nestSearch: Finds the closest pixel center to a point

Description

Finds the closest HEALPix pixel center to a given target point, specified in Cartesian coordinates, using an efficient nested search algorithm. HEALPix indices are all assumed to be in the "nested" ordering scheme.

Usage

nestSearch(target, nside, index.only = FALSE, save.dots = FALSE)

Arguments

target

A data.frame, matrix or vector of Cartesian (x,y,z) coordinates for the target point. If a data.frame is used then spherical coordinates can be specified with row names theta and phi.

nside

An integer, the target resolution at which the resulting pixels are returned.

index.only

A boolean indicating whether to return only the pixel index (TRUE), or cartesian coordinates as well (FALSE).

save.dots

A logical. A If TRUE then the dot product of each observation with the nearest child HEALPix pixel center will be returned as an attribute called "dot". Note that a 'child' pixel is any one of the four pixels contained in the current pixel in the nested scheme, at the next highest resolution. See children.

Value

if index.only = TRUE then the output will be a HEALPix index. If index.only FALSE then the output is the list containing the HEALPix index and Cartesian coordinate vector of the HEALPix point closest to target at resolution nside.

Examples

Run this code

# NOT RUN {
## Find the closest HEALPix pixel center at resolution j=2 for
## the point (0.6,0.8,0)

point <- c(0.6,0.8,0)
j <- 2
cpoint <- nestSearch(point, nside = 2^j)

## Plot the closest pixel center in blue and the point (0.6,0.8,0) in red
displayPixels(j, j, plot.j=j, spix=c(cpoint$pix),
              size=5, incl.labels =FALSE)
rgl::plot3d(point[1], point[2], point[3],
            col="red", size = 5, add = TRUE)


## Repeat the above for 4 points in a data.frame
points <- data.frame(x = c(1,0,0,0.6),
                     y = c(0,1,0,0.8),
                     z = c(0,0,1,0))
points
j <- 2
cpoints <- nestSearch(points, nside = 2^j)

## Plot the closest pixel center in blue and the point (0.6,0.8,0) in red
displayPixels(j, j, plot.j=j, spix=c(cpoints$pix),
              size=5, incl.labels =FALSE)
rgl::plot3d(points[,1], points[,2], points[,3],
            col="red", size = 5, add = TRUE)

# }

Run the code above in your browser using DataCamp Workspace