matlab-从矩阵获取向量（quiverplot）

Question

I am reading in an image and storing it into a 2d matrix. after doing some computation on it as shown here:

im = rgb2gray(imread('ellipse.png'));
im = im(:,:,1);
w = size(im,1);                   
h = size(im,2);                               
[dx,dy] = gradient(double(im));                
[x y] = meshgrid(1:h,1:w);                    
a = zeros(temp);
lambda = 1;
Ox =-1.^lambda.* -x;
Oy =-1.^lambda.* y;
hold on                                   
quiver(x,y,Ox,Oy)  

I get the following image from the quiverplot:

https://docs.google.com/file/d/0B0iDswLYaZ0zR2lUQ2NkZnd1QXM/edit?pli=1

My question is, how do I access those vectors (arrows) from the quiverplot? I need to use those vectors in a cross product later on. Thanks.

Answer 1

An easy way would be stack your matrices into a big NxMx3 block and call cross against it like this:

myVectors = cat(3, Ox, Oy, zeros(size(Ox)));
someOtherVectors = myVectors+1;
result = cross(myVectors,someOtherVectors );

Another would be to write your own cross function like this:

function result = cross2d(Ax, Ay, Bx, By)
      result = Ax.*By - Ay.*Bx;
end

And call it like this in your code:

Mx = Ox + 1;
My = Oy + 1;

result2 = cross2d(Ox, Oy, Mx, My)

(Note that I just made up a new set of vectors to take the cross product against by adding one... you would use your own)