Math Is Fun Forum

SamuraiDave

Guest

Dot notation problem. Not dot product??? Vector maths...

I have a problem understanding the following notation:

The problem is that to me the '.' means dot product, and I can't find any other definitions for this notation.

But v is a scalar value and (Q1 - P1) is a vector. Therefore, what am I supposed to do with this formula?

I'm trying to solve the bottom part of this equation (for X1):

Here, u and v are both used with the dot notation, but are both scalar values.

I'd appreciate any help! I'm lost.

Best Regards,

David

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Dot notation problem. Not dot product??? Vector maths...

Hi SamuraiDave;

As you know you can't dot product a scalar and a vector.

Are you sure that v is a scalar.

Last edited by bobbym (2009-11-27 01:22:24)

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

SamuraiDave

Guest

Re: Dot notation problem. Not dot product??? Vector maths...

v is definetly a scalar.

Equation 2's first line returns the dot product of (X - P) and the perpendicular of (Q - P). This is of course a scalar value.
It is then divided by || Q - P ||  which is the norm/length of the vector Q - P. Also a scalar value.

Therefore v is a scalar.

Thanks for the suggestion though...:)

I'm still stuck, so any further help is really appreciated!

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Dot notation problem. Not dot product??? Vector maths...

Hi;

Yes, you are correct. Could it be a typo? Have you tried just using multiplication instead of the dot prod in the spots where the formula has a scalar .  vector?

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

SamuraiDave

Guest

Re: Dot notation problem. Not dot product??? Vector maths...

I guess it could be a typo. I've tried scaling the vector using the scalar value v, but that doesn't seem to produce the correct results.

The equation is described more on this page: http://www.hammerhead.com/thad/morph.html

It's actually a siggraph paper, so i'd hope they wouldn't make typos...but we're all human (mostly) so I guess they could have slipped up.

SamuraiDave

Guest

Re: Dot notation problem. Not dot product??? Vector maths...

I had a slight error in my previous attempt with my perpenidcular function, i've now corrected this and can confirm that you were right.

The dot notation is a typo. In the equation where the dot notation is used with a scalar, it should be replaced by a vector multiplication.

Thanks again for the help! 

Regards,

David

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Dot notation problem. Not dot product??? Vector maths...

Hi SamuraiDave;

Those can be very frustrating. But it may not be a typo!

Looked at the link and even in the expalnations for u and v it is clear that thay are scalars.

2 Computer Algebra systems I have worked with, both default down to ordinary multiplication for
scalar . vector when they encounter it, so that is probably what was intended here.

Last edited by bobbym (2009-11-27 01:54:12)

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

SamuraiDave

Guest

Re: Dot notation problem. Not dot product??? Vector maths...

hehe you posted just 1 second after me, you are correct that I just had to multiply it.
I was not aware that in computing systems, scalar dot vector is a vector multiply.

Many thanks again,

David

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Dot notation problem. Not dot product??? Vector maths...

Hi SamuraiDave;

Glad to help. Thanks for the link!

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.