Algorithm 1.

Tripled Dictionary Training

1: Input: Training patch sets $\{{\mathit{x}}_1^{\mathit{MR}},\dots ,{\mathit{x}}_i^{\mathit{MR}},\dots ,{\mathit{x}}_{N_{\mathit{MR}}}^{\mathit{MR}}\}\in {ℝ}^{H\times N_{\mathit{MR}}}$, $\{{\mathit{x}}_1^L,\dots ,{\mathit{x}}_i^L,\dots ,{\mathit{x}}_{N_L}^L\}\in {ℝ}^{H\times N_L}$, and $\{{\mathit{x}}_1^S,\dots ,{\mathit{x}}_i^S,\dots ,{\mathit{x}}_{N_S}^S\}\in {ℝ}^{H\times N_S}$, and dictionary size P
2:	Initial: Initialize $D_{\mathit{MR}}^{(0)}$, $D_L^{(0)}$ and $D_S^{(0)}\in {ℝ}^{H\times P}$.
3.	Repeat
4:	For i = 1, 2, …, t (t is the number of tripled patches) do
5:	Compute ${\mathit{\alpha }}_i^{(C)}$ according to Eq. (12);
6:	End
7:	For i = t+1, t+2, …, NQ (Q can be replaced by MR, L or S) do
8:	Compute ${\mathit{\alpha }}_i^{(\mathcal{Q})}$ according to Eq. (11);
9:	End
10:	Update DMR, DL, and DS, according to Eq. 13.
11:	Until convergence
12:	Output: Tripled dictionaries DMR, DL, and DS.