defining a polygon area within an array

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Jonathan
Jonathan on 26 Mar 2020
Answered: Akira Agata on 26 Mar 2020
I have a 2D array of numbers.
I have a polygon with defined x and y coordinates.
Could someone please help me set the area within the array that the polygon overlaps to 0 please.

Answers (2)

Akira Agata
Akira Agata on 26 Mar 2020
How about the following solution?
% 100-by-100 sample matrix
Zin = peaks(100);
Zin = Zin - min(Zin(:));
% Example of polygon
pgon = polyshape([10 10 80 30], [10 30 30 10]);
% Create logical index which indicates inside/outsize the polygon
[colGrid,rowGrid] = meshgrid(1:size(Zin,2),1:size(Zin,1));
idx = isinterior(pgon,[colGrid(:),rowGrid(:)]);
idx = reshape(idx,size(Zin));
% Set elements inside the polygon to 0
Zout = Zin;
Zout(idx) = 0;
% Show the result
figure
subplot(2,2,1)
contourf(Zin)
title('Original','FontSize',14)
colorbar
subplot(2,2,2)
plot(pgon)
xlim([1 100])
ylim([1 100])
box on
title('Polygon','FontSize',14)
subplot(2,2,3)
contourf(Zout)
title('Result','FontSize',18)
colorbar

Walter Roberson
Walter Roberson on 26 Mar 2020
If you have the Computer Vision Toolbox, then you can use https://www.mathworks.com/help/vision/ref/insertshape.html#btppvxj-1-shape insertShape with 'filledpolygon' and 'Color', 'k' . This will write 0s into the part of the rectangle indicated by the coordinates. The resulting array will have been converted to RGB, but you can just take the first pane of it.
  1 Comment
Jonathan
Jonathan on 26 Mar 2020
Hi there is valuable information in the resulting array containing numbers that are essential for analysis. Is there anyway of using a function (maybe without the computer vision toolbox) that can preserve the resulting array?

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