# spBlkDiag

## PURPOSE

Creates a sparse block diagonal matrix from a 3D array.

## SYNOPSIS

function [B,inds] = spBlkDiag( A, inds )

## DESCRIPTION

     Creates a sparse block diagonal matrix from a 3D array.

     Given an [mxnxk] matrix A, construct a sparse block diagonal matrix B of
     dims [m*k x n*k], containing k blocks of size mxn each, where each block
     i is taken from A(:,:,i).

     When computing B, a time consuming step is to compute a series of
     indices. These indices are fixed for given dims of A and can be re-used.
     spBlkDiag's additional input/output can be used to cache these indices.

     USAGE
      [B, inds] = spBlkDiag( A, [inds] )

     INPUTS
      A       - [m x n x k] input matrix of k mxn blocks
      inds    - cached indices for faster computation

     OUTPUT
      B       - [m*k x n*k] sparse block diagonal matrix with k mxn blocks
      inds    - cached indices for faster computation

     EXAMPLE
      A=rand(3,4,2); B=spBlkDiag(A); full(B)

     See also SPARSE, BLKDIAG

     Piotr's Image&Video Toolbox      Version 2.35
     Copyright 2012 Piotr Dollar.  [pdollar-at-caltech.edu]
     Please email me if you find bugs, or have suggestions or questions!
     Licensed under the Simplified BSD License [see external/bsd.txt]