Sorry for the basic math question, but I'm trying to replicate the functionality of Softimage's “Get Angle Between”, which takes two normals and outputs the inbetween angle, and I'm blanking. I'm using VOP.

I'm guessing it's not as simple as just subtracting the two?

vangle() expression do what you're looking for?

float vangle (vector v0, vector v1)
Returns the angle between two vectors

What is that in VOP speak? Can I use expressions with vop?

negow
Sorry for the basic math question, but I'm trying to replicate the functionality of Softimage's “Get Angle Between”, which takes two normals and outputs the inbetween angle, and I'm blanking. I'm using VOP.
I'm guessing it's not as simple as just subtracting the two?

In VOPS land plug Dot Product VOP into Trigonometric Functions VOP selecting Arc Cosine as a function of it. This gives you an angle in radians, which you can convert to degrees with Radians to Degrees VOP. Note, vectors should be normalized!

hth,

ps. angle = acos(dot(normalize(v1),normalize(v2)))

Awesome, Thanks!

Would you mind explaining, for someone who missed that class, how the Arc Cosine works to output the inbetween angle in this scenario?

negow
Awesome, Thanks!

Would you mind explaining, for someone who missed that class, how the Arc Cosine works to output the inbetween angle in this scenario?

Well, I'm not the best person to explain math, but basically the magic thing is a dot product or scalar product, that relates to a projection of vector A on vector B. A' = |A|*cos(angle). Because arc cosine is an inverse function to cosine, you can use it to retrieve angle from above relation (and by using normalised vectors you cancel multiplication). Check out wiki: http://en.wikipedia.org/wiki/Dot_product [en.wikipedia.org]

cheers!
skk.

Don't you need to divide the dot by the magnitude before arcos? My notes have this.

cos i = v.w / |v||w|

This returns the cosine of the angle between the two vectors, so you'll need to use arcos (invert cos) to get the angle.

Alanw
Don't you need to divide the dot by the magnitude before arcos? My notes have this.

cos i = v.w / |v||w|

This returns the cosine of the angle between the two vectors, so you'll need to use arcos (invert cos) to get the angle.

unless you normalize v and w I guess… :roll:

two great resources for this:

Houdini specific:
http://www.3dbuzz.com/xcart/product.php?productid=58 [3dbuzz.com]
Peter Claes uses quite a bit of vector math and does a great breakdown of using the dot product and cross product for a few scenarios!

3DS MAX Specific - Which I don't use, but wanted to find some more material on 3D Math… The CG Academy's - MAXSCRIPT 3 DVD - The Matrix Explained…

Fantastic resource for getting a good understanding of 3D Math from vectors to matrices and their uses in 3d

Cheers,
Jonathan

SYmek

Alanw
Don't you need to divide the dot by the magnitude before arcos? My notes have this.

cos i = v.w / |v||w|

This returns the cosine of the angle between the two vectors, so you'll need to use arcos (invert cos) to get the angle.

unless you normalize v and w I guess… :roll:

doh!

Great advice guys, and thanks for the explanation SyMek!
Guess I'll go through those dvd's, once and for all )

thanks symek, just used you trick

symek
angle = acos(dot(normalize(v1),normalize(v2)))

I will just add
VEX
float angle = degrees(acos(dot(normal_pt, nn_pt)));

in case you need it on degrees

pelos
http://en.wikipedia.org/wiki/Dot_product [en.wikipedia.org]

what is normal_pt and nn_pt ?

what is normal_pt and nn_pt ?

They are two user-defined vectors for the dot function.

http://www.sidefx.com/docs/houdini/vex/functions/dot.html [www.sidefx.com]