[vtkusers] World Units

[Wheeler, Gavin]

Hi,

3D Slicer does indeed have a lot of functionality, and we use it within our department. However, the project I am working on is primarily concerned with Interaction and User Interface, so we are using VTK within Unity. By default Unity’s units are metres, and we stuck with that. However, since we made that decision we now know more about Unity, and their advice is to scale assets/worlds etc. as appropriate if metres isn’t appropriate. So effectively it’s unitless, but by default the unitless units are metres. If that makes sense. In the end metres works for us pretty well as we are developing room scale applications.

I think it is similar for VTK, for one application there may be a default unit, but for another application a different one. I imagine tracking these and ensuring they all work would be a bit of a nightmare/ basically impossible to ensure. So it and responsibility are left up to the user. My solution to this isn’t elegant – for variables with a unit I suffix them ‘m’ or ‘mm’ or ‘s’ etc.

Best regards,

Gavin


From: vtkusers <vtkusers-bounces at public.kitware.com> On Behalf Of Jean-Christophe Fillion-Robin
Sent: 21 November 2018 16:42
To: matthew.troke at avalonholographics.com
Cc: David Gobbi <david.gobbi at gmail.com>; Steve Pieper <pieper at isomics.com>; vtkusers at public.kitware.com
Subject: Re: [vtkusers] World Units

Hi Matt,

Units are intended to be used to describe the quantity of a given object. Units should help the application to be smarter about how the user can access/view/interact with the data. For example, units help the user by displaying the current unit in spinboxes as a suffix (1.3mm instead of 1.3).

For more details, see here<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.slicer.org%2Fwiki%2FDocumentation%2FNightly%2FDevelopers%2FUnits&data=01%7C01%7Cgavin.wheeler%40kcl.ac.uk%7Cc4187c8b6527455345b708d64fd051f7%7C8370cf1416f34c16b83c724071654356%7C0&sdata=SZNXij8JTJ3ygHwkERKMDuI7cZIAfvY9Z3gMbDM0jlo%3D&reserved=0>

Jc

On Wed, Nov 21, 2018 at 9:28 AM Matthew Troke <matthew.troke at avalonholographics.com<mailto:matthew.troke at avalonholographics.com>> wrote:
Could you elaborate a little more on support for units in Slicer? What do you mean by "built-in"?

Matt

On Tue, Nov 20, 2018 at 10:41 PM Jean-Christophe Fillion-Robin <jcfr at kitware.com<mailto:jcfr at kitware.com>> wrote:
Hi Gavin,

If you are working with medical data, you could look at 3D slicer (https://slicer.org<https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fslicer.org&data=01%7C01%7Cgavin.wheeler%40kcl.ac.uk%7Cc4187c8b6527455345b708d64fd051f7%7C8370cf1416f34c16b83c724071654356%7C0&sdata=Fyl%2Bs0UrSdZr5XCP3SUK4yTawu77%2FFIb7qPC7z1lM80%3D&reserved=0>), it leverages VTK and ITK and also has the notion of units built-in.

Hth
Jc

On Tue, Nov 20, 2018, 9:13 AM David Gobbi <david.gobbi at gmail.com<mailto:david.gobbi at gmail.com> wrote:
Hi Gavin,

Using mm for medical image data (with the exception of microscopy) is the most common choice.  I certainly wouldn't recommend using anything else.  I've also noticed the need to tweak volume rendering depending on resolution.

  David

On Tue, Nov 20, 2018 at 8:00 AM Wheeler, Gavin via vtkusers <vtkusers at public.kitware.com<mailto:vtkusers at public.kitware.com>> wrote:
We’ve not been using OpenVR directly, but have been using it indirectly with medical data. We ended up using m as the real world units, and then scaling loaded medical data by a factor of 1000 to convert from the commonly used mm, to m. This has worked well, but there are the odd gotchas when doing this– watch out for things like volume rendering step size which you may also need to scale.

Gavin Wheeler

From: vtkusers <vtkusers-bounces at public.kitware.com<mailto:vtkusers-bounces at public.kitware.com>> On Behalf Of Ken Martin
Sent: 19 November 2018 16:22
To: Dave Demarle <dave.demarle at kitware.com<mailto:dave.demarle at kitware.com>>
Cc: vtkusers at public.kitware.com<mailto:vtkusers at public.kitware.com>
Subject: Re: [vtkusers] World Units

Although the OpenVR module does use the notion of a physical to world transformation to map between meters in the VR room and the world coordinates.


On Mon, Nov 19, 2018 at 9:57 AM David E DeMarle <dave.demarle at kitware.com<mailto:dave.demarle at kitware.com>> wrote:
vtk itself is unitless. it is up to the application to keep track of that.

David E DeMarle
Kitware, Inc.
Principal Engineer
21 Corporate Drive
Clifton Park, NY 12065-8662
Phone: 518-881-4909
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