rcaiman (version 2.0.1)

estimate_sun_angles: Estimate sun angular coordinates

Description

Estimates the sun’s zenith and azimuth angles (deg) from a canopy hemispherical photograph, using either direct detection of the solar disk or indirect cues from the circumsolar region.

Usage

estimate_sun_angles(
  r,
  z,
  a,
  bin,
  g,
  angular_radius_sun = 30,
  method = "assume_obscured"
)

Value

Named numeric vector of length two, with names z and a, representing the sun’s zenith and azimuth angles (in degrees).

Arguments

r

numeric terra::SpatRaster of one layer. Typically the blue band of a canopy image.

z

terra::SpatRaster generated with zenith_image().

a

terra::SpatRaster generated with azimuth_image().

bin

logical terra::SpatRaster of one layer. Binary image where TRUE marks candidate sky pixels. Typically the output of binarize_with_thr().

g

single-layer terra::SpatRaster with integer values. Sky segmentation map produced by sky_grid_segmentation().

angular_radius_sun

numeric vector of length one. Maximum angular radius (in degrees) used to define the circumsolar region.

method

character vector of length one. Estimation mode: "assume_obscured" (default) or "assume_veiled".

Details

This function can operate under two alternative assumptions for estimating the sun position:

Veiled sun

The solar disk is visible or partially obscured; the position is inferred from localized brightness peaks.

Obscured sun

The solar disk is not visible; the position is inferred from radiometric and spatial cues aggregated over the circumsolar region.

When method = "assume_veiled", g and angular_radius_sun are ignored. Estimates refer to positions above the horizon; therefore, estimated angles may require further manipulation if the photograph was acquired under crepuscular light.

Examples

if (FALSE) {
caim <- read_caim()
z <- zenith_image(ncol(caim), lens())
a <- azimuth_image(z)
m <- !is.na(z)
r <- caim$Blue

bin <- binarize_by_region(r, ring_segmentation(z, 15), "thr_isodata") &
  select_sky_region(z, 0, 88)

g <- sky_grid_segmentation(z, a, 10)
sun_angles <- estimate_sun_angles(r, z, a, bin, g,
                                  angular_radius_sun = 30)
row_col <- row_col_from_zenith_azimuth(z, a,
                                       sun_angles["z"],
                                       sun_angles["a"])
plot(caim$Blue)
points(row_col[1,2], nrow(caim) - row_col[1,1], col = "yellow",
       pch = 8, cex = 3)
}