[Radiance-general] specifying sources

[william reynolds]

Hi
thanks for your suggestions greg, though i'm afraid i'm still having a 
bit of trouble getting to grips with this one!

I have my hdr capture sources working fine, and now i've got them scaled 
to emit a physically correct brightness.

but i'm still struggling with the colour rendering.
i have converted my CIE XYZ values to radiance RGB primaries, and 
rendered a test scene. the colour is a bit out for what the filter 
should be, but not wildly i dont think.
i have read your paper though, and i would like to try the rendering in 
a diferent colour space, for the sake of comparision, and also to 
correct the white point.

converting to sharp RGB:
-i think i can just change some constants in the xyz_rgb.cal file, 
(values for sharp RGB are given in Sharp.cal) to change the clour space.
-i assume that all colours (ie of materials as well as sources) have to 
be converted?
-the rendered picture will then be in th sharp RGB colour space, which 
ximage does not use, so every pixel in the image then has to be 
converted to back to the radiance RGB primaries. is this right? if so, 
is there a utility to apply the transformation to a .pic file or do i 
need to write a script?

white point adjustment:
-presumably the white point that radiance uses is the same as most 
monitors (D65 i think) so i need to transform from the white point of 
the XYZ colour space to that of the sharp RGB (or radiance RGB) colour 
space, but only because of this change in colour space.
-can anyone start me off with how to apply the white point 
transformation? i assume it uses the vonKries.cal file, and that the 
inputs (initial and final white point cramaticities) are standard values 
for the colour spaces in question, but how is it used!?
-the XYZ data (and the xy coordinates) i have are given for both a C 
source (the CIE standard?) and for a tungsten lamp with colout temp 
3200K, so i presume i should use the tungsten readings (as my lamps are 
tungsten).
-the sources i am modelling are tungsten, but i want an accurate 
representation of what the colour will look like, so presumably i need 
some sort of white point transformatin to correct their slight 
orangeness, as the eye is quite good at correcting this, i believe. how 
should i specify this? i know the lamps' colour temperature (about 3200K).



and finally, how should i apply the colour to my sources?
-i have XYZ tristimulus and xy cromaticity coordinate data for the 
filters i want to use, but i'm not sure of the best way to apply it.
(data comes from eg.  http://www.leefilters.com/LPFD.asp?PageID=248  )
-i could either the calculated RGB values to scale the output from my 
source primative (though would i have to take the Y value 
(reflectance/transmission depending on the context i believe) into 
account, or is this done automatically in the conversion?). This is what 
i did for my first test run earlier (converting XYZ values to radiance 
RGB primaries, and then not using the Y value again), and the result 
seemed to bright for the filter i was using (i had 2 sources in the 
scene, one coloured, the other not, and they were both of similar 
brigtness).
-or i could use the RGB values to define the transmission of a thin 
piece of glass, and place this in front of the source (as a real filter is).


i'm sorry to keep asking long questions, but thanks once again for the 
help, its proved invaluable in building up my understanding Radiance, 
but its also really kindled my interest in the whole area!
will




Gregory J. Ward wrote:
> Hi Will,
> 
>> firstly, i want to specify the brightness of my light sources  
>> accurately. i am using photos (hdr images assembled from several  
>> photos) to specify the output distribution of the source (theatre  
>> spotlights) and i know the "light output in lux" at various  distances 
>> from the lens (from here: http://www.seleconlight.com/ 
>> english/support/english/acclaim%20pc.pdf  ) and i assume that they  
>> give the figure for the centre of the beam, rather than the average  
>> over the whole field.
>> what i'm stuck on is what figures i should use for the (rgb)  
>> brightness in the rad file description of the "light" material.
> 
> 
> I don't have a ready example to offer, but the basic idea is to apply  
> the HDR capture as a pattern to your light source, indexing the image  a 
> perspective projection:
> 
> void colorpict spot_dist
> 7 red green blue capture.hdr .
>         (-Dx/Dz)/A1+.5  (Dy/Dz)/A1+.5
> 0
> 1 0.404026
> 
> spot_dist light spot_output
> 0
> 0
> 3 3881 3881 3881
> 
> The value given in the colorpict's first argument (A1) is the tangent  
> of the subtended horizontal angle.  The image (capture.hdr) is  assumed 
> to be square.
> 
> To get the right output value for your lamp, you can create as scene  to 
> match the lux measurement you have and render it in rvu with the - i 
> option, having it report on the illuminance for a surface at the  same 
> distance with the 'trace' command.  Then, apply a correction  equal to 
> the ratio between this value and the measured one to your  light color 
> (3881 in the example above).
> 
>> i need the light output of these sources to be physically accurate,  
>> because i want to compare them to models using ies-data described  
>> distributions.
>> i would like to be able to colour correct these sources (i believe  
>> the ies2rad will do this automatically) given the colour  temperature 
>> of the bulb.
> 
> 
> Rendering with an RGB color isn't very accurate and won't look right  
> unless you apply a more sophisticated spectral technique, such as the  
> one described in the paper:
> 
>     http://www.anyhere.com/gward/papers/egwr02/
> 
>> secondly i want to colour my lights, as though they had colour  
>> filters in front of the lense, and i dont really understand the  
>> colour space that radiance uses. can anyone point me towards any  
>> resources on this?
>> i want to model the colours of commercially available filters (such  
>> as this one:  http://www.leefilters.com/LPFD.asp?PageID=248  ) but  i 
>> cant find any RGB transmission data for them. is it possible to  
>> convert the XYZ values given on the data sheet to RGB?
> 
> 
> The following command will take XYZ values on its input (triplets)  and 
> produce Radiance RGB values on its output:
> 
>     rcalc -f ray/src/cal/xyz_rgb.cal -e '$1=R($1,$2,$3);$2=G($1,$2,$3); 
> $3=B($1,$2,$3);'
> 
> Expect to be disappointed by the results if you don't apply white- 
> balancing afterwards.
> 
> -Greg
> 
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