This topic is used in spectral methods, for collocation grid.
Literature mentions Chebyshev interpolation on a grid (defined by $\xi_j = cos(\pi \cdot j/N)$, $x_j = (\xi_j+1) L/2$, $j=0,...,N$) includes 2 steps
1/ Calculation of Chebyshev coefficients (sum over all points $x$)
$c_n = \sum_{j=0}^N f(x_j) \cdot T_n (\xi_j)\cdot w_j$ — In the $x$ direction. Does not depend on a specific $x$, only on $n$...
2/ The interpolation itself
$result(x) = \sum_{n=0}^{N} c_n \cdot T_n\left ( 2\cdot {x \over L}-1\right )$
How can the same be done in 2-dimensional space?
I could come up with these steps, but no literature supporting such point.
(This is $N_x + N_y$ coefficients, as mentioned in one of the answers.)
A. Calculation of Chebyshev coefficients (sum over all points $x$)
- $c_n^{(y_0)} = \sum_{j=0}^{N_x} f(x_j, y_0) \cdot T_n (\xi_j)\cdot w^{(x)}_j$ — in the $x$ direction
- $d_n^{(x_0)} = \sum_{k=0}^{N_y} f(x_0, y_k) \cdot T_n (\eta_k)\cdot w^{(y)}_k$ — in the $y$ direction
B. The interpolation itself $$ result(x,y) = \sum_{n=0}^{N_x} \sum_{k=0}^{N_y} c_n^{(y_k)} d_n^{(x_n)} \cdot T_n\left ( 2\cdot {x \over L_x}-1\right ) \cdot T_k\left ( 2\cdot {y \over L_y}-1\right ) $$
(Or these two steps, which are a quite different thing.)
(This is $N_x \cdot N_y$ coefficients, as mentioned in one of the answers.)
A. Calculation of Chebyshev coefficients (sum over all points $x$) $$ c_{n,k} = \sum_{j=0}^{N_y} \sum_{i=0}^{N_x} f(x_i, y_j) \cdot T_n (\xi_i)\cdot T_k (\eta_j)\cdot w^{(x)}_i \cdot w^{(y)}_j $$ B. The interpolation itself $$ result(x,y) = \sum_{i=0}^{N_x} \sum_{j=0}^{N_y} c_{i,j} \cdot T_i\left ( 2\cdot {x \over L_x}-1\right ) \cdot T_j\left ( 2\cdot {y \over L_y}-1\right ) $$