Patch won't fill my polygon and inpolygon won't find points inside

I'm trying to do two things with the coordinates of a shape that was created using slice_stl_create_path from the FEX. First, I just want to plot one of the slices as a filled patch, but I can only get part of it filled. I think it's because it kind of appears as two polygons, but I'm not sure how to avoid that given the method for creating the coordinates. I've attached a file with the x and y coordinates of the polygon as derived from the FEX script.

Second, I have another grid of data and would like to identify points on the grid that are within this polygon. I know from looking at the numbers that some are within it, but inpolygon returns all zeros. The grid points are also in the attachment. Here's some code:

load('coords')

% this doesn't fill anything
patch(hhx,hhz,'k') % hhx and hhz are found with slice_stl_create_path.m

% this fills part of it
[shhx,idx] = sort(hhx);
shhz = hhz(idx);
patch(shhx,shhz,'k')

% to find points inside
inshape = inpolygon(xdim(31:31+length(zdim)-1),zdim,hhx,hhz); % xdim and zdim are the grid points, xdim is longer than zdim

[Daniel]

Solved the first problem by getting rid of NaNs in hhx and hhz. That had no effect on the second problem, though.

[Daniel]

I managed a simple workaround using find, but I'm going to leave it open in case someone can say why inpolygon wasn't working (or how I was misusing it).

[Cris LaPierre]

I tried running your code and got the following errer:

Unrecognized function or variable 'ycoords'.

[Daniel]

Sorry, had an error. Fixed now.

Answer 1

[Cris LaPierre]

For question 2, it doesn't work for a couple reasons.

1. xdim(31:length(zdim)-1) must be the same length as zdim. They are not, so you get an error.
2. Modifying the data so they are the same length, none of the points fall within the polygon, hence, inpolygon is not returning anything.

load('coords')
% Remove nans 
hhx(isnan(hhx)) = [] ;
hhz(isnan(hhz)) = [] ;
% GEt the boundary 
idx = boundary(hhx,hhz) ; 
hhx = hhx(idx) ; hhz = hhz(idx) ; 
patch(hhx,hhz,'k') 

hold on
plot(xdim(31:length(zdim)+30),zdim,'r.')
hold off

Perhaps you have not created the grid of data you intended. Can you describe what you expected the gridded points to be?

[Daniel]

Apologies. I had a typo that I have now fixed. The lengths were not the issue. Perhaps I need a meshgrid? xdim and zdim are meant to be the x and z coordinates of a grid on which data exist. So all pairs of x and z coordinates should be considered, but I thought that inpolygon did that. Does it not?

[Cris LaPierre]

No, it will not create the combinations for you. Look to the documentation.

• The size of xq must match the size of yq. (and vice versa)

You must pass in the data with all the pairings already made. If you want to create a grid using the points you already have,then yes, use meshgrid.

load('coords')
% Remove nans 
hhx(isnan(hhx)) = [] ;
hhz(isnan(hhz)) = [] ;
% GEt the boundary 
idx = boundary(hhx,hhz) ; 
hhx = hhx(idx) ; hhz = hhz(idx) ; 
patch(hhx,hhz,'k') 

[xq,yq] = meshgrid(xdim,zdim);
inshape = inpolygon(xq,yq,hhx,hhz);

hold on
plot(xq,yq,'b.')
plot(xq(inshape),yq(inshape),'r.')
hold off

Answer 2

[KSSV]

load('coords')
% Remove nans 
hhx(isnan(hhx)) = [] ;
hhz(isnan(hhz)) = [] ;
% GEt the boundary 
idx = boundary(hhx,hhz) ; 
hhx = hhx(idx) ; hhz = hhz(idx) ; 
patch(hhx,hhz,'k') 

[Daniel]

Thanks for the answer. I had already figured out this part of the question, but I'm leaving it "unanswered" until I get an answer on the second part regarding inpolygon. Can you tell me why that is not working even though some of the grid points (xdim and zdim) in the attachment are within the polygon?

Answer 3

[Matt J]

Aside from what others have said, your hhx and hhz data does not define a polygon, but rather two polygons. I supsect that this will confuse inpolygon() even if you test points that are within one of the polygons.

load coords
pgon=polyshape(hhx,hhz,'Simplify',false);

plot(pgon)

I'm assuming the shape you actually intend is closer to the following:

d=0.0003;

pgon=polybuffer( polybuffer(pgon,d), -d); %morphological close
pgon=rmslivers(pgon,3e-6);
plot(pgon)

[Daniel]

I tested your code but do not get the same result. See below. How did you pick the d and the input to rmslivers? Also, this creates a polyshape that cannot be used as an input to inpolygon. Is there a way to use this method to convert to an input to inpolygon?

[Matt J]

You tested the code on the same data?

[Daniel]

Honestly, I'm not totally sure, but that's the less important part. How would you extend this to use inpolygon?

[Matt J]

You can either use isinterior()

https://www.mathworks.com/help/matlab/ref/polyshape.isinterior.html

or you can get the vertices from the Vertices property of the polyshape object,

pgon.Vertices

whereupon you can use inpolygon() as normal.