239 |
|
* All vectors are assumed to be normalized. |
240 |
|
* This function is an implementation of the method proposed by |
241 |
|
* Robert Levin in his 1975 JIES article. |
242 |
+ |
* This calculation presumes the view direction and up vectors perpendicular. |
243 |
|
* We return a value less than zero for improper positions. |
244 |
|
*/ |
245 |
|
|
253 |
|
d = DOT(sd,vd); |
254 |
|
if (d <= 0.0) |
255 |
|
return(-1.0); |
256 |
+ |
if (d >= 1.0) |
257 |
+ |
return(1.0); |
258 |
|
sigma = acos(d) * (180./PI); |
259 |
< |
tau = acos(DOT(sd,vu)/sqrt(1.0-d*d)) * (180./PI); |
259 |
> |
d = DOT(sd,vu)/sqrt(1.0-d*d); |
260 |
> |
if (d >= 1.0) |
261 |
> |
tau = 0.0; |
262 |
> |
else |
263 |
> |
tau = acos(d) * (180./PI); |
264 |
|
return( exp( sigma*( (35.2 - tau*.31889 - 1.22*exp(-.22222*tau))*1e-3 |
265 |
|
+ sigma*(21. + tau*(.26667 + tau*-.002963))*1e-5 ) |
266 |
|
) ); |