[vtkusers] Dragging an object in the x-y plane

[David Gobbi]

Hi Dov,

When you do the Display-to-World, what are you using as the input "Z"
display coordinate? Really, I don't think I understand your situation.

Generally, if you know that your objects are constrained to move
within a certain plane, you should do the following:

Let X,Y be the "display" coordinates, in pixel units.
Let Z be the display "depth", with a range of [0,1].

Convert display coordinate (X, Y, 0.0) to world coordinate (x_near,
y_near, z_near)
Convert display coordinate (X, Y, 1.0) to world coordinate (x_far, y_far, z_far)

Now "far" and "near" define the two ends of a line segment in world
coordinates.  These are where the view ray intersects the near and far
clipping planes for the vtkCamera.

At some point along the segment between "near" and "far", the segment
will intersect your constraint plane.  Obviously it does not have to
be a plane, it can be whatever kind of constraint surface you want.
Let's call this intersection point "surface" because it lies on the
constraint surface.

So now you have three points in world coordinates: near, surface, and
far.  Use Pythagoras to compute:

Z = (surface - near)/(far - near)

Now Z is the depth at which the view ray at (X,Y) intersects your
constraint surface.  Of course, maybe you don't need to compute "Z" at
all, maybe you can just leave the surface intersection point in world
coordinates.

I don't know if VTK has any special functions to do this, but even if
it did, I probably wouldn't use them.  I always write my interaction
code by thinking about how the view ray intersects my world space, and
then by applying a constraint to achieve the desired 2D-to-3D
coordinate conversion (or vice-versa).

  David






On Tue, Apr 8, 2014 at 2:48 PM, Dov Grobgeld <dov.grobgeld at gmail.com> wrote:
> Thanks David,
>
> I got it to work perfectly as long as I'm looking at the scene from the top.
> But when i rotate the scene, e.g. tilt it forward around the x-axis, then
> the dragging of the pieces lags behind the mouse. Geometrically I somehow
> understand why, as I'm constraining to only move the piece in the x-y plane
> and the Display2World transformation is also returning a z-component that
> I'm ignoring. Is there a shortcut in vtk to do the necessary z to x,y
> projections, or should i just figure it out on my own? Again, my requirement
> is that the mouse should stay around the same position on the piece, as I'm
> moving it around.
>
> Regards,
> Dov
>
>
>
> On Tue, Apr 8, 2014 at 11:19 PM, David Gobbi <david.gobbi at gmail.com> wrote:
>>
>> Hi Dov,
>>
>> I use a function like this:
>>
>> void DisplayToWorld(vtkRenderer *renderer,
>>   double x, double y, double z, // window coordinates (the input)
>>   double world[3]) // world coordinates (the output)
>> {
>>   // Use the vtkViewport interterface for conversions.
>>   renderer->SetDisplayPoint(x, y, z);
>>   renderer->DisplayToWorld();
>>   double hcoord[4];
>>   renderer->GetWorldPoint(hcoord);
>>   world[0] = hcoord[0]/hcoord[3];
>>   world[1] = hcoord[1]/hcoord[3];
>>   world[2] = hcoord[2]/hcoord[3];
>> }
>>
>> It is not enough to have just the xy window coordinates.  You also
>> need a z window coordinate, which is a depth value in the range [0,1].
>> Usually you get the depth from the pick.
>>
>>   David
>>
>>
>> On Tue, Apr 8, 2014 at 2:03 PM, Dov Grobgeld <dov.grobgeld at gmail.com>
>> wrote:
>> > I'm trying to create a toy x-y board which has pieces, realized as
>> > Actors,
>> > that I can drag around in the x-y plane. Through the vtkPicker() I have
>> > figured out how to choose a piece, and I can get the render window
>> > coordinates through GetInteractor().GetEventPosition(). Further, if I
>> > now
>> > move the mouse I can calculate a Dx,Dy shift in the window coordinate.
>> > But I
>> > would now like to translate the Window Dx,Dy to an Actor Dx,Dy shift.
>> > I.e. I
>> > would like to take a two pairs of xy coordinates in window coordinates
>> > and
>> > calculate their respective positions in actor coordinates, according to
>> > the
>> > current actor to dispay matrix. How can I do that?
>> >
>> > Thanks in advance!
>> > Dov
>
>