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

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Cillian
Cillian 2012-5-4
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

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回答（2 个）

Daniel Shub
Daniel Shub 2012-5-4
The error seems pretty clear to me your n and x are different sizes.
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Cillian
Cillian 2012-5-4
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
Jan 2012-5-4
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.

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Cillian
Cillian 2012-5-4
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.

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