[vtk-developers] BUG: vtkImageActor - set of ComputedDisplayExtent variable.

[Karthik Krishnan]

This behaviour has always existed in vtkImageActor - due to the way VTK
deals with point data and cell data and the fact that the tips of the render
window delineate the available screen pixels for display. What this means is
that the tips of the clipped rectangle you are seeing (and the centers of
any other rectangle) correspond to the positions of the centers of
vtkImageData voxels. Because the display extent, in word coordinates runs
from the Word position of the left edge voxel center to the world position
of the right edge voxel center, you end up with clipping.

This has been the topic of discussion on the list before.

I don't believe that the fix you proposed can resolve that. Does it ? It
would simply cause one to update the whole input needlessly leading to
slowness when one renders a vtkImageActor for the very first time, without
setting the DisplayExtent.

Thanks
--
karthik

On Wed, Jul 14, 2010 at 7:43 PM, Dave Partyka <dave.partyka at kitware.com>wrote:

> Hi David Gobbi,
>
> It appears from git blame that you are the author, or at least, the last
> person to modify this code. What are your thoughts? Is this a bug or
> intended as Karthik described?
>
> The blobs of code are:
>
>  // if the display extent has not been set, then compute one
>   int *wExtent = input->GetWholeExtent();
>   if (this->DisplayExtent[0] == -1)
>     {
>     this->ComputedDisplayExtent[0] = wExtent[0];
>     this->ComputedDisplayExtent[1] = wExtent[1];
>     this->ComputedDisplayExtent[2] = wExtent[2];
>     this->ComputedDisplayExtent[3] = wExtent[3];
>     this->ComputedDisplayExtent[4] = wExtent[4];
>     this->ComputedDisplayExtent[5] = wExtent[4];
>     }
>
> and
>
>  // if the display extent has not been set, then compute one
>   int *wExtent = this->Input->GetWholeExtent();
>   if (this->DisplayExtent[0] == -1)
>     {
>     this->ComputedDisplayExtent[0] = wExtent[0];
>     this->ComputedDisplayExtent[1] = wExtent[1];
>     this->ComputedDisplayExtent[2] = wExtent[2];
>     this->ComputedDisplayExtent[3] = wExtent[3];
>     this->ComputedDisplayExtent[4] = wExtent[4];
>     this->ComputedDisplayExtent[5] = wExtent[4];
>     }
>
>
> On Wed, Jul 14, 2010 at 9:54 AM, Karthik Krishnan <
> karthik.krishnan at kitware.com> wrote:
>
>> Dave:
>>
>> I would recommend holding off that commit. I'm not convinced that its a
>> bug. Specifically, the fragment of code reads :
>>
>>
>> int vtkImageActor::RenderOpaqueGeometry(vtkViewport* viewport)
>> {
>> ....
>>
>>  int *wExtent = input->GetWholeExtent();
>>   if (this->DisplayExtent[0] == -1)
>>     {
>>     this->ComputedDisplayExtent[0] = wExtent[0];
>>     this->ComputedDisplayExtent[1] = wExtent[1];
>>     this->ComputedDisplayExtent[2] = wExtent[2];
>>     this->ComputedDisplayExtent[3] = wExtent[3];
>>     this->ComputedDisplayExtent[4] = wExtent[4];
>>
>>     this->ComputedDisplayExtent[5] = wExtent[4];
>>     }
>>   input->SetUpdateExtent(this->ComputedDisplayExtent);
>>   ...
>> }
>>
>> An ImageActor should have flattened extents along some dimension. It does
>> not make sense for the ImageActor to update the WHOLE input. It appears that
>> with the fix, "this->ComputedDisplayExtent[5] = wExtent[4];", it will lead
>> to updating the entire image.
>>
>> I think the author of the class simply intended to set the display extent
>> to the first axial slice if they haven't been set.
>>
>> Pavel: I'm also interested in knowing how you uncovered/ concluded that
>> this is a bug. We use vtkImageActor quite heavily and haven't had any
>> issues; Maybe we haven't covered this "if" block, but still, its good to
>> know..
>>
>> Thanks
>> --
>> karthik
>>
>>
>> On Wed, Jul 14, 2010 at 7:09 PM, Dave Partyka <dave.partyka at kitware.com>wrote:
>>
>>> Hi Pavel, I am curious how you discovered this bug. Were you just
>>> browsing through the file or do you have code that is affected by this? If
>>> so would you mind sharing it? I would like to make a test out of it if
>>> possible.
>>>
>>>
>>> On Wed, Jul 14, 2010 at 9:26 AM, Dave Partyka <dave.partyka at kitware.com>wrote:
>>>
>>>> Those lines are definitely covered. Though it appears the only are used
>>>> when the extents haven't been set yet.
>>>>
>>>>
>>>> http://www.cdash.org/CDash/viewCoverageFile.php?buildid=663956&fileid=10980615
>>>>
>>>>
>>>> On Wed, Jul 14, 2010 at 9:18 AM, David Cole <david.cole at kitware.com>wrote:
>>>>
>>>>> This sort of error looks like it should produce some sort of seriously
>>>>> incorrect results.
>>>>>
>>>>> Do we not have a test that covers this code? Is the coverage for the
>>>>> lines in question "0" or do we just not have a *good* test that runs through
>>>>> that code...?
>>>>>
>>>>> I would think vtkImageActor is a heavily used class.
>>>>>
>>>>>
>>>>> ?
>>>>>
>>>>>
>>>>> On Wed, Jul 14, 2010 at 8:51 AM, Dave Partyka <
>>>>> dave.partyka at kitware.com> wrote:
>>>>>
>>>>>> Thanks for the fix! I have just committed it.
>>>>>>
>>>>>> On Wed, Jul 14, 2010 at 4:19 AM, Pokutnev, Pavel (GE EntSol, SensInsp)
>>>>>> <Pavel.Pokutnev at ge.com> wrote:
>>>>>>
>>>>>>>  Hello vtk-devs,
>>>>>>>
>>>>>>> it looks like there is a copy & paste bug in vtkImageActor line 250
>>>>>>> and 293:
>>>>>>>
>>>>>>> The line:
>>>>>>>   this->ComputedDisplayExtent[5] = wExtent[4];
>>>>>>> should be:
>>>>>>>   this->ComputedDisplayExtent[5] = wExtent[5];
>>>>>>>
>>>>>>> Best regards!
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
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>>>>>>
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>>>>>>
>>>>>
>>>>
>>>
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