How to count pixel which are coming on the edge of the polygon?

Question

Subhodh Sharma 2021-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/1583279-how-to-count-pixel-which-are-coming-on-the-edge-of-the-polygon

编辑： Subhodh Sharma 2021-11-11

Hello community,

I am facing issue in masking the data according to shape file. I took Indian latitude and longitude information. I have masked using the inpolygon function. Each pixel has size 2.5 * 2. I have a polygon, I want to select each pixel that placed in or on my polygon.The issue is that the pixel which is on the polygon is not considered by inpolygon command, however from the plot (left panel: after mask, right panel:after mask), one can readily see some pixel are cutting the polygon. I have shared the data and polygon info in the attachment.

I want each pixel which inside or atleast cutting the ploygon.

I would really appreciate any sort of help.

Thanking you in advance.

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Answer 1

KSSV 2021-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/1583279-how-to-count-pixel-which-are-coming-on-the-edge-of-the-polygon#answer_828309

在 MATLAB Online 中打开

Your data from .mat file already has lot of NaN values. Thee is no data in the mat file. Check your data.

The same code for some dummy data works fine.

lat1=0:2:40; 
lon=60:2.5:100;
% data = load ('data.mat') ;
shape=xlsread('indiaonlylatlon.xls');
[x,y]=meshgrid(lon,lat1);
[isin,ison] =inpolygon(y,x,shape(:,2),shape(:,1));
id=isin|ison;
% var1=data.ans;
var1=rand(size(x));
var1(~id)=NaN;
pcolor(x,y,var1)
hold on
plot(shape(:,1),shape(:,2),'r')

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KSSV 2021-11-10

That case go for knnsearch.

KSSV 2021-11-10

在 MATLAB Online 中打开

lat=0:2:40; 
lon=60:2.5:100;
[nodes,X,Y,P] = Mesh(lon,lat) ;
var1 = load ('data.mat') ;
var1 = var1.ans ; 
shape=xlsread('indiaonlylatlon.xls');
[x,y]=meshgrid(lon,lat);
nel = size(nodes,1) ; 
iwant = zeros(nel,1) ;
for i = 1:size(nodes,1)
    [isin,ison] =inpolygon(shape(:,1),shape(:,2),X(:,i),Y(:,i));
    if any(isin|ison)
        iwant(i) = 1 ;
    end
end
idx = logical(iwant) ;
figure
hold on
plot(shape(:,1),shape(:,2),'r')
patch('faces',nodes(idx,:),'vertices',P,'facevertexcdata',var1(:),'facecolor','interp','edgecolor','k') ;
function [nodes,X,Y,P] = Mesh(lon,lat) 
Nx = length(lon)-1 ;   
Ny = length(lat)-1 ;       
% From here dont change
nel = Nx*Ny ;        % Total Number of Elements in the Mesh
nnel = 4 ;           % Number of nodes per Element
% Number of points on the Length and Breadth
npx = Nx+1 ;
npy = Ny+1 ;
nnode = npx*npy ;      % Total Number of Nodes in the Mesh
% Discretizing the Length and Breadth of the plate
[xx, yy] = meshgrid(lon,lat) ;
% To get the Nodal Connectivity Matrix
coordinates = [xx(:) yy(:)] ;
NodeNo = 1:nnode ;
nodes = zeros(nel,nnel) ;
% If elements along the X-axes and Y-axes are equal
if npx==npy
    NodeNo = reshape(NodeNo,npx,npy);
    nodes(:,1) = reshape(NodeNo(1:npx-1,1:npy-1),nel,1);
    nodes(:,2) = reshape(NodeNo(2:npx,1:npy-1),nel,1);
    nodes(:,3) = reshape(NodeNo(2:npx,2:npy),nel,1);
    nodes(:,4) = reshape(NodeNo(1:npx-1,2:npy),nel,1);
% If the elements along the axes are different
else%if npx>npy
    NodeNo = reshape(NodeNo,npy,npx);
    nodes(:,1) = reshape(NodeNo(1:npy-1,1:npx-1),nel,1);
    nodes(:,2) = reshape(NodeNo(2:npy,1:npx-1),nel,1);
    nodes(:,3) = reshape(NodeNo(2:npy,2:npx),nel,1);
    nodes(:,4) = reshape(NodeNo(1:npy-1,2:npx),nel,1);
end
P = [xx(:) yy(:)] ; 
%
% Plotting the Finite Element Mesh
% Initialization of the required matrices
X = zeros(nnel,nel) ;
Y = zeros(nnel,nel) ;
% Extract X,Y coordinates for the (iel)-th element
  for iel = 1:nel
      X(:,iel) = coordinates(nodes(iel,:),1) ;
      Y(:,iel) = coordinates(nodes(iel,:),2) ;
  end
end