[vtkusers] units in VTK

David Gobbi david.gobbi at gmail.com
Sat Oct 25 19:52:25 EDT 2014

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
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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