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

[David Gobbi]

I should add a bit more explanation to my last email...

When you move a 3D object with the mouse, you usually want
the exact point on the surface of the object that you pick to be
the point that moves with the mouse.  This means that the
"constraint plane" that you want to use for the motion is the plane
that:
1) contains all the vectors that describe allowed directions of motion, and
2) contains the point on the object that you want to move with the mouse

In order for the interaction to be intuitive, it is not good enough to use
just any point within the object.  It is necessary to use the point on
the closest surface of the object.

   David


On Thu, Apr 10, 2014 at 3:45 PM, David Gobbi <david.gobbi at gmail.com> wrote:
> Hi Dov,
>
> The DisplayToWorld transformation definitely takes the zoom factor
> into account.  The WorldToDisplay and DisplayToWorld methods are used
> throughout VTK: in the picking, in the interaction, and in the
> rendering.  If they didn't work, then neither would VTK.
>
> The only thing in your code that looks questionable to me is that you
> always assume that you world coordinate has Z=0.  Your objects are
> not two dimensional, so obviously when you click on them the "pick"
> position (in world coordinates) will not have Z=0, since the surface
> of the object lies above Z=0.  I believe that the problems that you
> are seeing are due to parallax: the motion at the Z=0 plane is dx,
> but the motion at the surface of your object is in fact slightly higher
> due to perspective.
>
> Instead of using Z=0, you should use the actual Z of the surface of
> the object under the cursor.
>
>   David
>
> On Thu, Apr 10, 2014 at 1:29 AM, Dov Grobgeld <dov.grobgeld at gmail.com> wrote:
>> Hi David,
>>
>> Though I suspected that the errors are negligible, I changed the code to
>> remember the coordinate at the point of the picking and then translate
>> relative to it. As I expected there was no noticeable difference. The
>> problem at high zooms remain. It appears like the current zoom factor is not
>> taken properly into account the DisplayToWorld transformation. Do you have
>> any idea of what is wrong?
>>
>> Thanks,
>> Dov
>>
>>
>>
>>
>> On Thu, Apr 10, 2014 at 12:34 AM, David Gobbi <david.gobbi at gmail.com> wrote:
>>>
>>> Hi Dov,
>>>
>>> I took a quick look at your code, and I notice that on every MouseMove
>>> you compute the dx,dy since the last MouseMove.  I advise that you do
>>> not do this, because it leads to the accumulation of errors.  Instead,
>>> on the ButtonDown, you should save the Display and World coordinates
>>> (i.e. the coordinates at which the drag action started).  Then, in
>>> MouseMove, you should do all your offset calculations relative to this
>>> start position, _not_ relative to the previous MouseMove.  I.e. always
>>> compute dx,dy relative to the position at the start of the drag.
>>>
>>>   David
>>>
>>> On Wed, Apr 9, 2014 at 2:28 PM, Dov Grobgeld <dov.grobgeld at gmail.com>
>>> wrote:
>>> > Thanks David. With your help I almost solved the interaction problem. My
>>> > resulting example creates a board with three pieces on it that may be
>>> > moved
>>> > around with the right mouse button. The example may be found here:
>>> >
>>> > https://gist.github.com/dov/10310484
>>> >
>>> > It gives a very intuitive feeling for moving the pieces around on the
>>> > board
>>> > no matter how the board is rotated. But there is still a dragging
>>> > problem at
>>> > high zoom ins. Then the pieces don't follow the mouse anymore. I'm still
>>> > at
>>> > lost why his happens.
>>> >
>>> > Feel free to add this example to the vtk example programs.
>>> >
>>> > Regards,
>>> > Dov
>>> >
>>> >
>>> >
>>> > On Wed, Apr 9, 2014 at 12:36 AM, David Gobbi <david.gobbi at gmail.com>
>>> > wrote:
>>> >>
>>> >> 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
>>> >> >
>>> >> >
>>> >
>>> >
>>
>>