{"id":285117,"date":"2023-05-22T11:37:46","date_gmt":"2023-05-22T11:37:46","guid":{"rendered":"\/forum\/forums\/topic\/topology-inverse-design-mesh-interpolation\/"},"modified":"2023-05-22T11:37:46","modified_gmt":"2023-05-22T11:37:46","slug":"topology-inverse-design-mesh-interpolation","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/topology-inverse-design-mesh-interpolation\/","title":{"rendered":"Topology inverse design mesh interpolation"},"content":{"rendered":"

Dear ansys team .
TLDR: why does topological optimization use the spatial interpolation of “specified position”, instead of “nearest mesh cell”, it creates errors. <\/p>\n

Long:
I am learning how to use the topological optimization in lumopt + FDTD. when working on my project, I stumbled upon FDTD mesh grid vs optimised geomtry error, which I also found how to recreate using the following example.<\/p>\n

following the 2d example in y splitter topology example  link <\/a>the example runs ok as-is. 
However, when changing line 26 in file “splitter_base_2D_TE_topology.lsf” to
set(‘x max’,-opt_size_x\/2 – 0.5e-7); 
which means making the input waveguide to be outside of the optimization area, I get this error:
  File “C:\\Program Files\\Lumerical\\v231\\api\\python\\lumopt\\geometries\\topology.py”, line 158, in check_param_count_matches_bounds <\/span>
UserWarning: Project file structure ‘initial_guess’ reports 13832 parameters where expected 13741. This can occur when the number of x, y coordinates specificed to LumOpt do not match the dimensions of the FDTD grid. Try using similar or divisible delta values (size of a pixel) across all the axes.<\/span><\/p>\n

Note that the amount of displacment is important, -0.5e-7 will cause the error, whereas -1e-7 wont. 
Also note, I didnt change the optimization area, the FDTD size, the mesh grid setting or anything relevant. just the WG
my debugging:
<\/span>after adding a breakpoint in topology.py, line 185
I noticed that indeed the “opt_fields” x resolution is of 152, whereas the x_geo resolution is of 151.
I think that somehow changing the wg position changes the mesh.
possible fix I found:
<\/span>change lines 366-367 in topology.py to: <\/p>\n

set_spatial_interp(sim.fdtd, ‘opt_fields’, ‘nearest mesh cell’)
set_spatial_interp(sim.fdtd, ‘opt_fields_index’, ‘nearest mesh cell’).
Then optimization runs OK.<\/p>\n

My question<\/span>: why does the spatial interpolation of opt_fields & opt_fields_index set to “specifed postion”? The goal should be to force that three data positions are identical:  <\/p>\n