> On Nov 2, 2020, at 1:35 AM, Mandy Xia <[email protected]> wrote:
> My high-level understanding of the near to far field calculation is that it
> uses dft to compute equivalent current on the near field box and convolve
> with the green's function to compute the field in the far field. When I
> looked into the implementation of the function
> dft_near2far::farfield_lowlevel, I was wondering where the integration over
> current sources is done. It seems that the code it's adding up the field
> contribution from each equivalent current source on the near field box. Maybe
> I'm missing a place where the numerical integration is performed or I have
> some misunderstanding.
That's right, this line in farfield_lowlevel is adding up the contribution of
each near-field source point computed by the preceding call to the "green"
function:
https://github.com/NanoComp/meep/blob/accad0a827f95e89bd05da08d92f4b4d6ecb7f5e/src/near2far.cpp#L380
<https://github.com/NanoComp/meep/blob/accad0a827f95e89bd05da08d92f4b4d6ecb7f5e/src/near2far.cpp#L380>
In fact, this sum is a numeric integral, because the "dV" integration weight is
already included in the Fourier transformed fields array f->dft. That's done
by passing "true" for the 5th argument of add_dft here:
https://github.com/NanoComp/meep/blob/accad0a827f95e89bd05da08d92f4b4d6ecb7f5e/src/near2far.cpp#L633
<https://github.com/NanoComp/meep/blob/accad0a827f95e89bd05da08d92f4b4d6ecb7f5e/src/near2far.cpp#L633>
which creates the object that accumulates the Fourier-transformed E and H
fields, which corresponds to the include_dV_and_interp_weights argument here:
https://github.com/NanoComp/meep/blob/master/src/dft.cpp#L158-L161
<https://github.com/NanoComp/meep/blob/master/src/dft.cpp#L158-L161>
which leads the DFT object to include the dV factor when it accumulates the
Fourier transforms here:
https://github.com/NanoComp/meep/blob/master/src/dft.cpp#L232-L233
<https://github.com/NanoComp/meep/blob/master/src/dft.cpp#L232-L233>
The basic reason for all of this is that the knowledge of how to compute dV
factors and interpolation weights for a Yee grid point contributing to a
user-specified volume is all concentrated in a single place in the code, the
loop_in_chunks function defined here:
https://github.com/NanoComp/meep/blob/master/src/loop_in_chunks.cpp
<https://github.com/NanoComp/meep/blob/master/src/loop_in_chunks.cpp>
which is used to loop over the Yee grid when updating the Fourier transforms,
and which is described in section 7.2 of the Meep paper
(http://math.mit.edu/~stevenj/papers/OskooiRo10.pdf
<http://math.mit.edu/~stevenj/papers/OskooiRo10.pdf>)._______________________________________________
meep-discuss mailing list
[email protected]
http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
