[vtkusers] Questions for picking.

[An Jiye]

Hi. It's me again.

Through most of my questions seem to be solved. There are still some 
phenomena that confuse me.

With a carema lies right in front of the screen, the position and focal 
point is:

aCamera->SetViewUp (0, 0, 1);
aCamera->SetPosition (32, -1, 46.5);    <-- In front the center of the 
front surface of A.
aCamera->SetFocalPoint (32, 63, 46.5);  <-- The center of the back surface 
of A.
aCamera->ComputeViewPlaneNormal();
aCamera->ParallelProjectionOn();

Now when I perform pickings, the value of y-axis will only be 1 or 20. It 
means the picking points are from either the front surface of A or the 
front surface of B. That's great!

But the coordinates like (0.349618, 1, 16.0183), (53, 1, 16.8), (16.4, 20, 
27.2) and(43.6565, 20, 25.5363) occur. Note the x axis value of these four 
results.

In my example the x-axis range of the front surface of A shoulbe be [5, 
49], and for B [20, 39]. Why some points will exceed these ranges?

The same question occurs when goes to the z-axis. That is, I will get 
coordinates like -4.44537 and 95.9695. But actually, the range should be 
[0, 92].


Regards,
An Jiye

> Hi.
> 
> I have understood why the picking results are not as expected. Here are 
the 
> explanations.
> 
> There is a section in the vtk textbook introducing the picking functions 
of 
> vtk. Unfortunately, I only have the 2nd edition for vtk 2.0. But I think 
> the principle should be the same. Here I quote some contents from it.
> 
> "The Visualization Toolkit provides four classes to perform actor, point, 

> cell and world point picking. These are the classes vtkPicker, 
> vtkPointPicker, vtkCellPicker and vtkWorldPointPicker. These pickers vary 

> in speed and functionality as described in the following."
> 
> "The object vtkPicker intersects a ray defined from camera position to a 
> screen (i.e., pixel coordinate) against the bounding box of all pickable 
> and nontransparent actors. (An actor is pickable if its Pickable instance 

> variable is true.) The result of the vtkPicker pick operation is to 
return 
> a list of the actors whose bounding box is intersected. The actor closest 

> to the camera position is also returned."
> 
> "The object vtkPointPicker intersects the ray against the points defining 

> each actor, and returns the point coordinate closest to the camera 
> position, as well as the actor that the point belongs to. Since screen 
> resolution prevents precise selection of a point, a tolerance around the 
> ray must be specified. The tolerance is expressed as a fraction of the 
> rendering window size. (Rendering window size is measured across the 
window 
> diagonal) Points must lie within this tolerance to be picked."
> 
> 
==========================================================================================

> 
> According to the above introduction to vtkPointPicker, my problems can be 

> easily explained.
> 
> First, the returning points y coordinate is not necessarily one of the 1, 

> 20, 39 and 49. This is because I set the camera position to (0, 0, 0). So 

> when I perform a picking, the ray is from origin to the cursor position. 
It 
> doesn't run parallel with the y-axis.
> 
> Second, I will get one or two points as a result. This is because the 
> vtkPointPicker will return both the point coordinate closest to the 
camera 
> position, and the actor that the point belongs to. When these two points 
> are actually the same, one point is returned.
> 
> Third, the points from the back surface are picked sometimes. This is 
> because the vtkPointPicker only pick the points defining each actor. So 
the 
> ray will traverse all the surfaces, no matter it is front or back. If it 
> doesn't intersect any points on the front surface, a back surface point 
> will be returned of course. And do not forget that the ray comes from the 

> origin (0, 0, 0).
> 
> Combining the second and third situation, you will probably get two 
points. 
> And neither of them is on the front surface.
> 
==========================================================================================

> 
> 
> Let's go on with other vtk pickers.
> 
> "The object vtkCellPicker intersects the ray with the cells defining each 

> actor, and returns the point of intersection, as well as the actor that 
the 
> cell belongs to. If you are trying to select a unique actor, 
vtkCellPicker 
> is the object to use because it performs surface (or cell) intersection. 
> Picking actors using vtkPicker can yield undesirable results because of 
the 
> overlap of bounding boxes; the same is true of vtkPointPicker because of 
> tolerance problems. Unfortunately, vtkCellPicker is the slowest object of 

> the three because of greater computational requirements."
> 
> 
==========================================================================================

> 
> So I decided to use vtkCellPicker for my picking application. Since the 
> camera affects the picking results directly. Remember to manipulate it 
> carefully. In my application, I set its position to ahead of the screen, 
> and at the middle of the z-axis, (32, -1, 46.5). Also I set the focal 
point 
> to (32, 63, 46.5), which lies in the center of the back most surfaces in 
my 
> former example. It works well for picking if user does not change the 
> camera or actor positions by vtkRenderWindowInteractor. But if this 
> happens, I still don't know how to deal with it.
> 
==========================================================================================

> 
> 
> For completeness, please allow me to introduce vtkWorldPointPicker at 
last.
> 
> "The calss vtkWorldPointPicker returns the (x,y,z) coordinate value of a 
> pick in the rendering window. To determine this information, it combines 
> the display (x,y) values with the z-buffer depth values. Of all the 
pickers 
> it is the fastest, but it cannot determine the actual cell, point or 
actor 
> that is selected. (Note: on some system z-buffer operations are 
inoperative 
> and this object will not function properly)"
> 
> 
> Hope this will help some newbie who are performing picking in vtk, just 
> like me.
> 
> Best Regards,
> Jiye An

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