rcaiman (version 1.2.2)

extract_sun_coord: Extract sun coordinates

Description

Extract the sun coordinates for CIE sky model fitting.

Usage

extract_sun_coord(r, z, a, bin, g, max_angular_dist = 30)

Value

Object of class list with two numeric vectors of length two named row_col and zenith_azimuth. The former is the raster coordinates of the solar disk (row and column), and the other is the angular coordinates (zenith and azimuth angles in degrees).

Arguments

r

SpatRaster. A normalized greyscale image. Typically, the blue channel extracted from a canopy photograph. Please see read_caim() and normalize().

z

SpatRaster built with zenith_image().

a

SpatRaster built with azimuth_image().

bin

SpatRaster. This should be a preliminary binarization of r useful for masking pixels that are very likely pure sky pixels.

g

SpatRaster built with sky_grid_segmentation() or chessboard().

max_angular_dist

Numeric vector of length one. Angle in degrees to control the potential maximum size of the solar corona.

Details

This function uses an object-based image analyze framework. The segmentation is given by g and bin. For every cell of g, the pixels from r that are equal to one on bin are selected and its maximum value is calculated. Then, the 95th percentile of these maximum values is computed and used to filter out cells below that threshold; i.e, only the cells with at least one extremely bright sky pixel is selected.

The selected cells are grouped based on adjacency, and new bigger segments are created from these groups. The degree of membership to the class Sun is calculated for every new segment by computing the number of cells that constitute the segment and its mean digital number (values taken from r). In other words, the largest and brightest segments are the ones that score higher. The one with the highest score is selected as the sun seed.

The angular distance from the sun seed to every other segments are computed, and only the segments not farther than max_angular_dist are classified as part of the sun corona. A multi-part segment is created by merging the sun-corona segments and, finally, the center of its bounding box is considered as the sun location.

See Also

Other Tool Functions: colorfulness(), correct_vignetting(), defuzzify(), extract_dn(), extract_feature(), extract_rl(), extract_sky_points_simple(), extract_sky_points(), find_sky_pixels_nonnull(), find_sky_pixels(), masking(), optim_normalize(), percentage_of_clipped_highlights(), read_bin(), read_caim_raw(), read_caim(), write_bin(), write_caim()

Examples

if (FALSE) {
caim <- read_caim()
r <- caim$Blue
caim <- normalize(caim, 0, 20847, TRUE)
z <- zenith_image(ncol(caim), lens())
a <- azimuth_image(z)
m <- !is.na(z)
plotRGB(caim*255)
bin <- ootb_obia(caim, z, a, m, HSV(239, 0.85, 0.5), gamma = NULL)
g <- sky_grid_segmentation(z, a, 10)
sun_coord <- extract_sun_coord(r, z, a, bin, g, max_angular_dist = 30)
points(sun_coord$row_col[2], nrow(caim) - sun_coord$row_col[1],
        col = 3, pch = 10)
}