[vtkusers] vtkImageSlice question

[David Gobbi]

Typo: I meant to write "spacing of the voxels", not "spacing and voxels".


On Thu, May 3, 2012 at 9:08 AM, David Gobbi <david.gobbi at gmail.com> wrote:
> Hi Siddharth,
>
> If you just want to map your image pixels directly to screen pixels,
> then there's hardly any reason to use a computer graphics toolkit like
> VTK at all.  The goal of vtkImageActor and vtkImageSlice is to place
> an image into a 3D scene, so that the vtkCamera can then be set up to
> view that scene from a particular viewpoint.
>
> There are several coordinate transformations that occur along the way.
>
> First, the sampling information for the image is applied, i.e. the
> spacing and voxels and the position of the lower-left corner voxel of
> the image volume (or upper-left in the case of DICOM).  This
> transformation maps the voxels to a physical block of space (usually
> using millimetres as units).
>
> Next, the prop3D transformation is applied.  This establishes the
> position of the aforementioned block of space within the world
> coordinate system. For medical applications, I always equate the world
> coordinate system with the DICOM patient coordinate system of my
> primary image series, in order to keep things simple.
>
> Next, there are two camera transformation that are applied (a view
> transformation and a projection transformation) that map the world
> coordinates to the view coordinates, followed by a viewport
> transformation that maps the viewport coordinates to screen pixels.
>
> None of these transformations are specific to VTK.  The same basic
> approach is used in virtually all modern computer graphics libraries.
>
> There are recipes that you can apply to achieve basic effects (e.g
> like viewing a 2D slice of an image at a specific zoom factor), but in
> order to get the most out of VTK, it is important understand the
> computer graphics fundamentals that underlie the toolkit.
>
>  - David
>
>
> On Thu, May 3, 2012 at 7:32 AM, sidd_vtk <siddharthvikal at gmail.com> wrote:
>> Hi David & all vtk users,
>>
>> I'm using the newly added vtkImageSlice class (I've used vtkImageActor also
>> with the same issue that I'm describing below). I set up the pipeline, and
>> the image shows up. The issue is that the image that comes up does not
>> preserve its original size in pixels unlike how vtkActor2D does. To be more
>> specific and clear, e.g. if the image size in pixels is 512x512, the use of
>> vtkActor2D results in displaying the image of size 512x512 pixels, but use
>> of vtkImageSlice or vtkImageActor displays in size other than true size of
>> the image. And I can't seem to figure out the scale factor that has been
>> applied.
>>
>> I need to use vtkImageSlice or vtkImageActor, because they work well with
>> vtkDistanceWidget. But I also require 1:1 display of the image. If the scale
>> factor is known, I can re-transform my image to get what I want.
>>
>> Can you please point me the code where this re-scaling of the image happens
>> before it gets rendered? or How to fetch the scaling that has been applied
>> to the image when vtkImageSlice or vtkImageActor has been used?
>>
>> Any help is greatly appreciated.
>>
>> regards,
>> Siddharth