[Radiance-general] Problems using hdrgen for hdri generation

[Despina Michael]

Hi all,

I am trying to calibrate camera response function.
(unfortunately withh no good results - i belief still problems with 
alignment although I used a tripod)

I would like to ask you if  the scene in the photo
www2.cs.ucy.ac.cy/~cs99dm1/calib.zip
is appropriate for that purpose.

According to Photosphere quickstart... is an inside scene looking out a 
window.
Of course I took a series of photos of this scene with different exposures.

Thanks,
Despina

----- Original Message ----- 
From: "Greg Ward" <gregoryjward at gmail.com>
To: "Radiance general discussion" <radiance-general at radiance-online.org>
Sent: Friday, January 07, 2005 2:53 AM
Subject: Re: [Radiance-general] Problems using hdrgen for hdri generation


Hi Despina,

Thanks to the helpful folks on the mailing list, most of your questions
have already been answered, but I'll try to add a few points...

> From: "Despina Michael" <despina_m81 at hotmail.com>
> Date: January 6, 2005 9:07:46 AM PST
>
> 1) First of all, I would like to know which is the best way to take 
> photographs with different exposure.
>  If I understand correctly there are two ways. Either by changing F-number 
> and let fixed the shutter speed or either by varying shutter speed and let 
> the f-number fixed.
> I would like to know witch of two ways is the best?

You should definitely vary the shutter speed (i.e., exposure time)
rather than the aperture (i.e., f-number, f-stop).  You should read the
attached tips, taken from the quickstart_pf.txt file distributed with
Photosphere from <www.anyhere.com>.

> 3) Where is the fault
>  when there is a warning: Trouble finding HDR patches***** ?
> when ther is a warning: Poor convergence for order 1 fit? (is 1 or any 
> other number X appear there refers to a problem with the Xth image in 
> command arguments?)

Warnings are mostly there as a kind of excuse for when things don't
turn out.  If they turn out, then you shouldn't lose any sleep over
them.

> 4) Where is the fault when there is an error:
> "Cannot solve for response function" ?
> Is that because it can not generate file for response function of camera, 
> with specific series of photographs?

It's probably because the sequence didn't capture enough dynamic range,
or there were no smooth gradients.  The best strategy is to use a good
sequence to get the camera response, then store it and reuse it via
hdrgen's -r option.  (See the related tips in the attachment.)

> 5) One problem of resulting hdr images (in cases that there was a 
> generation of image)
> was that they were somehow "green". What can cause that?

I don't know.  I would have to see the source sequence, but there's
probably not much I could do to fix it.

> 6) My photographs are not perfectly aligned.
>  As I understood, the hdrgen uses an algorithm to align the photographs.
> But as I noticed when the uses of alighnment algorithm is enable (NOT use 
> of -a option) the "alignment" getting worst..
> and when I disabled it (Using -a option) the result is better.. but still 
> no good (I guess because that original photographs are not aligned)

The automatic alignment algorithm is not fool-proof, but I don't know
of one that is.  It does not take care of rotation, and images that are
very far out of alignment will not work, either.  (The maximum computed
shift between adjacent exposures is +/-64 pixels in X and Y.)  The
final solution is to use a tripod.  Using a tripod AND performing
automatic alignment usually gives the best results.  I have good luck
myself with auto-bracketed hand-held exposures, and practice does help.
  I still get bad sequences, though -- usually in portrait mode, as I
have a hard time not leaning during the exposures....

-Greg

Tips on HDR image creation taken from Photosphere quickstart blurb:

12) To create a high dynamic-range image, you need to start with
a set of "bracketed" exposures of a static scene.  It is best if
you take a series of 10 or so exposures of an interior scene looking
out a window and containing some large, smooth gradients both inside
and outside, to determine the camera's natural response function.
Be sure to fix the camera white balance so it doesn't change, and
use aperture-priority or manual exposure mode to ensure that only
the speed is changing from one exposure to the next.  For calibration,
you should place your camera on a tripod, and use a small aperature
(high f-number) to minimize vignetting.  Take your exposure series
starting from the longest shutter time and working to the shortest in
one-stop increments.  Make sure the longest exposure is not all white
and the darkest exposure is not all black.  Once you have created your
image series, load it into Photosphere directly -- DO NOT PROCESS THE
IMAGES WITH PHOTOSHOP or any other program.  Select the thumbnails,
then go to the "File -> Make HDR..." menu.  Check the box that says
"Save New Response", and click "OK".  The HDR building process
should take a few minutes, and Photosphere will record the computed
response function for your camera into its preferences file, which
will save time and the risk of error in subsequent HDR images.
You will also have the option of setting an absolute calibration
for the camera if you have a measured luminance value in the scene.
This option is provided by the "Apply" button submenu when the
measured area is selected in the image.  (Click and drag to select.)
Once an HDR image has been computed, it is stored as a temporary
file in 96-bit floating-point TIFF format.  This file is quite
large, but the data will only be saved in this format if you
select maximum quality and save as TIFF.  Otherwise, the 32-bit
LogLuv TIFF format will be preferred (or the 24-bit LogLuv format
if you set quality to minimum).  You also have the option of saving
to the more common Radiance file format (a.k.a.  HDR format), or
ILM's 48-bit OpenEXR format.  If you choose not to save the
image in high dynamic-range, the tone-mapped display image can be
written out as a 24-bit TIFF or JPEG image.


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