mapscanner (version 0.1.1)

ms_rectify_map: Rectify one map to another

Description

Rectify two previously scanned-in pdf or png maps with RNiftyReg, and return the modifications in map_modified as spatial objects in sf format.

Usage

ms_rectify_map(
  map_original,
  map_modified,
  nitems = NULL,
  non_linear = 1,
  type = "hulls",
  downsample = 10,
  concavity = 0,
  length_threshold = 10,
  quiet = FALSE
)

Value

An sf object representing the drawn additions to map_modified.

Arguments

map_original

File name of the original map without anything drawn over it (either a .pdf or .png; extension will be ignored).

map_modified

File name of the modified version with drawings (either a .pdf or .png; extension will be ignored).

nitems

Optional parameter to explicitly specify number of distinct items to be extracted from map; if possible, specifying this parameter may improve results.

non_linear

Integer value of 0, 1, or 2 representing degree of non-linearity in modified image - see Note.

type

Currently either "points", "polygons", or "hulls", where "points" simply reduces each distinct object to a single, central point; "polygons" identifies individual groups and returns the polygon representing the outer boundary of each; and "hulls" constructs convex or concave polygons around each group.

downsample

Factor by which to downsample type = "polygons", noting that polygons initially include every outer pixel of image, so can generally be downsampled by at least an order or magnitude (n = 10). Higher values may be used for higher-resolution images; lower values will generally only be necessary for very low lower resolution images.

concavity

For type = "hulls", a value between 0 and 1, with 0 giving convex hulls and 1 giving highly concave hulls.

length_threshold

For type = "hulls", the minimal length of a segment to be made more convex. Low values will produce highly detailed hulls which may cause problems; if in doubt, or if odd results appear, increase this value.

quiet

If FALSE, display progress information on screen

Examples

f_orig <- system.file ("extdata", "omaha.png", package = "mapscanner")
f_mod <- system.file ("extdata", "omaha-polygons.png",
    package = "mapscanner"
)

# reduce file sizes to 1/4 to speed up these examples:
f_orig2 <- file.path (tempdir (), "omaha.png")
f_modified2 <- file.path (tempdir (), "omaha-polygons.png")
magick::image_read (f_orig) %>%
    magick::image_resize ("25%") %>%
    magick::image_write (f_orig2)
magick::image_read (f_mod) %>%
    magick::image_resize ("25%") %>%
    magick::image_write (f_modified2)

# then rectify those files:
if (FALSE) {
xy_hull <- ms_rectify_map (f_orig2, f_modified2, type = "hull")
xy_poly <- ms_rectify_map (f_orig2, f_modified2, type = "polygons")
xy_pts <- ms_rectify_map (f_orig2, f_modified2, type = "points")
}