[Radiance-general] lighting fixtures data

[william reynolds]

greg
thanks for the suggestion for the mapping.
i'm afraid i dont quite understand it though!

i've put a rendering up at http://users.ox.ac.uk/~orie1226/rad/ again. 
its called lantern_2.2.pic (or .jpg) using your suggestions for the .cal 
file. it seems to go a bit strange though, making a sort of star pattern!

the idea seems right - that the brightness of a ray is dependant on the 
angle to the normal of the source (rather than on the point it leaves 
the source from, as i originally had), but i'm lost with the idea of a 
square picture covering 20 degrees! what does this mean?

please could you also explin the bit of code you gave? particularly the 
divisor, and why there is an offset. also, what does the DEGREE word do? 
i guess its a constant - perhaps to convert deg -> radians?

thanks very much
will



Greg Ward wrote:
> Hi Will,
> 
> I think your problem is in "source_pic.cal":
> 
> mr(r,g,b) = r;
> mg(r,g,b) = g;
> mb(r,g,b) = b;
> 
> myu = Px/128;
> myv = Px/128;
> 
> First off, you are using Px as indices for both u and v picture  
> coordinates, which is bound to be wrong.  Second, you should probably  
> be using something based on the ray direction rather than the  
> intersection point, which is with the light source surface and  
> unrelated to the output distribution.  Something more along the lines  of:
> 
> myu = atan2(-Dx, Dz)/(20*DEGREE) + 0.5;
> myv = atan2(Dy, Dz)/(20*DEGREE) + 0.5;
> 
> The above assumes that your light is pointed in the negative Z  
> direction (as it seems to be) and that your distribution is oriented  
> with the horizontal scanlines along the X-axis in world space.  It  also 
> assumes that the picture is square and covers 20 degrees  horizontally 
> and vertically.
> 
> A paper that covers this idea in great detail, including local  
> photometry modelling, is:
> 
>     M. Goesele, X. Granier, W. Heidrich, and H.-P. Seidel
>     "Accurate Light Source Acquisition and Rendering"
>     SIGGRAPH '03
>     <http://www.cs.ubc.ca/~heidrich/Papers/Siggraph.03.pdf>
> 
> -Greg
> 
>> From: william reynolds <william.reynolds at oriel.ox.ac.uk>
>> Date: March 5, 2006 5:54:03 AM PST
>>
>> hi everyone
>> i've been trying out this hdr stuff, and i've run into a bit of a  
>> dead end with my understanding!
>>
>> i've left this on the general list, as it seems a slightly more  
>> general use of hdr imaging. correct me if im wrong!
>>
>> i've tried what you suggested greg, using an image i found (its not  
>> hdr, thoug i converted it to .pic) rather than a photo of the real  
>> lantern i want to use (hopefully i'll be able to go in and take  some 
>> photos early in the week). but the images i can generate are  turning 
>> out a bit messy - i wasn't sure that its just noise because  the 
>> patteern seems very regular, and there are 2 sorts of "mess" on  the 
>> image as well. i've put my files up at http://users.ox.ac.uk/ 
>> ~orie1226/rad/ if someone wouldn't mind having a quick look at what  
>> i've done. (files.txt explains what everything is).
>>
>> basically i've used the image to define the output distribution of  a 
>> source, and am then using that to light a room. the likely  problems 
>> seem to me to be either:
>> -some badly set arguments in my .rif file (i dont feel like i  
>> completely know what im doing, so this seems quite possible!), or
>> -a problem with the quality of the image im using to describe the  
>> source distribution. it's not that good quality, but i didn't think  
>> it was that bad.
>>
>> also, all i've been able to find about using hdr images in  rendering 
>> is about using an image showing the light distbution  across a whole 
>> scene so that a entire scene can be lit by a single,  large sperhical 
>> source surrounding it. (at least, i think thats the  idea!).
>> what im trying to do is define the complete output of source as i'm  
>> trying to model the effect of different combinations of sources on  my 
>> scene. does anyone know of any literature out there that might  be 
>> relevant to this (i think simpler) approach?
>>
>> thanks for your help.
>> will
> 
> 
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