[Radiance-general] HDR

[Greg Ward]

Hi Jack, Axel,

I agree that a separate HDRI mailing list makes sense, if Peter A-B  
is willing to set one up.  I would certainly participate, and it  
would save those who may not be interested from our noise.

Regarding color in HDR images, this really falls under the category  
of tone-mapping.  Most film and digital cameras increase color  
saturation according to well-worn practices, which may be found in  
R.W.G. Hunt's classic tome on color management, "The Reproduction of  
Colour" (ISBN 0-470-02425-9).  Unfortunately, few tone-mapping  
algorithms actually pay much attention to preference with respect to  
color, and I know of at least two reasons why not:

1) People prefer increased gamma or saturation on their colors, which  
happens to correspond to less dynamic range in the reproduction -- in  
other words, boosting saturation makes it difficult to represent your  
larger luminance range.

2) Increasing saturation, or even leaving it alone, while compressing  
an image's dynamic range makes colors look *too* saturated in many  
images, because a loss in contrast is often read as haze by the human  
visual system, and your brain compensates by imagining the colors to  
be more saturated than they appear.

That said, you can of course play around with the gamma when you  
convert to 24-bit/pixel formats, and get a boost in contrast and  
saturation that way.  Sometimes the results will be nicer, oftentimes  
they will be worse.  To boost contrast/saturation, try:

	pcond capture.hdr | ra_tiff -g 1.6 - boosted.tif

To reduce contrast/saturation try:

	pcond capture.hdr | ra_tiff -g 2.7 - reduced.tif

There are more sophisticated ways to play around with contrast and  
saturation separately using pcomb, but I'm not sure you gain that  
much by them.

-Greg

> From: Jack de Valpine <jedev at visarc.com>
> Date: January 16, 2006 7:53:24 AM PST
>
> Hi Axel,
>
> It seems to me that HDRI is quite relevant to this community, as  
> there are a variety of people interested in the topic. Perhaps we  
> should ask:
> is this topic appropriate for this particular radiance list  
> (radiance-general)
> if not would it be worth it to set up a radiance-hdri at radiance- 
> online.org
> -Jack
--------------------
> From: "Axel Jacobs" <a.jacobs at londonmet.ac.uk>
> Date: January 15, 2006 8:26:53 PM PST
>
> Hullo again,
>
> I would like to move this off-list because I do realise that all these
> questions about HDRI are only of peripheral interest to the RADIANCE
> community at large. However, a quick Google tought me that the  
> majority of
> HDR discussion seem to be going on at
> http://www.hyperfocaldesign.com/,
> and frankly, it doesn't seem to cut the custard for me. So please  
> DO tell
> me if there is a more appropriate List to direct this one to...
>
> And 'ere we go:
>
> I've just put up a few more examples on the WebHDR Gallery page, and
> noticed that all of the tonemapped images are rather desaturated,  
> compared
> to what you get when hitting the old 'Do-it-all-for-me-Button' in Auto
> Mode. I understand that most if not all digital consumer cameras do  
> some
> jigger-pokery colour gamut stretching, so the photos look a bit nicer.
> However, my understanding is that once you've done an HDR, you can  
> do all
> sorts of things to it, probablay including those colour transform
> things...
>
> Since I am processing all results of WebHDR with RADIANCE tools, I was
> wondering if there is a way of putting the colour-vibrancy back  
> into the
> tone mapped imaged.
>
> Cheers
>
> Axel