[vtkusers] vtkImageSlice question

[David Gobbi]

The recipe for getting image pixels to match screen pixels is, roughly:

render_window->Render();
int *rsize = renderer->GetSize();
int height = rsize[1];
vtkCamera *camera = renderer->GetActiveCamera();
camera->ParallelProjectionOn();
camera->SetParallelScale(0.5*height*yspacing);
render_window->Render();

If the image is not yet calibrated, then it should be fine to use
a yspacing of 1.

 - David


On Thu, May 3, 2012 at 11:33 AM, Siddharth Vikal
<siddharthvikal at gmail.com> wrote:
> Thanks David for detailed response.
>
> Thanks, yes I do understand now more clearly the series of transformations
> that the image goes through when vtkImageSlice or vtkImageActor is in use.
> And I completely agree and am for, equating the world coordinate system with
> the DICOM patient coordinate system. It is the camera transformations and
> view transformation I'm interested in (world --> view --> screen).
>
> Perhaps, it would make more sense to you if I explain the particular
> scenario a bit better. Consider the situation where the image that one has
> acquired are just off the hardware, where the spacing of voxels is not yet
> known. A manual calibration step is then needed to actually determine the
> pixel spacing. In this case, user prefers to the see the image as and how it
> comes out of the hardware i.e. the native pixel size equated to computer
> monitor's pixels. Based on that calibrated pixel size, the measurements have
> to added to image.  My thought was that since vtkDistanceWidget works well
> with vtkImageActor/vtkImageSlice, it would help to know the exact scaling so
> that I can accurately calculate calibrated pixel size and enable
> vtkDistanceWidget display correct measurements.
>
> regards,
> Siddharth
>
>
> On Thu, May 3, 2012 at 8:38 PM, 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
>
>