[Radiance-general] False color and luminance values

[Greg Ward]

Hi Brian,

How did you determine the "small area mean" of 131 nits using falsecolor?

The discrepancy between the two programs has to do with a bug in the "pextrem" program, which uses the intensity rather than the luminance to determine the brightest pixel.  In your case, the red shelves have higher intensity than the top shelf, despite the fact that the top of the bookcase has a higher luminance value.  Therefore, pextrem (used by falsecolor) thinks the brightest point is on a red shelf rather than on top, and both the luminance scale (using "auto") and the max value are incorrectly determined.

It is possible to correct this flaw in falsecolor without too much difficulty by replacing some lines of code, at a small expense in computation time.  This hasn't come up before, to my knowledge, as people tend not to use such strong primary colors.

Best,
-Greg

> From: "Brian Karr" <brianakarr at yahoo.com>
> Subject: [Radiance-general] False color and luminance values
> Date: January 4, 2015 9:00:31 AM PST
> 
> Hello all,
>  
> A question regarding false color and luminance values. Starting with an image created via rpict, and using the photosphere histogram, the max luminance value is 351 nits. Selecting a small area around the top outer corner of the shelf results in a mean around 302 nits. This agrees fairly close to a measurement with a spot meter on the outer corner of a physical version of the scene, so far so good.
>  
> Command: rpict -vtv -vp 0.75 .75 5.5 -vd 0.3 1.0 -0.6 -vu 0 0 1 -vh 45 -vv 45 -vo 3 -va 0 -vs 0 -vl 0 -av .1 .1 .1 -x 2000 -y 2000 -ab 2 -aa .1 shelf.oct > shelf_rpict.hdr
>  
> File: https://dl.dropboxusercontent.com/u/91366188/shelf_rpict.hdr
>  
> With a false color version, the histogram max luminance however is 177 nits, the small area mean is 131 nits. Im not sure why the luminance values between the two versions are different?
>  
> Command: falsecolor -ip shelf_rpict.hdr -e -s auto -n 16 -lh 1000 -lw 225 -log 4 -l > shelf_false_log_norm.hdr
>  
> File: https://dl.dropboxusercontent.com/u/91366188/shelf_false_log_norm.hdr
>  
> Thoughts?
>  
> Thank you!
> Brian
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