[HDRI] Re: Photosphere question

[Greg Ward]

Hi Chris,

Thanks for writing.  If you want to subscribe to the HDRI mailing  
list at radiance-online, here is the URL:

	http://www.radiance-online.org/mailman/listinfo/hdri

It's generally pretty quiet, but this sort of discussion is very  
welcome.

The warnings you are seeing in Photosphere are just that -- they are  
just my way of saying, "If the output looks funny, it's not my  
fault."  In particular, the 'Trouble Finding HDR Patch' warning is  
nothing to worry about.  It just means some of your exposures are  
very dark or very light, and there's not much information to be  
gleaned from them in deriving a camera response.  The same is usually  
true of the alignment warning, though small, bright objects like  
light sources may show double-images if the camera wasn't mounted on  
a tripod.  If it was, then you should probably turn alignment off  
altogether.  In the latest version of Photosphere, turning alignment  
off kicks in a new feature to remove "hot pixels" from the output.

If you are not deriving the camera response for the first time,  
during on 'Skip Extra Exposures' in the dialog is a good idea, since  
it will avoid most of these problems.  In fact, you shouldn't see the  
'Finding Patches' error at all if you use a saved response function.   
Hopefully, this is working for you, as following the hints in  
quickstart_pf.txt (part 12) and reusing a saved response is the best  
way to avoid problems.

I should mention that it doesn't really help the process to include  
exposures that have no dark pixels (values all over 20) and no bright  
pixels (values all under 200).  You can use Photosphere's histogram  
viewer to check this and exclude extreme exposures.  This will help  
reduce the number of warnings.

Best,
-Greg

> From: "Christopher Simmons, Mr" <christopher.simmons at mail.mcgill.ca>
> Date: May 27, 2008 11:33:37 AM PDT
>
> Dear Dr. Ward,
>
> First of all, let me begin this e-mail by saying I am a huge fan of  
> your work. I was wondering if you could refer me to some resources  
> that address an issue I am having with your photosphere program. I  
> am creating HDR images, and for some I have the recurring problem  
> of "Trouble Finding HDR Patch" and "Exposure too extreme for  
> alignment." When I took the photo series used in the HDR image, I  
> went from 1/4000 s to 30 s exposure times. That is, I double the  
> exposure time for each photo between 1/4000s to 30s (standard  
> procedure), but I took a few extra exposures in between to be on  
> the safe side and included these in the final HDR image. Does the  
> "Exposure too extreme for alignment" message indicate that there  
> was an extra exposure, so it was removed? Or does it mean that an  
> exposure was brighter or darker than necessary for the HDR image  
> and was eliminated? And how do these messages affect the final  
> output? When I get these messages, how does it affect the luminance  
> values obtained, or relative luminances between different pixels in  
> the same image? If it has a negative impact on luminance, should I  
> create the HDR in another program (photoshop, HDR shop, etc) and  
> then analyze it in yours?
>
> Any resource you may be able to mention discussing these issues  
> would be greatly appreciated. Thank you for your time and attention.
>
> Sincerely,
>
> Chris Simmons
>