Christopher James Huff wrote:

> In article <403c951a@news.povray.org>, Wolfgang Wieser <wwi### [at] gmxde>
> wrote:
> 
>> static DBL radial_pattern (VECTOR EPoint)
>> {
>>   register DBL value;
>> 
>>   if ((fabs(EPoint[X])<0.001) && (fabs(EPoint[Z])<0.001))
>>   {  value = 0.25;  }
>>   else
>>   {  value = 0.25 + (atan2(EPoint[X],EPoint[Z]) + M_PI) / TWO_M_PI;  }
>> 
>>   return(value);
>> }
>> 
>> Since atan2() will return values in range -Pi..Pi, the return value
>> of the pattern will be in range 0.25 .. 0.25+2*Pi and not in range
>> 0..1 as stated in the description of the function.
> 
> Note that the code adds pi to atan2() and divides the result by 2*pi
> before adding the 0.25. So the range of the function is [0.25, 1.25]...
>
[ Note that I corrected this mistake of mine about 3.5 hours before your 
post :) ]

> I don't know why. It gets wrapped to [0, 1] later anyway...maybe it was
> done to make the pattern start at +x. Seems unnecessary to me, but
> changing it would affect every scene that uses radial. Though you could
> make it dependent on version.
> 
Well, but instead of adding 0.25 one could use atan2(z,-x) instead of 
atan2(x,z). Why not write: 

value = (atan2(EPoint[Z],-EPoint[X]) + M_PI) / TWO_M_PI;

Now, the values are back in range 0..1 and it behaves exactly like 
the current one (after being wrapped to 0..1). 

Wolfgang

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