Re: How to downsample a image matrix using left and right

• To: mathgroup at smc.vnet.net
• Subject: [mg100888] Re: How to downsample a image matrix using left and right
• From: Ray Koopman <koopman at sfu.ca>
• Date: Wed, 17 Jun 2009 04:45:02 -0400 (EDT)
• References: <h19i7r\$p7g\$1@smc.vnet.net>

```On Jun 16, 6:51 pm, tenveer <ten.v... at gmail.com> wrote:
> HI all
>
> i am just stuck with a unique problem ...
>
> What i want is to downsample a matrix (image matrix) using two left
> and right matrices....
> e.g if i have a 100 by 100 image matrix X what i want is to downsample
> X to 25 by 25 matrix Y using two downsampling matrices U and V which
> are on left and right respectively i.e Y=U*X*V
>
> than what will the contents of the U and V matrices.
>
> Any help will be appreciated

If you just want to take means then you can use U = Flatten /@
(IdentityMatrix[25] /. {1 -> {.25,.25,.25,.25}, 0 -> {0,0,0,0}})
and V = Transpose@U. However, you can also get U.X.Transpose@U as
Nest[ Transpose[Mean/@Partition[#,4]]&, X, 2].

```

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