Re: Help with matrix replication

[Carnë Draug]

On 20 December 2012 19:28, Richardson, Anthony <address@hidden> wrote:
>> On Behalf Of Carnë Draug
>> > Subject: Help with matrix replication
>> >
>> > By the way, imresize(a, 2, 'nearest') will do what I want, but this is a
>> function from the image package and I'm trying to write some routines that
>> duplicate the functionality of the image package without using any of those
>> functions.
>>
>> Why are you avoiding the image package? It's the second time this week
>> someone tries to solve something that is already on a package but does not
>> want to use it. I don't understand the aversion.
>>
>> You know that you can at least just look into imresize, see how it works and
>> use it.
>
> I'm not averse to using it.  I'm using it a lot, but I'm also studying image 
> processing and most of the study involves implementing many image processing 
> algorithms in Octave.  Most (all?) of the algorithms are already implemented 
> in the image package.  We aren't allowed to use routines from the image 
> package except to compare the results from our routines to those from the 
> image package.

Oh! Ok, so it's an assignment, I understand that.

> Also, imresize appears to fit the image matrix to a grid and then uses one of 
> several different interpolation methods (user selectable) to generate a new 
> (larger or smaller) image matrix.  I'm interested only in the specific case 
> of generating a larger matrix by pixel replication (at this point) and 
> thought that clever indexing might provide a "better" solution than the more 
> general interpolation method used by imresize.  That appears to be the case.  
> On my machine resizing a 2x3 double matrix by a factor of 1000 using indexing 
> is over 50 times faster than using imresize.  Also, I can increase the same 
> matrix by a factor of 5000 using indexing, but get a "memory exhausted" error 
> when using imresize with a factor of 2000.  (I'm not complaining about 
> imresize.  It is great, but it might not be the best method to use when you 
> are interested only in pixel replication.)

It's perfectly acceptable to have imresize use different methods for
different cases. It doesn't need to have a single solution, whatever
is faster is better. Would you be able to supply a patch for us?

Carnë