[vtkusers] vtkImageViewer2 show zoom factor and visible coordinates

[Dean Inglis]

Hi Christian,

I wrote a class for querying image pixel/voxeld data a while back and I
could make it available
to you off-line at your request.
In the meantime, have a look at http://www.vtk.org/Wiki/VTK/Examples/Cxx in
particular
http://www.vtk.org/Wiki/VTK/Examples/Cxx/Images/PickingAPixel2
and
http://www.vtk.org/Wiki/VTK/Examples/Cxx/Images/PickingAPixel

For displaying a minimap, I would try adding a second vtkRenderer with a
smaller viewport
in your main vtkRenderWindow and place a vtkImageActor within it.

To compute zoom, have a look at vtkCoordinate class, which allows conversion
between numerous kinds of coordinate types (viewport, normalized viewport,
world etc.),
probably make use of the extrema of your viewing window and those of the
image.

Dean



On Thu, Sep 12, 2013 at 4:47 AM, Christian Lackas <lackas at invicro.com>wrote:

> Hi Everybody,
>
> I display a large 2D image (larger then typical screen resolutions) in a
> a vtkImageViewer2 and would like to give the user some guidance as to
> which part she is currently looking at.
>
> My questions are:
> * What is the easiest way to get the zoom (ratio of image to world pixels)?
> * How can I get the coordinates (in image pixels) of the par of the
>   image that is currently visible?
>
> Details:
>
> In a first steps, I would like to show the zoom level (100% = one pixel
> on screen contains exactly one pixel of the image). I looked at
> vtkCamera and found the Zoom(double) function, pointing me to
> ParallelScale() (ParallelProjection: On), and this variable seems to be
> indeed related to the zoom (larger values indicate lower zoom), however,
> I was not able to derive an absolute number from this (e.g. 50%, so two
> image pixels are shown in 1 pixel on screen). In my example 100% is
> around ParallelScale=140.
> Also vtkCamera::Get(Model)ViewTransformObject()->GetScale() does not
> work (always returns 1,1,1).
> I tried to use vtkCamera's Print(stream) method, however, could not find
> any other values that change with the zoom, and I could not figure out
> how to use GetProjectionTransformObject(double,double,double) properly.
>
> In a next step, I would like to show a small minimap in one corner, just
> consisting of two rectangles showing the full image and the current view
> port. For this I would need to know which coordinates (in image pixels)
> are currently shown.
> Unfortunately, ImageActors DisplayExtent just contains the full
> dimensions, and I have not found any other value that seems to be useful
> (also not in vtkCamera).
>
> Any help on this is highly appreciated.
>
> Thanks,
>  Christian
>
> --
> Dr. Christian Lackas, Managing Partner
> inviCRO, LLC -- In Imaging Yours
> http://www.invicro.com/  http://www.spect-ct.com/
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