[vtkusers] units in VTK

[Sam Raby]

Very helpful! Thank you!

-S

On Sat, Oct 25, 2014 at 10:16 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> Hi Sam,
>
> Yes, your summary is correct, with one caveat: the STL file format (unlike
> DICOM) does not specify units, so you have to trust that whoever created
> the file was using millimeters as their units.
>
>  - David
>
>
> On Sat, Oct 25, 2014 at 8:51 PM, Sam Raby <rabysam28 at gmail.com> wrote:
>
>> Thanks David.
>>
>> OK. Then, if the actor is not coming from a VTK class such as
>> vtkTubeFilter, but instead it is coming from an STL file (for example it is
>> a model of catheter designed in SolidWork), then once this actor is shown
>> in 3D scene next to the DICOM slice, the size of the actor (which is from
>> the STL file) should make sense without having to be worried about any sort
>> of calibration (unless the DICOM is corrupted).
>>
>> Did I get it right?
>>
>> -S
>>
>> On Sat, Oct 25, 2014 at 6:52 PM, David Gobbi <david.gobbi at gmail.com>
>> wrote:
>>
>>> Hi Sam,
>>>
>>> You shouldn't have to perform any size calibration.  If you load a DICOM
>>> file that uses millimeter units into VTK, then VTK ipso facto measures
>>> all
>>> physical distances in millimeters.
>>>
>>> If you have to set the radius to 1.4 in order to make it "fit" one
>>> millimeter,
>>> that suggests that the DICOM metadata has become corrupted somehow,
>>> either before being loaded into VTK, or after being loading into VTK.
>>>
>>> Make sure that the DICOM PixelSpacing is correct, and make sure that
>>> the after the DICOM is loaded into VTK, that the image data "Spacing"
>>> is also correct.
>>>
>>>  - David
>>>
>>>
>>>
>>> On Sat, Oct 25, 2014 at 5:15 PM, Sam Raby <rabysam28 at gmail.com> wrote:
>>>
>>>> Thanks David.
>>>> If a slice of DICOM is shown in 3D scene, and a tube actor is shown
>>>> along with it, now I am not sure how to set the radius of the tube to be
>>>> actually 5 mm, because DICOM image is also present there in the same scene
>>>> and DICOM has a sense of unit.
>>>>
>>>> Should I "calibrate" the unit of the Tube? For example, I may end up
>>>> concluding that if I set the radius of Tube to 1.4 then it is equal to 1 mm
>>>> in DICOM image.
>>>>
>>>> -S
>>>>
>>>> On Sat, Oct 25, 2014 at 5:43 PM, David Gobbi <david.gobbi at gmail.com>
>>>> wrote:
>>>>
>>>>> On Sat, Oct 25, 2014 at 4:11 PM, Sam Raby <rabysam28 at gmail.com> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Consider an instant of vtkTubeFilter which has a method named
>>>>>> "SetRadius". I was wondering how this Radius is related to millimeter?
>>>>>>
>>>>>> I am using vtkTubeFilter as an example but in general I wanted to
>>>>>> know how units are involved in actors.
>>>>>>
>>>>>
>>>>> The VTK filters have no units.  It's the same as looking at the
>>>>> equation y = m*x + b and asking what the units are:  all you can
>>>>> tell for certain is that 'y' and 'b' have the same units, and that 'm'
>>>>> has units of 'y/x'.  Other than that, it's up to whoever is using the
>>>>> equation to decide on the units.  VTK is the same.
>>>>>
>>>>> If you want to use millimeters, then set the radius in millimeters.
>>>>> Just make sure that you use millimeters everywhere else, too.
>>>>>
>>>>> Also make sure that all the files you load use millimeters.  If they
>>>>> don't, you'll have to convert their units before you use them in your
>>>>> pipeline.
>>>>>
>>>>>  - David
>>>>>
>>>>
>>>
>>
>
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