# [vtkusers] units in VTK

Sam Raby rabysam28 at gmail.com
Sat Oct 25 19:15:58 EDT 2014

```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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