Error using dot(A,B) - size/dimension problem

Hello.

I get the error "Error using dot (line 15) A and B must be same size." (Which is dot(A,B) ) I have tried numbers of time now, and I still don't know what I am doing wrong here.

I have the normal vector. 'n' and a point 'x' in the plane and the output is 's' in my formula.

I have attempted to get same dimension with size, but it appears not to been working so well.

...

n = vector

x = point in a plane

size(x,dim)

size(n,dim)

s=x+2*(d-dot(x,n))*n;

...

Regards

Cillian

Answer 1

[Daniel Shub]

The error seems pretty clear to me your n and x are different sizes.

[Cillian]

Yes, I know. But do you know how I can get them to the same size?

[Daniel Shub]

What does size(x) and size(n) return?

[Cillian]

I changed my code a little:

a=size(x)

b=size(n)

s=x+2*(d-dot(a,b))*n;

a =

1 1

b =

3 1

ans =

-1.2815

-2.4222

-3.5629

[Jan]

The DOT product is defined for vectors of the same size only. If you want an operation between e.g. a [1 x 3] and a [1 x 4] vector, it cannot be a DOT product. Therefore you have either a bug in your code, or you want something else.

[Daniel Shub]

What do you think x and n represent?

[Jan]

Now your "a" is a scalar. Then using "*" instead of "dot" is recommended.

[Cillian]

so, the multiplication sign "*" should be put on this way:

s=x+2*(d-(a*b))*n; ?

I tested it, and still got:

a =

1 1

b =

3 1

And some erros:

Error using *

Inner matrix dimensions must agree.

Error in spegel (line 8)

s=x+2*(d-(a*b))*n;

[Jan]

While the multiplication between "a" and "b" works now, I guess that "b" and "n" have different sizes and the matrix multiplication fails.

However, Cillian, it is time to stop the confused guessing. Please post the complete relevant part of the code and explain what you want to compute. It seems like there is a problem in the mathematical part already.

Answer 2

[Cillian]

sorry that I haven't been so clear abut this. here is the code:

function s = mirror(x,A,B,C,D) % x is a point in the room.

normvect = [A;B;C;];

length = norm(normvect);

d = D/length;           % d is a point in the plane
n = normvect/length;    % n is a normal vector 

s = x + 2*(d - dot(x,n))*n

end

I want to calculate the reflection image "s" of the point "x" in the plane.