# Find the surface area of an object using stereo vision.

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Naseeb Gill on 6 Jan 2020
Edited: Adam Danz on 7 Jan 2020
Hello,
I want to find out the surface area of tree (only green part) using stereo vision. I know stereo vision and using the same I got the (X,Y,Z) values of tree, (of course one side only) (file is attached). I used delaunay function to calculate area but the problem is that I have some negative values of X and Y which cause error in surface area. How I can get the correct value of surface area using stereo vision?
It will be good if I can calculate volume also.
Thank You.
Adam Danz on 7 Jan 2020
Edited: Adam Danz on 7 Jan 2020
"Say for example I have A = [-2,-7,0.5,14,3] by doing offset It should be convert into B ="[0,0,2.5,21,3,]. "
Why would A be converted to that? A would be offset by it's minimum value which is -7.
A = [-2,-7,0.5,14,3]
A = A - min(A)
A =
5 0 7.5 21 10
For C
C = [-1,-9,-5,14,3]
C - min(C)
ans =
8 0 4 23 12
Note that even if A or C do not have negative values, applying the same proceedure (A - min(A)) does not affect the area.

KSSV on 7 Jan 2020
After using delaunayTriangulation, you have vertices and nodal connectivity in hand. Then you can use this function to get the area of each triangular element........sum the total areas, you will get the total sutgace area.
Have a look on function triarea from the package.
Naseeb Gill on 7 Jan 2020
Yes I tried with triarea, it shows error.
Error using triarea (line 29)
Incorrect input dimensions.
Mine pp array is 18x3 and tt array is 24x4.
Also, code I wrote in above comment is same as code written in triarea. Some of its part is:
%--------------------------------------- compute signed area
ev12 = pp(tt(:,2),:)-pp(tt(:,1),:) ;
ev13 = pp(tt(:,3),:)-pp(tt(:,1),:) ;
switch (size(pp,2))
case +2
area = ev12(:,1).*ev13(:,2) ...
- ev12(:,2).*ev13(:,1) ;
area = 0.5 * area;
case +3
avec = cross(ev12,ev13);
area = sqrt(sum(avec.^2,2)) ;
area = 0.5 * area;
otherwise
error('Unsupported dimension.') ;
end
After observing both codes, there is no difference. I'm not changing order of vertices, I'm using them as it as.

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