 75; do gensky -ang $i 0 +s >sk_$i.rad; oconv -f
> sk_$i.rad >sk_$i.oct; rpict -vp 0 0 0 -vd 0 0 1 -vu 0 -1 0 -vta -vv 180 -vh
> 180 -x 5000 -y 5000 -dc 0 -dj 0 -ps 0 sk_$i.oct >sk_$i.hdr & done
> for i in 5 15 25 45 75; do pcomb -f /usr/local/radiance/lib/fisheye_corr.cal
> -o sk_$i.hdr | getinfo2 -a "VIEW= -vta -vh 180 -vv 180" >sk_$i"corr.hdr"; done
>
> The solid angles I calculated with pcomb (and checked also with evalglare).
> The amount of pixels I calculated with pvalue -H -h -o -b image.hdr |awk
> '{if ($3>1) print $0}'|wc -l and checked also with evalglare.
> cheers
> Jan
>
>
>
> On 27/06/17 18:13, Gregory J. Ward wrote:
>> Hi Jan,
>>
>> What is the resolution of your image, and how many pixels does the sun
>> cover? Could this be simply due to quantization error? The function moves
>> pixels around, not changing their magnitude, and the accuracy can never be
>> better than +/- 1/(number of pixels in sun).
>>
>> Cheers,
>> -Greg
>>
>>> From: Jan Wienold <[email protected]>
>>> Date: June 27, 2017 7:06:03 AM PDT
>>> Hi all,
>>>
>>> I'm a bit puzzled by the result of applying fisheye_corr.cal to an fish-eye
>>> image.
>>>
>>> I have a simple example:
>>> I created a fish-eye image with a sun at 5 degree altitude and kept the sky
>>> black (just the sun). Let's assume now this image is equi-solid-angle and
>>> we want to transfer it to a equi-distant (-vta).
>>> For this I applied pcomb -f fisheye_corr.cal -o fisheye.hdr > corrected.hdr.
>>>
>>> In the next step I counted the amount of pixels for the sun in the two
>>> images. They are exactly the same! What has changed is only the position
>>> in the image. Neither the size of the sun nor the luminance has changed.
>>> But: The difference between the solid angles of a pixel at 85° from the
>>> center between equi-solid-angle and equi-distant projection is around 20%!
>>> (in my example the solid angle per pixel for a 5000x5000 image at 85° is:
>>> 3.2e-7sr vs 2.58e-7sr)
>>> I would have expected also a change in size, accounting for the difference
>>> in solid angles per pixel for the different projection methods, the
>>> luminance of course should be the same.
>>> Am I doing sth. wrong?
>>> thx for the help.
>>>
>>> Jan
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