R: Apply a function over (multi-band) pixel time series

apply_time {gdalcubes} | R Documentation

## Apply a function over (multi-band) pixel time series

### Description

This generic function applies a function on pixel time series of a data cube, an R array, or other classes if implemented. The resulting object is expected to have the same spatial and temporal shape as the input, i.e., no reduction is performed.

### Usage

    apply_time(x, ...)

### Arguments

`x` | input data
`...` | additional arguments passed to method implementations

### Value

return value and type depend on the class of x

### See Also

`apply_time.cube`

`apply_time.array`

### Examples

    # 1. input is data cube
    # create image collection from example Landsat data only
    # if not already done in other examples
    if (!file.exists(file.path(tempdir(), "L8.db"))) {
      L8_files <- list.files(system.file("L8NY18", package = "gdalcubes"),
                             ".TIF", recursive = TRUE, full.names = TRUE)
      create_image_collection(L8_files, "L8_L1TP", file.path(tempdir(), "L8.db"))
    }

    L8.col = image_collection(file.path(tempdir(), "L8.db"))
    v = cube_view(extent=list(left=388941.2, right=766552.4,
                  bottom=4345299, top=4744931, t0="2018-01", t1="2018-06"),
                  srs="EPSG:32618", nx = 497, ny=526, dt="P1M")
    L8.cube = raster_cube(L8.col, v)
    L8.cube = select_bands(L8.cube, c("B04", "B05"))
    L8.ndvi = apply_pixel(L8.cube, "(B05-B04)/(B05+B04)", "NDVI")

    # Apply a user defined R function
    apply_time(L8.ndvi, names="NDVI_residuals",
       FUN=function(x) {
          y = x["NDVI",]
          if (sum(is.finite(y)) < 3) {
             return(rep(NA,ncol(x)))
          }
          t = 1:ncol(x)
          return(predict(lm(y ~ t)) -  x["NDVI",])})

    # 2. input is array
    d <- c(4,16,32,32)
    x <- array(rnorm(prod(d)), d)
    z <- apply_time(x, function(v) {
      y = matrix(NA, ncol=ncol(v), nrow=2)
      y[1,] = (v[1,] + v[2,]) / 2
      y[2,] = (v[3,] + v[4,]) / 2
      y
    })
    dim(z)

[Package gdalcubes version 0.6.0 Index]