[vtkusers] vtkImageReslice zoomed performance - no clipping?

[David Gobbi]

Hi Henning,

The behaviour of vtkImageReslice can be counterintuitive, but be
assured that its behaviour is correct.

Probably the best way to think about it, is that vtkImageReslice
presents a "viewport" into the image that you are reslicing.  This
"viewport" is a rectangle with its lower-left corner set by
reslice->SetOutputOrigin(), and its size set by
reslice->SetOutputExtent().  The "pixel size" of the viewport is set
by reslice->SetOutputSpacing().

After this basic "viewport" has been set up with
SetOutputOrigin()/Extent()/Spacing(), the ResliceAxes() are used to
set its position and orientation with respect to the volume, i.e.
ResliceAxes() applies a transform to the "viewport".  The transformed
viewport is the slice that you will end up slicing from the volume.

It is most useful to define the "viewport" so that (0,0,0) is at the
center of the viewport, e.g. if you want to produce a 256x256 slice
with pixel spacing of (1mm, 1mm) you would do this:

reslice->SetOutputExtent(0,255,0,255,0,0);
reslice->SetOutputSpacing(1,1,1);
reslice->SetOutputOrigin(-127.5,-127.5,0.0);

Then you would set the ResliceAxes to transform this "viewport" to
somewhere inside the volume.

Basically, you should always set the OutputExtent(), OutputSpacing(),
and OutputOrigin() whenever you want to do anything sophisticated with
vtkImageReslice.

The default Extent/Origin/Spacing values are set that way as a flag
for vtkImageReslice to create its own viewport according to some
simplistic rules that usually don't give people what they want.  That
is why you should always set these values.

   David


On Fri, Jun 25, 2010 at 7:33 AM, Henning Meyer <tutmann at gmail.com> wrote:
> Hi David,
>
> thank you for your explanation.
> I still can't help but get the impression something is odd there.
> When I call
> m_reslice->SetOutputExtent(0,100,0,100,0,0);
> I get a nice square clipped area, which seems to be scaled up in my QVTKWidget.
>
> And when I start to use
> m_reslice->SetResliceAxes(myTransform);
> this clipped area will move inside (even outside) my QVTKWidget.
>
> So - what is going on here? Can I help to solve this issue?
>
> Henning
>
> Btw: When I never call SetOutputExtent, I get an OutputExtent of:
> -2147483648 ,2147483647 ,-2147483648 ,2147483647 ,-2147483648 ,2147483647
>
>
> 2010/6/24 David Gobbi <david.gobbi at gmail.com>:
>> Hi Henning,
>>
>> The vtkImageReslice filter will clip the image if you call
>> SetOutputExtent(), or if you don't set the OutputExtent, then clipping
>> is done according the UpdateExtent set by the pipeline.  So those are
>> the only ways to change the clipping: either set it manually with
>> SetOutputExtent, or fiddle with the downstream filters to change the
>> UpdateExtent.
>>
>> I've wanted to write an easier-to-use alternative to vtkImageReslice
>> for a very long time (there really isn't any way to make
>> vtkImageReslice "smarter" without breaking backwards compatibility in
>> one way or another).
>>
>>   David
>>
>>
>> On Thu, Jun 24, 2010 at 9:13 AM, Henning Meyer <tutmann at gmail.com> wrote:
>>> Hi!
>>>
>>> When using vtkImageReslice, I get very low performance, once I zoom
>>> into an image. It seems, that vtkImageReslice is rendering a huge
>>> virtual image, and is clipping after that, is that true? Could it be
>>> changed to be clipped before? Also I found, the clipping is not always
>>> correct, as sometimes with certain transforms the output image gets
>>> clipped inside the render window.
>>> Is there anything I could do here?
>>>
>>> Thank you,
>>>
>>> Henning
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>