If you're using imwarp, you might as well use imrotate, no? You'd be using the function imrotate() to rotate an image, if that's really your goal.
fa = 45;
I = imread('cameraman.tif');
imrot = imrotate(I,fa);
imshow(imrot)
If you want an anonymous function, like you're using...
rotateImage = @(im, ang) imrotate(im, ang);
imrot = rotateImage(I, fa);
imshow(imrot)
You could also make a standalone function, in a file of the same name or at the bottom of a script, that accepts inputs and outputs the rotated image, and does whatever else you want with the input.
imrot = rotImFun(I, fa);
imshow(imrot);
function out = rotImFun(im, ang)
out = imrotate(im, ang);
end
