affine2d in Octave - matlab

Question

In MATLAB I make a cylinder r and shift it to the position I want which is Pos. In MATLAB I use affine2d and imwarp but unfortunately Octave doesn't have these functions.
Does anybody know how I can do this in Octave without affine2d and imwarp?
The MATLAB code is
% get image limits
limX = size(image,1)/2;
limY = size(image,2)/2;
steps = 1;
% add 30% to the inner diameter to make sure it covers the complete sparse area
largeradius = 1.5*diaStart/2;
smallradius = diaStart/2;
% generate x/y points
[x,y] = meshgrid(-limX:steps:limX,-limY:steps:limY);
% calculate the radius values:
r = sqrt(x.^2 + y.^2);
r(r>largeradius) = 0;
r(r<smallradius) = 0;
% Shift translate circle in place
r = im2bw(r);
xPos = (Pos(1)-limX);
yPos = Pos(2)-limY;
tform = affine2d([1 0 0; 0 1 0; xPos yPos 1]);
r_trans = imwarp(r,tform,'OutputView',imref2d(size(image)));

Answer 1

It should be easy to do. affine2d is not a problem, as it is a function that changes the data type, but doesn't modify anything.
imwarp does [x y 1] = [u v 1] * T (being T the affine transformation Matrix) for each of the pixels.
So, if you know that you want the values of the pixels in some specific locations in the transformed image, then its easy to know them
In other words: you have an image r (composed by [u v 1] pixels, a transformation tform and you know that you want to know how a new image is created by that. The new image r_trans is composed by [x y 1] pixels, and you know [x,y,1] values.
Basically, you want to get r(u,v) for each [u,v,1]=[x y 1]*T^(-1);.
x will be 1:size(r,1), and y=1:size(r,2).
Therefore computing [u,v,1]=[x y 1]*T^(-1); is not a problem.
Now, you want to access r(u,v), and u and v wont be integers, they will be floating point values. To be able to get ther` values you will need interpolation.
For that, you need to use this simple piece of code;
[X,Y]=meshgrid(1:size(r,1),1:size(r,2));
value=interp2(X,Y,r,ui,vi,method); %chose method from https://www.gnu.org/software/octave/doc/interpreter/Multi_002ddimensional-Interpolation.html
r_trans(xi,yi)=value;
I didn't give you the whole code, but hopefully you understand how to do it.