View Full Version : blinn half vector

tellaman

03-14-2003, 02:09 PM

i still dont get it, everywhere i go i see H defined as (V+L)/2 , with V and L normalized vecs

but, shouldnt H be normalized as well?

if so, what's the use of dividing it by 2?

thanks for your help

pkaler

03-14-2003, 02:27 PM

H is an approximation of an apporximation.

H' ~ R = 2(N.L)N - L

Where

H' = (L + V)/ | L + V |

H = (L +V)/2 ~ H'

Because

1/2 ~ | L + V |

When

|L| = |V| = 1

I think the equations are right. Double check in your favorite copy of Foley et al.

[This message has been edited by PK (edited 03-14-2003).]

castano

03-14-2003, 02:33 PM

You are perfectly right. H should be normalized. The equation is usually written that way, to state that H is the average of L and V, and after that you should normalize H. Of course, you can skip the division, since in that case it's mathematically equivalent.

tellaman

03-14-2003, 09:54 PM

oh thanks it finally makes some sense

pk: i think the approximation only works in the viewable range of R since i just dont see how | L + V | could be ~ 2 even if |L|=|V|=1, and i wouldnt say that H (or H') ~ R because H is usually far away from R and it's being dotted with N, while R has to be dotted with V

anyway thanks again, this H thing was just driving me crazy

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