| 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 |
|
) ); |
