Discrete cosine transform matrix
If you have an n
-by-n
image,
A
, then
is
the DCT of the columns of D
*AA
and D'*A
is
the inverse DCT of the columns of A
.
The two-dimensional DCT of A
can be computed as
D*A*D'
. This computation is sometimes faster than using
dct2
, especially if you are
computing a large number of small DCTs, because D
needs to
be determined only once.
For example, in JPEG compression, the DCT of each 8-by-8 block is computed. To
perform this computation, use dctmtx
to determine
D
, and then calculate each DCT using
D*A*D'
(where A
is each 8-by-8 block).
This is faster than calling dct2
for each individual
block.