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

• To: mathgroup at smc.vnet.net
• Subject: [mg100973] Re: How to downsample a image matrix using left and right
• From: Ray Koopman <koopman at sfu.ca>
• Date: Thu, 18 Jun 2009 20:49:55 -0400 (EDT)

```On Jun 17, 1:44 am, Ray Koopman <koop... at sfu.ca> wrote:
> 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].

Map[Mean@Flatten@#&,Partition[X,{4,4}],{2}]]  is better.

```

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