| Re: Accurate/fast interpolation [message #55718 is a reply to message #55717] |
Tue, 04 September 2007 07:47  |
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Mike[2]
Messages: 99
Registered: December 2005
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On Sep 4, 7:32 am, Steve <f...@k.e> wrote:
> What I am trying at the moment is to set one image as a common
> reference, covert all the others to sub pixel positions on that
> reference frame and then use triangulate and trigrid to interpolate
> image values onto this common reference frame. This seems to work but
> is painfully slow [trigrid is fine triangulate takes many seconds].
> I just wondered since my data is nearly on the right grid to start with
> if there were a quicker way to do this?
One way is to use interpolate afte transforming the homogeneous
coordinates of the points of your image with the !p.t matrix. Suppose
your image data is in an [Nx,Ny] array. You can set up the !p.t
matrix with something like
saved_pt = !p.t
t3d, /reset
t3d, rotate=[0.0, 0.0, angle], translate=[dx, dy, 0]
matrix = !p.t
!p.t = saved_pt
This sets up !p.t as a transformation matrix for rotation by angle
around the z-axis (or is that -angle?) with translations of dx and dy
along x and y (again, I may be dropping a sign here). Your array of
homogeneous coordinates will be [Nx*Ny,4], call it p0. Then you can
transform those points with the !p.t matrix:
p1 = p0 # matrix
and use the new points (in the rotated and translated coordinate
system) to interpolate the image like this:
new_image = reform(interpolate(image, p1[*, 0], p1[*, 1], p1[*, 2]),
Nx, Ny)
I have never timed this against triangulate and trigrid, but I suspect
it will be faster.
Mike
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