Date: Mon, 20 Feb 2012 14:48:37 -0600
From: "Tyukhova, Yulia" <ytyukhova@unomaha.edu>
To: hdri@radiance-online.org
Subject: [HDRI] HDRI capture of LED
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Everybody,
Thank you for fast responses/tools and suggestions!
Greg,
Thank you for your suggestions and files!
I am new to Radiance, and I assume that this is what I need to have
installed on my computer in order to use suggested Perl scripts.
If you can provide me with the link/info how to run it, that would be
really helpful!
Let me restate the question about the compression of the curve in
Photosphere.
1. Do manufactures compress the response curve or maybe it is limited by
camera/optics/sensor saturation itself on the upper end?
2. And I'm still curious, how CF is applied in Photosphere?
I've been using ND filter t=0.0094 on the luminance meter, because
otherwise it is impossible to measure such high luminances. I assume, you
suggest to use it on the camera as well.
I'm looking forward to analyze my images with the suggested
hdrgen. Luckily, I've been taken them in both formats jpeg and raw.
Greg, will you recommend to have regular calibration scene calibrated at
the grey card instead of using brighter scene?
Thank you,
Yulia
Hello Yulia,
Seems your question has spawned quite a bit of interesting discussion...
My main recommendation is to use camera RAW images for critical photometry,
especially when there are saturated colors involved. It is impossible to
correct the color of JPEG images and undo what the camera maker has done,
so you need to start from the sensor data.
Photosphere does not accept camera RAW as input, but I have written a Perl
script that uses dcraw with the command-line HDR image builder hdrgen to
overcome this limitation. It also requires the use of another third-party
program, exiftool, which I have packaged together for you at:
http://www.anyhere.com/gward/pickup/raw2hdr.tgz
Unfortunately, I don't have a good set of documentation to go with it.
Typing "raw2hdr" by itself shows the basic syntax:
Usage: raw2hdr [hdrgen opts][-h][-w][-C calib][-c cspace] -o
output.hdr input1.raw ..
If your images are taken on a tripod (aligned exposures), you can use the
default settings:
raw2hdr -o output.hdr expos1.cr2 expos2.cr2 expos3.cr2 ...
The hdrgen settings can be found in the included HTML man page, and so can
the -h and -w option meanings in the included dcraw man page. The -C
option is to provide a linear factor to correct the overall exposure based
on previous calibrations. The -c option is to specify an output color
space. The default is "sRGB" which is actually linear CCIR-709 primaries.
The only other output color space I would recommend is AdobeRGB. There is
a CIE XYZ space supported by dcraw, but I have found it to be somewhat
unreliable, and I don't know where the fault lies in this.
Regarding Axel's mention of camera flare, this is less of an issue for
sources that are brighter than the rest of the scene. It mostly affects
darker, surrounding regions. The -f option will attempt to estimate the
camera/lens PSF and remove it, but it cannot be relied upon to remove this
source of error completely. Your problem with the accuracy of the LED
sources is due no doubt (as others have said) to limitations in your short
exposures combined with the color issues inherent to JPEG processing.
Other responses inline....
> From: "Tyukhova, Yulia" <ytyukhova@unomaha.edu>
> Date: February 19, 2012 9:34:25 PM PST
>
> Hello everybody!
> I?ll provide the summary of my research and have questions within the
summary. I would appreciate any of your help!
> My research investigates if HDRI technique can precisely capture
luminances of small bright light sources (e.g. LED garage fixtures) with
narrow light distributions.
> I was able to figure out luminance values for a single LED, which can be
compared to the ones from HDR images. But I have a couple of
questions/concerns on HDRI technique and Photosphere.
> At first, I?ve used ?regular? scene to retrieve response curve of the
camera (large smooth gradients with very dark and bright areas, and had
reflectance standards for the absolute calibration).
> Camera: EOS T1i Rebel with 28-105mm lens, at 28mm
> Calibrated at the grey reflectance sample 186.45 cd/m2
> CF=0.957
>
> I?ve got the following RC for RGB:
> red(r) = -6.434199e-03+ 4.518039e-01*r + 1.291426e+00*r^2 +
1.802896e+00*r^3;
> green(g) = -5.804720e-03+ 4.175837e-01*g + 1.176582e+00*g^2 +
1.721643e+00*g^3;
> blue(b) = -4.376831e-03+ 3.784418e-01*b + 1.075695e+00*b^2 +
1.658471e+00*b^3
> If I look at the histogram of the scene, maximum luminance within the
scene is 60,291 cd/m2.
> Then I use this RC to analyze HDRI of a captured LED. The value is
230,000 cd/m2 for a single LED, which is low (it?s has to be around 7*106
cd/m2). So, it underestimates the luminance.
> It seems like calibration point is critical here. I?ve decided to try to
capture a different scene for deriving RC with a wider range. It would make
sense that camera has to see higher luminance values in order to accurately
measure them later. The dynamic range has to cover measured values.
> 1. How does Photosphere deals/approximates/calculates the upper end of
the curve? I assume it gives more weight to mid tone values? But what
happens with high luminance values?
Photosphere (and hdrgen) use all the brightest pixels from the shortest
exposure and all the darkest pixels from the longest exposure. Middle
exposures have their brightest and darkest pixels downgraded.
> So, the new brighter scene was picked with the direct sun! But in order
to avoid the damage of the camera?s sensor, measurements were taken before
the sunset.
> In the new brighter captured scene without the calibration all values for
reflectance standards were overestimated, while the value for the sun
underestimated. Then I decided to calibrate my scene at the sun!
> But when I apply absolute calibration, it simply multiplies CF to all
values.
> 2. I assumed when CF is applied, it does not equally change all
values, but does it proportionally to RC (since it is not linear). Why
does it do it equally for the whole range?
> Lsun=80*106 cd/m2. And of course CF is very big 391.
> New RC:
> red(r) = 3.219064e+00+ -2.655078e+01*r + 9.351069e+02*r^2 +
-2.115052e+03*r^3+1.594538e+03*r^4;
> green(g) = 2.094164e+00+ -1.468109e+00*g + 7.306838e+02*g^2 +
-1.720743e+03*g^3+1.380693e+03*g^4;
> blue(b) = 1.049078e+00+ 1.591820e+01*b + 5.848958e+02*b^2 +
-1.461635e+03*b^3+1.251033e+03*b^4
> But then something interesting happened. When I analyze LED, it gives a
value of 79*106 cd/m2. So, it jumps to this upper limit calibrated with the
sun previously.
> (I had similar results for EOS 7D with the lens 16-35mm, at 16mm)
>
I don't think your shortest exposure properly captured the LED, and maybe
didn't capture the sun, either.
> 3. Does photosphere compress the response curve, so at the upper end
all values above certain threshold will have the same number?
Photosphere does not compress the curve.
> 4. Any additional suggestions on properly obtaining and calibrating
HDRI for this purpose?
I would only reiterate others' suggestion to use a neutral density filter,
and using raw2hdr rather than Photosphere.
> --
> Thank you,
> Yulia Tyukhova
Best,
-Greg
--
Thank you,
*Yulia Tyukhova*
*
*
Fulbright Scholar, "Intern LC"
Architectural Engineering Graduate Student, UNL-Omaha, NE, USA
B.E. and M.E. in Lighting Engineering (MPEI), Moscow, Russia
jtyukhova@yahoo.com
ytyukhova@unomaha.edu
+1 (402) 996 0910
PKI 247
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Message: 2
Date: Mon, 20 Feb 2012 13:14:15 -0800
From: "Gregory J. Ward" <gregoryjward@gmail.com>
To: High Dynamic Range Imaging <hdri@radiance-online.org>
Subject: Re: [HDRI] HDRI capture of LED
Message-ID: <7B0CB6E2-C816-4FB9-A3E0-9672B4101EC0@gmail.com>
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Responses inline...
> From: "Tyukhova, Yulia" <ytyukhova@unomaha.edu>
> Date: February 20, 2012 12:48:37 PM PST
>
> Everybody,
>
> Thank you for fast responses/tools and suggestions!
>
> Greg,
>
> Thank you for your suggestions and files!
> I am new to Radiance, and I assume that this is what I need to have
installed on my computer in order to use suggested Perl scripts.
> If you can provide me with the link/info how to run it, that would be
really helpful!
Actually, you don't need to have Radiance installed. You just need to
move the executables (non-HTML files) from the unpacked directory to
/usr/bin or /usr/local/bin or some other directory in your shell's PATH
variable. These are command-line tools that must be run from the Terminal
application under /Applications/Utilities. I.e., start Terminal and copy
the files from your Downloads folder with:
cd Downloads
tar xzf raw2hdr.tgz
cd raw2hdr
cp raw2hdr dcraw exiftool /usr/bin
cd
raw2hdr
This should give you the usage message I wrote you earlier if it all goes
well. Some basic commands and pointers for Unix are available many places
online. Googling "basic unix tutorial" gave this page at the head of the
list:
http://www.ee.surrey.ac.uk/Teaching/Unix/
> Let me restate the question about the compression of the curve in
Photosphere.
> 1. Do manufactures compress the response curve or maybe it is limited by
camera/optics/sensor saturation itself on the upper end?
Some camera makers do compress the top end of the response curve, and do
funny things at the bottom as well. Photosphere attempts to discover the
tone curve and correct for these manipulations, but it isn't perfect and if
the camera is changing the tone curve dynamically, it's pretty hopeless.
There are settings you can use on a DSLR to disable such manipulations,
but using RAW files bypasses the problems entirely because the data is
linear.
> 2. And I'm still curious, how CF is applied in Photosphere?
A calibration factor is applied equally to all coefficients in the
polynomial, which is exactly the same as applying a linear scale factor
after the HDR merge operation.
> I've been using ND filter t=0.0094 on the luminance meter, because
otherwise it is impossible to measure such high luminances. I assume, you
suggest to use it on the camera as well.
Whatever gives you a short exposure that is past the integration time of
your source (1/60th second is acceptable) and not saturated is OK.
Specifically, all values in the short exposure's histogram should be be
below 245.
> I'm looking forward to analyze my images with the suggested hdrgen.
Luckily, I've been taken them in both formats jpeg and raw.
> Greg, will you recommend to have regular calibration scene calibrated at
the grey card instead of using brighter scene?
The best scene for calibration is a white card in a scene with no bright
sources directed at the camera. The calibration should hold in other
scenes where lens flare is not problematic.
> Thank you,
> Yulia
Certainly,
-Greg
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