[vtkusers] undo changes to binary vtkImageData

[David Gobbi]

On Fri, Jan 28, 2011 at 7:45 PM, Jonathan Morra <jonmorra at gmail.com> wrote:

> I have a binary vtkImageData that a user can modify through some tools I
> provide them.  However, the user has requested an undo stack be made so they
> can go back through their changes.  In my case, storing a new binary
> vtkImageData every time the user changes it costs too much memory (these are
> large 3D volume sets).  Is there a way to store a difference between two
> binary vtkImageData's?  Ideally I'd like to have just one vtkImageData which
> is the current image and then a stack of vtkImageDifferences (I made that
> class up) which are themselves small, but say how to go to either the next
> or previous state of a binary vtkImageData.
>
> Does such a method exist?
>

No, but it shouldn't be too hard to do this.  You can get a pointer to the
data by calling GetScalarPointer() on the image data.  The size of this data
(in bytes) will be product of the Dimensions, the NumberOfScalarComponents,
and the ScalarSize.

So, you should easily be able to compute a bytewise difference between two
images.  There's no reason to write a new filter for this, since it's just a
simple for() loop.  Then, you can use vtkDataCompressor::Compress() to
compress the diff so that it can be efficiently stored.  By doing everything
bytewise, you don't have to worry about different datatypes, and you
shouldn't lose anything in terms of efficiency.

If the image changes in size, then you could store a compressed copy of the
full image rather than dealing with the difficulty of diffing images of
different sizes.

I hope this helps.

  - David
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