[vtkusers] vtkThinPlateSplineTransform propogation

[Dean Inglis]

Hi David,

I sort of understand how the spline transform works.
As an example, say the reference lines are stored
as vtkPoints in XYcurvedLinePoints and XYstraightLinePoints ,
and all points are at location z = 0

transform->SetTargetLandmarks( XYcurvedLinePoints )
transform->SetSourceLandmarks( XYstraightLinePoints )

tempZ = queryPoint[ 2 ]
queryPoint[ 2 ] = 0
transform->TransformPoint( queryPoint, transPoint )
transPoint[ 2 ] = tempZ

here the queryPoint lies on the flat surface and the transPoint
now lies on the curved surface.

I dont understand how I can use vtkGridTransform since it takes
a vtkImageData as input and two sets of points.  Would I make a 2D
image of floats (3 components) repesenting the dx,dy,dz's from each
pair of corresponding curve/straight point locations over both surfaces?
What effects would the displacment shift and scale have?

thanks,
Dean

----- Original Message ----- 
From: "David Gobbi" <david.gobbi at gmail.com>
To: "Dean Inglis" <dean.inglis at camris.ca>
Cc: <vtkusers at vtk.org>
Sent: Wednesday, May 04, 2011 12:26 PM
Subject: Re: [vtkusers] vtkThinPlateSplineTransform propogation


Hi Dean,

Since your points are evenly spaced and one of your surfaces is
flat, you might be better off with the vtkGridTransform.  It is much
faster, it does not slow down as you add more points, and it should
be well-behaved over all of 3D space even if contains control points
mapping just a single straight line to a single contour.

 - David


On Wed, May 4, 2011 at 5:27 AM, Dean Inglis <dean.inglis at camris.ca> wrote:
> I have a ruled surface composed of a spline curve defined in
> an x-y plane and extruded along the z axis. I have a corresponding
> ruled surface which is essentially the same surface but ironed flat;
> a rectangular plane, hence, the corresponding curve is a straight line.
> The points are regularly spaced along the
> curve and parallel curves along the z axis are equally spaced.
> The number of points (can be tens to hundreds) along any x-y cut through
> either surface
> (ie a curve or line) is the same. I want the transform that relates
> coordinates of points
> forming the flattened surface to those of the curved surface. If I use
> vtkThinPlateSplineTransform for
> relating points in the bottom line of the flat surface to the 
> corresponding
> points
> in the curved suface, the transform is fast and works perfectly, but as I
> traverse upward
> to other areas of the surface the correspondance diminishes. If I include
> additional points, as in from a pair of middle curves and a pair of top
> curves,
> the correspondance is better, but diminishes in between those regions, 
> and,
> furthermore,
> the number of points significantly increases, slowing the transformation
> calculations drastically.
> Is there a way to propogate the transformation from only one set of curve
> pairs?
> I have looked at the code in vtkThinPlateSplineTransform but I have no 
> idea
> where
> to begin making modifications.
>
> thanks,
> Dean
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