[Radiance-general] Re: exposure value

[Greg Ward]

Ximage does the division by the exposure value for you when you use the 
'l' command.  If you use the 'c' command, it reports the RGB from the 
file without exposure correction.  The "right" value in terms of 
absolute numbers is the one reported by the 'l' command.  The one 
reported by 'c' is what is actually at that point in the image, which 
has been premultiplied by the value(s) reported by getinfo.

To save an image without changing it's exposure in rview, set 'e =1' 
prior to writing it out.  Rpict never adjusts the exposure of its 
output -- you have to run pfilt yourself for that.

Hope this helps!
-Greg

> From: "Alexa I. Ruppertsberg" <a.i.ruppertsberg@Bradford.ac.uk>
> Date: December 17, 2003 8:04:56 AM PST
>
> Hi Greg,
>
> thanks for your explanation about the weights for computing luminance. 
> That has helped.
>
> Now, our next problem deals with the exposure value:
>
> If you use rview on the octree, trace returns among other values a 
> luminance value. As far as we understand the octree should be free of 
> any image processing stuff, like exposure values. So, we regard this 
> luminance value as a reflection of the reading of a photometer.
>
> If we then use ximage on the picture, generated from the very same 
> octree, we read the very same luminance value as under rview, even 
> though getinfo returns an exposure value (Note, we have not set any 
> exposure values in rpict and pfilt (apart from the default values).
> According to the luminance equation and other notes, one should 
> actually divide each radiance value by the exposure value, or divide 
> the luminance reading by the exposure value.
>
> So, which luminance reading is the 'right' one in ximage? The one 
> returned from the image or the one that one obtains by dividing the 
> returned one by the exposure value?
>
> And how do I avoid any exposure setting or correction when producing a 
> picture (radiance picture that is)?
>
> THanks for all your help in the hectic pre-christmas time.......
>
> alexa and marina
> =dumb and dumber   :-)