fields (version 1.2)

interp.surface: Fast bilinear interpolator from a grid.

Description

Uses bilinear weights to interpolate values on a rectangular grid to arbitrary locations.

Usage

interp.surface(obj, loc)

Arguments

obj

A list with components x,y, and z in the same style as used by contour, persp, image etc. x and y are the X and Y grid values and z is a matrix with the corresponding values of the surface

loc

A matrix of (irregular) locations to interpolate. First column of loc isthe X coordinates and second is the Y's.

Value

• An vector of interpolated values. NA are returned for regions of the obj$z that are NA and also for locations outside of the range of the grid.

Details

Here is a brief explanation of the interpolation: Suppose that the location, (locx, locy) lies in between the first two grid points in both x an y. That is locx is between x1 and x2 and locy is between y1 and y2. Let ex= (l1-x1)/(x2-x1) ey= (l2-y1)/(y2-y1). The interpolant is

( 1-ex)(1-ey)*z11 + (1- ex)(ey)*z12 + ( ex)(1-ey)*z21 + ( ex)(ey)*z22

Where the z's are the corresponding elements of the Z matrix.

See Also

image.smooth, as.surface, as.image, image.plot, krig.image,Tps

Examples

#
# evaluate an image at a finer grid
# 

data( lennon)
# create the surface object
obj<- list( x= 1:20, y=1:20, z= lennon[ 201:220, 201:220])

# sample at 50 equally spaced points 
temp<- seq( 1,20,,50)
make.surface.grid( list( temp,temp))-> loc
interp.surface( obj, loc)-> look
# take a look
image.plot( as.surface( loc, look))