[vtkusers] Issues with coordinate conversion, possible VTK bugs?

Andras Lasso lasso at queensu.ca
Wed May 24 12:17:49 EDT 2017

There are many free, open-source, VTK-based medical image computing software applications that do exactly these and much more (3D Slicer, ITK-Snap, MITK, etc). Just have a look at their source code and copy-paste what you need (at least from 3D Slicer and MITK, as they have BSD-type license).

Or, even better, save yourself a couple of years of work and implement your application as custom plugins in one of these applications. The advantage of building on an existing application framework is that all the features that you don’t even know that your users will request, are already implemented, documented, and tested by thousands of users. It made sense 10-15 years ago to develop medical applications from scratch, but now that there are very good, established platforms, you should build on top of those.


From: vtkusers [mailto:vtkusers-bounces at vtk.org] On Behalf Of Bill Q
Sent: Wednesday, May 24, 2017 10:35 AM
To: Dan Lipsa <dan.lipsa at kitware.com>
Cc: VTK Users <vtkusers at vtk.org>
Subject: Re: [vtkusers] Issues with coordinate conversion, possible VTK bugs?

Hi Dan,
Thanks a ton for the example. The example used 4 different viewports. However, for my application, I need to set all renderers under the same viewport. The problem is that if I add a second renderer into the render window and using the default viewport, it will cover the first renderer and nothing on the first renderer will show up. I am wondering if I did the whole thing wrong.

My purpose is to display some DICOM images, some texts, and scales on the same screen. Also, I would like to make the scales zoom with the DICOM images but the texts stay put. Most importantly, I would like to get the world coordinates on the DICOM image when users click on the screen, so I can mark the click properly.

What should be done in order to get this working?

Many thanks.


On Wed, May 24, 2017 at 12:02 AM, Dan Lipsa <dan.lipsa at kitware.com<mailto:dan.lipsa at kitware.com>> wrote:
Not sure what is going on. Do the objects you are rendering have the same world coordinates? Are both renderers added to the rendering window?
Here is an example with multiple renderer (they are over different viewports but they could have the same viewport as well)

On Tue, May 23, 2017 at 11:39 AM, Bill Q <bill.q.hdp at gmail.com<mailto:bill.q.hdp at gmail.com>> wrote:
I just gave it a try. But when I added another renderer into the window that I used for text actors, it overlaps and covered the first renderer. So the first renderer won't show up. Is there a solution?

Many thanks.


On Tue, May 23, 2017 at 10:16 PM, Dan Lipsa <dan.lipsa at kitware.com<mailto:dan.lipsa at kitware.com>> wrote:
I think moving those text actors on a different renderer will solve your problem.


On Tue, May 23, 2017 at 9:40 AM, Bill Q <bill.q.hdp at gmail.com<mailto:bill.q.hdp at gmail.com>> wrote:
Hi Dan,
Thanks a lot for the reply. Yes, that's exactly the case. If I remove the text actors, the world picker would return the correct coordinates. So, what method should I use in order to get the correct coordinates no matter how many actors I put in with the dicom image reader actor?

Many thanks.


On Mon, May 22, 2017 at 10:04 PM, Dan Lipsa <dan.lipsa at kitware.com<mailto:dan.lipsa at kitware.com>> wrote:
For display_to_world, the picker picks up only what is visible (closes to the camera). You might have several objects (several values in world coordinates) that end up over the same display coordinate. This might explain why adding those text actors changes what you pick.


On Sun, May 21, 2017 at 5:25 AM, Bill Q <bill.q.hdp at gmail.com<mailto:bill.q.hdp at gmail.com>> wrote:
Can anybody help? Many thanks.

Many thanks.


On Fri, May 19, 2017 at 11:20 PM, Bill Q <bill.q.hdp at gmail.com<mailto:bill.q.hdp at gmail.com>> wrote:
Hello Everyone,
I encountered a weird problem in coordinate conversion.

In order to check the accuracy, I converted from display to world, and from world back to display. Using the following ways:

self.world_picker = vtk.vtkWorldPointPicker()

def display_to_world(self, pt):
    self.world_picker.Pick(pt[0], pt[1], 0, self.ren)
    picker_pt = self.world_picker.GetPickPosition()
    picker_pt = (picker_pt[0], picker_pt[1], 0)
    return picker_pt

def world_to_display(self, pt):
    if len(pt) == 3:
        pt = pt + (1.0,)
    elif len(pt) == 2:
        pt = pt + (0.0, 1.0)
    pt = self.ren.GetDisplayPoint()
    pt = (pt[0], pt[1])
    return pt

pt = [5, 5]

world_pt = self. world_to_display(pt)

display_pt = self. world_to_display(world_pt)
In some cases, it's quite accurate, which means pt equals display_pt. But, in some cases, the result is simply wrong. For example, for (5, 5), after conversions back to display, the coordinate becomes (20.89, 20.89).

Any ideas?

Many thanks.


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