2D Discrete laplacian (del2) in C++

I am trying to figure out how to port the del2() function in matlab to C++.

I have a couple of masks that I am working with that are ones and zeros, so I wrote code liket his:

for(size_t i = 1 ; i < nmax-1 ; i++)

{
    for(size_t j = 1 ; j < nmax-1 ; j++)

    {
        transmask[i*nmax+j] = .25*(posmask[(i+1)*nmax + j]+posmask[(i-1)*nmax+j]+posmask[i*nmax+(j+1)]+posmask[i*nmax+(j-1)]);

    }
}

to compute the interior points of the laplacians. I think according to some info in "doc del2" in matlab, the border conditions just use the available info to compute, right? SO i guess I just need to write cases for the border conditions at i,j = 0 and nmax

However, i would think these开发者_JAVA技巧 values from the code I have posted here would be correct for the interior points as is, but it seems like the del2 results are different!

I dug through the del2 source, and I guess I am not enough of a matlab wizard to figure out what is going on with some of the code for the interior computation

You can see the code of del2 by edit del2 or type del2. Note that del2 does cubic interpolation on the boundaries.

The problem is that the line you have there:

transmask[i*nmax+j] = .25*(posmask[(i+1)*nmax + j]+posmask[(i-1)*nmax+j]+posmask[i*nmax+(j+1)]+posmask[i*nmax+(j-1)]);  

isn't the discrete Laplacian at all.

What you have is (I(i+1,j) + I(i-1,j) + I(i,j+1) + I(i,j-1) ) / 4

I dont' know what this mask is, but the discrete Laplacian (assuming the spacing between each pixel in each dimension is 1) is:

(-4 * I(i,j) + I(i+1,j) + I(i-1,j) + I(i,j+1) + I(i,j-1) )

So basically, you missed a term, and you don't need to divide by 4. I suggest going back and rederiving the discrete Laplacian from its definition, which is the second x derivative of the image plus the second y derivative of the image.

Edit: I see where you got the /4 from, as Matlab uses this definition for some reason (even though this isn't standard mathematically).

I think that with the Matlab compiler you can convert the m code into C code. Have you tried that?

I found this link where another methot to convert to C is explained.

http://www.kluid.com/mlib/viewtopic.php?t=337

Good luck.