Trying to understand HDR . . .  

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Dcameraman
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Trying to understand HDR . . .

by Dcameraman » Thu Apr 06, 2006 2:48 pm

Hello All,

I'm pretty new to all this, so please bear with me. I went through all of the posts and the two tutorials and I think that I understand what to do pretty well. The one question that I have is this. When I am setting the HDR "window", where do I set it? If the actual "float" picture is wider than 0...255 then no matter where I put the "window" some portion will always be outside. Do I center the window on the image? or am I missing something?

Thanks for your time.

Josh

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GURL
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by GURL » Thu Apr 06, 2006 11:11 pm

Dcameraman wrote:When I am setting the HDR "window", where do I set it? If the actual "float" picture is wider than 0...255 then no matter where I put the "window" some portion will always be outside. Do I center the window on the image? or am I missing something?

Two possible solutions

a) Every part of the image is processed using the same rule, for example a 0..1024 "float window" is compressed to 0..255. This is posible because the steps between 0 and 1, 1 and 2, etc, in the final image correspond to a larger variation in the original subject. When using this method the result tends to be to flat because local contrast is lower.

b) The different parts of the image are processed differently and a given float value is rendered by diffent values in the final image: the value of a final image pixel depends of the sourrounding pixels values.

Those points are very well explained on the Photomatix website: http://www.hdrsoft.com/index.html

Photomatix wrote:Tone Mapping algorithms vary from a simple gamma curve (which is often what cameras are doing when converting 12-bit RAW data to 8-bit JPEGs) to more complex operators commonly divided into two categories:
· Global operators: mapping depends on pixel's intensity and global image characteristics, but not on spatial location
· Local operators: mapping takes into account the pixels' surroundings (in addition to intensity and image characteristics).

The main advantage of global operators is fast processing. Local operators require longer processing times but they are better at producing a "good-looking" photograph (the human eye adapts to contrast locally). The Tone Mapping algorithm of Photomatix Pro belongs to the category of local operators
Georges


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