[Radiance-general] Slow rendering with transparant textures

[Greg Ward]

Hi Lars,

Although it helps up to a point to reduce the complexity of the .cal  
expressions, it's more important to minimize the total number of such  
modifiers in your rendering, since it's the overhead of the call  
itself that's most significant.  For example, adding a functional  
pattern to another functional pattern that uses a functional texture  
and surface normal interpolation and a mapped picture with its own  
(admittedly unavoidable) coordinate transformation, that's going to  
be a lot slower than combining all the patterns into a single  
precomputed image and having just one functional texture.

Since Iebele had many mixfunc calls, most of which rendered  
transparent, this cost quite a bit in overhead.  In many cases it's  
unavoidable, but bear in mind that any calculations using the  
function files, while wonderfully programmable, cost quite a bit in  
overhead (i.e., a few times the cost of a ray intersection) plus 2x  
or more longer than the equivalent compiled math calls.

Does this make sense?

-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: July 28, 2008 9:32:33 AM BDT
>
>> One of the slowest things in Radiance is the .cal file  
>> evaluations, so minimizing expression evaluations is the key to  
>> faster renderings in this case.  Having many surfaces for each  
>> ray, all having function evaluations, is bound to be slow.   
>> Combining or eliminating such calls would help a lot.  I don't  
>> know enough about your situation to recommend "how."
> Greg hi,
>
> I got curious now when I hear about eliminating such calculations.  
> As far as I know, I alsways have to at least rotate anything mapped  
> that I want to appear on a non-horizontal surface. I can avoid  
> scaling, mirroring etc by applying these tranforms to the images  
> using image processing tools such as those coming with radiance.  
> But for avoiding the rotation, I do not see a way (besides rotating  
> the whole model with light sources and views instead of the images  
> if e.g. I want to map most onto a vertical plane). So did you mean  
> something like that when saying "eliminating such calls", or were  
> you thinking about structuring them more efficient?
>
> CU Lars.