img_orientation: Orientation of object on image

In jiho/morphr: Analyse the Morphology of Objects on Images

Orientation of object on image

Description

Compute the orientation angle of the best fitting ellipse based on image moments.

Usage

img_orientation(x, direction = FALSE)

Arguments

x	a greyscale image, of type imager::cimg()
direction	try to resolve the 180º uncertainty in orientation: orient the ellipse (and therefore the angle) towards the denser part of the object. On a binary image, the denser part is the largest. On a greyscale image, the denser part is the lightest (because black=0, white=1).

Details

Image moments only make sense for light on dark images. If your image is dark on light, invert it before computing the orientation with this function.

On an image displayed with img_show(), the angle is given from the horizontal, in clockwise direction, in degrees. The computation gives it in trigonometric convention (from the horizontal, towards the positive side of the y axis, in radians) but (i) in an image, the y axis increases when going down, (ii) the angle is converted in degrees for easiness.

Value

The orientation angle.

Examples

# read an image of an asymetric shape
x <- img_read(system.file("extdata", "shape.png", package="morphr"))
x <- imager::imrotate(x, 45)
img_show(x)
img_orientation(x)
# 45º from horizontal, clockwise
img_orientation(x, direction=TRUE)
# the highest density is in the same direction
# rotate it to make the object horizontal, with highest density on the right
imager::imrotate(x, -45) %>% img_show()

# now if we work with the mask of the object
img_show(x>0)
img_orientation(x>0)
img_orientation(x>0, direction=TRUE)
# the highest density is now at the opposite end
# so the angles are opposed by 180º
# and to make the object horizontal and with highest density on the right
imager::imrotate(x>0, -225) %>% img_show()

# try with a real image
# (NB: inverted to allow the computation of moments)
x <- img_read(system.file("extdata", "amphipoda/33463695.jpg",
              package="morphr"), invert=TRUE)
img_show(x)
a <- img_orientation(x, direction=TRUE)
imager::imrotate(x, -a) %>% img_show()

jiho/morphr documentation built on May 11, 2024, 9:32 p.m.