Image rotation and NaN elements

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Sordin
Sordin 2017-4-6
编辑： KSSV 2017-4-6
The following code rotates an image by a given angle θ. I do not understand the function and significance of the last line:
n = size(Image,1);
x = linspace(-1,1,n);
[X1,Y1] = meshgrid(x,x);
% Coordinate rotation
X = cos(theta)*X1 + -sin(theta)*Y1;
Y = sin(theta)*X1 + cos(theta)*Y1;
% Interpolation
RotatedImage = interp2(X1,Y1,Image,X,Y);
RotatedImage(isnan(RotatedImage)) = 0;
If I understand correctly, it eliminates the elements in the final image that are NaN. But why would the rotation or interpolation introduce elements which are not-a-number?
When I omit this line, the resulting image looks nothing like the original image. Without this line, is there any alternative way of avoiding NaN elements?
Any explanation would be appreciated.

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KSSV
KSSV 2017-4-6
编辑：KSSV 2017-4-6
RotatedImage = interp2(X1,Y1,Image,X,Y);
It introduces NaN's at the points, which are not in range. As it is not using extrapolation. So those NaN's are replaced with zeros. Read about interp2, how it is associated with extrapolation.

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