[Tig] Why isn't HDR(-like) rendering more widespread?

Dan C. Tatut dtatut
Thu Mar 30 00:17:27 BST 2006


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Thanks to oktobor for supporting the TIG
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Bob,

32-bit color grading is not just to correct mistakes. Try this simple example: 

step 1: lift the luminance of all your colors until the image on screen looks white. 

step2: Add a secondary color correction and decrease the luminance of the selected region. 

If you get a grey shade, then too bad, your system is not HDR capable. 

Now imagine that step 1 and step 2 are animated and go from one extreme to the other... if your system is not HDR capable, you will get white or black most of the time... ugly and not correct.

Best, Dan

Dan Tatut
CEO
CHROME Imaging
105 Rue de Lyon
CH-1203 Geneva
Switzerland

Phone: +41 22 807 23 60
Fax: +41 22 807 23 70
Mobile: +41 78 659 11 04
WWW: http://www.chrome-imaging.com


-----Original Message-----
From: Bob Friesenhahn [mailto:bfriesen at simple.dallas.tx.us]
Sent: jeudi, 30. mars 2006 01:13
To: glenn chan
Cc: tig at tig.colorist.org
Subject: Re: [Tig] Why isn't HDR(-like) rendering more widespread?


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Thanks to oktobor for supporting the TIG
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On Wed, 29 Mar 2006, glenn chan wrote:
> I'm curious if there are any color correction systems on the market that do
> this.  The latest version of After Effects supports this, although I
> wouldn't consider it a serious color correction system.
>
> Why HDR?
> It allows more natural-looking corrections.  Saturated colors will retain

You are mixing many issues together and entitling them "HDR".  All HDR 
means is that the encoding supports subtantial range above white, and 
substantial range below black.  So, yes, if an intermediate algorithm 
would have caused normal-ranged values to "blow out", "saturate", or 
become zeroed, then extended coding can help prevent that.  The Cineon 
log encoding is considered to be an extended range format (let's call 
it "EDR").

There is extensive use of "HDR" encodings in the rendering and 
compositing areas and has been for years.

It would be wrong to assume that traditional image processing uses the 
same storage size for its computations as is used in the file format. 
For example, pixel values can be converted to 32-bit float or double 
for crunching, and then the final values can be appropriately 
translated back into the simple integer form required by the majority 
of file formats.  The end result can be the same as you are 
describing.

The traditional image processing approach does not help prevent the 
user of the software from destroying the image by using an algorithm 
which promotes the values outside of the storage range.  From my point 
of view, HDR with linear-light values is necessary for rendering in 
order for the math to work, but for the colorist, the main advantage 
of HDR is to allow mistakes to be corrected later.

There is a disadvantage of HDR formats in that the files must be much 
larger so file I/O becomes much slower and schedules must therefore be 
longer.  There is also the disadvantage that there are only a few well 
supported HDR formats.

Bob
======================================
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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