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Bug 437 - [feature request] Add Reshape Operation
Summary: [feature request] Add Reshape Operation
Alias: None
Product: Eigen
Classification: Unclassified
Component: Core - general (show other bugs)
Version: unspecified
Hardware: All All
: Normal enhancement
Assignee: Nobody
Depends on:
Blocks: 3.4 712
  Show dependency treegraph
Reported: 2012-03-19 10:38 UTC by brian
Modified: 2018-10-08 20:33 UTC (History)
5 users (show)


Description brian 2012-03-19 10:38:19 UTC
I often want to have a reshaped view of an existing array or matrix. For example, view a 3xK matrix as a 3K x 1 vector to perform a linear operation on the vertices of a model. This can be achieved with the Eigen::Map object, but is rather verbose.

It would be great, if eigen had a .reshape(h,w) function which would return an Eigen::Map with a new view on the data.

Could this be implemented? Do other people also need this funcitonality? I believe, that it is quite common, when you are coming from a matlab mindset.

kind regards,

Comment 1 brian 2012-03-20 12:22:04 UTC
After working  a bit more on my problem, I see that it would really be best to get as the return a reshape expression, not an Eigen::Map, as the reshape might happen on a non-evaluated expression. How complicated is it to implement this?
Comment 2 Christoph Hertzberg 2012-03-20 13:06:27 UTC
I think basically you can implement it the same way as Block is implemented -- of course with different index calculations.
Comment 3 brian 2012-03-22 12:54:02 UTC
Thank you for the feedback. I currently have no spare time to dig into this, is anyone willing to take it up?
Comment 4 brian 2012-04-02 07:45:36 UTC
Would anyone be willing to walk me through this. The Eigen code is quite complex, but if there is someone holding my hand I think I could make the necessary adaptations without breaking anything. Christoph maybe?
Comment 5 Christoph Hertzberg 2012-04-02 12:48:00 UTC
OK, the quick hack would be to copy Eigen/src/Core/Block.h and literally search and replace all Block by Reshape, then adapt all index calculations and the constructors. Most likely you also need forward-declarations in Eigen/src/Core/util/ForwardDeclarations.h and then include the new file somewhere from Eigen/Core.

Next and final step is to implement some reshape member functions -- also similar to the way the block member functions are implemented (Eigen/src/plugin/BlockMethods.h) -- and add them to Eigen/src/Core/DenseBase.h.

This solution will most likely not break anything, so have no fear *g*.

Overall, this solution will result in a lot of code duplication, so I guess the cleaner solution would be to make the Block-class more general, maybe also fixing bug 58, by making Map essentially be a (trivial) Block.

W.r.t Eigen's code complexity, I guess it is more a problem of the poor internal documentation (see bug 138).
Comment 6 brian 2012-04-09 21:51:36 UTC
Hello Christoph,

so after looking at Block.h I feel a bit intimidated. Can you explain the purpose and way of functioning of the template specialization at the end? And will I need to change anything inside the traits block? How about the packet access, how does that work, what do I have to keep in mind while tinkling with it?

thank you,

Comment 7 Hilton Bristow 2013-12-12 06:38:46 UTC
+1. reshape() with a vec() specialization would be greatly appreciated. Perhaps a more general "View" interface which implements block(), diagonal(), diagonalize(), reshape() and vec() would be the way to go.
Comment 8 Christoph Hertzberg 2013-12-12 11:40:52 UTC
@Brian: Sorry for not responding (for very long). I may have time between the years to look into that.

@Hilton: Yes, Bug 712 will basically fall of this feature, using (size(), 1) as parameters.

Generally, I think a clean solution should return something equivalent to a Map if possible. However, things like
  MatrixXd M;
will have changing inner strides, so they could not be represented by a map -- and of course it only works for DirectAccess types.
Comment 9 yoco 2014-01-17 22:21:08 UTC
Hi all :)

I am new to the eigen project.

I have done some work about this issue. It works basically.

Except the specialization of the "direct access". I currently have no idea what that is.

Please give me any suggestion if I done anything wrong.

thanks in advance.
Comment 10 Gael Guennebaud 2018-10-08 20:33:55 UTC
Merged in PR 301:

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