compare this tiling scheme

[mdsumner]

started a blog post, compare with .tilescheme

raster_url <- "ftp://ftp.data.pgc.umn.edu/elev/dem/setsm/REMA/mosaic/v1.1/200m/REMA_200m_dem.tif"

readfrom_file <- tempfile(pattern = "readfrom", fileext = ".tif")
writeto_file <- tempfile(pattern = "writeto", fileext = ".tif")
## file size is 1.3Gb
curl::curl_download(raster_url, readfrom_file)

## this now makes a copy of the 1.3Gb file, so we have two of them
fs::file_copy(readfrom_file, writeto_file)

Now we want the tiling

info <- vapour::vapour_raster_info(readfrom_file)
tiling <- list(dimension = info$dimXY, tiles = info$tilesXY)
fac <- 100
fac * tiling$tiles
if (tiling$tiles[2] == 1) {
  ## let's take fac scanlines at a time
  tiling$tiles[2] <- fac
}
calc_steps <- function(dimension, tiles) {
  bounds_x <- seq(0, dimension, by = tiles)
  steps_x <-  rep(tiles, length.out = length(bounds_x)-1)
  dangle_x <- sum(steps_x) -dimension
  if (dangle_x > 0) steps_x[length(steps_x)] <- steps_x[length(steps_x)] - dangle_x

  list(head(bounds_x, -1), steps_x)
}
x_step <- calc_steps(tiling$dimension[1], tiling$tiles[1])
y_step <- calc_steps(tiling$dimension[2], tiling$tiles[2])
y_step
system.time({
for (i in seq_along(x_step[[1]])) {
  startx <- x_step[[1]][i]
  countx <- x_step[[2]][i]
  for (j in seq_along(y_step[[1]])) {
  starty <- y_step[[1]][j]
  county <- y_step[[2]][j]

  ## now read
  offset <- c(startx, starty)
  dimension <- c(countx, county)
  vals <- vapour:::vapour_read_raster_block(readfrom_file, offset = offset, dimension = dimension, band_output_type = info$datatype, band = 1)[[1L]]

  ## do something to the values
  if (any(na.omit(vals) > info$nodata_value)) {
    vals <- vals * -1
    vapour:::vapour_write_raster_block(writeto_file, vals, offset, dimension, band = 1, overwrite = TRUE)
  }
  }
}