[Radiance-general] Re: Radiance RGB definition

[Greg Ward]

> Thank you, this was very useful.
>
> May I ask some related questions (sorry, they may be trivial but I did 
> not
> find answers from the documentation):
>
> Since Radiance has its own RGB definition, I tried to find out what is 
> the
> RGB gamut produced by ra_tiff. Is it sRGB of something else? (pcond 
> seems
> to produce sRGB as a default - am I right?). I am using the DOS/Windows
> executables. Is the resulting RGB gamut same for all systems?
>
> Regards, Markku
>
Unfortunately, the RGB coordinates I chose for Radiance predated 
(slightly) those chosen for sRGB, and they don't quite agree.  In fact, 
the red and blue primaries are the same, only the green primary differs 
slightly.  The gamut of the Radiance color space is therefore slightly 
larger than sRGB, but not significantly larger.  It would be much better 
of course if they were the same, but it was a couple of years before I 
noticed the difference as sRGB came into vogue, and I didn't want to 
make a change that would affect the colors of all my rendered images, 
however slightly.  If you like, you may redefine the primaries in the 
main src/common/color.h to match the sRGB primaries, like so:

*** color.h     Wed Oct 21 17:48:07 1998
--- color_srgb.h        Fri Feb 15 07:53:50 2002
***************
*** 60,73 ****
   #define  CIE_x_w              0.3333          /* use true white */
   #define  CIE_y_w              0.3333
   #else
! #define  CIE_x_r              0.640           /* nominal CRT 
primaries */
   #define  CIE_y_r              0.330
! #define  CIE_x_g              0.290
   #define  CIE_y_g              0.600
   #define  CIE_x_b              0.150
   #define  CIE_y_b              0.060
! #define  CIE_x_w              0.3333          /* use true white */
! #define  CIE_y_w              0.3333
   #endif

--- 60,74 ----
   #define  CIE_x_w              0.3333          /* use true white */
   #define  CIE_y_w              0.3333
   #else
! #define  CIE_x_r              0.640           /* sRGB primaries */
   #define  CIE_y_r              0.330
! #define  CIE_x_g              0.300
   #define  CIE_y_g              0.600
   #define  CIE_x_b              0.150
   #define  CIE_y_b              0.060
! #define  CIE_x_w              0.3127          /* Illuminant D65 */
! #define  CIE_y_w              0.3290
!
   #endif
-------------------
You'll notice above that I also use "equal energy white" for the 
Radiance white point, i.e., neutral.  I chose this rather than the sRGB 
standard of D65 because it avoids the need to specify non-white sources 
to achieve color-balanced pictures.  This is less of a problem with 
3.4's incorporation of the von Kries transform, so using D65 in Radiance 
should work OK now so long as you render things with this in mind.  
Thinking about it now, I'm still a little muddy on what the exact 
procedure should be; I've gone so long skirting these issues in my 
work....

-Greg