img_extract_largest: Extract largest object from a greyscale image

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

Extract largest object from a greyscale image

Description

Extract the largest object from the image background.

Usage

img_extract_largest(x, threshold = 2/255, quiet = FALSE)

Arguments

x	a greyscale image, of type imager::cimg()
threshold	grey value threshold to detect the object over the background. Either a number in ⁠[0,1]⁠ where 0 is pure black and 1 is pure white, or a string with a grey level quantile value (e.g. "50%" to keep 50% of the greys in the foreground).
quiet	when TRUE, do not display the message about image inversion when providing an image on a light background.

Details

Threshold the input image, detect connected particles, and keep only the largest one. Note that, for ease of use, when the image has an obviously white/light background (based on img_guess_background()), the image is automatically inverted. Indeed, thresholding select pixels whose value is above the threshold = it supposes that the object of interest has values higher (i.e. lighter) than the background, which is therefore supposed to be dark.

Value

The extracted object, on a pure white background, as a imager::cimg() object.

Examples

# get an image with lots of background space, several blobs and a legend
# NB: invert it so that the background is black
x <- img_read(system.file("extdata", "blob.jpg", package="morphr"), invert=TRUE)
img_show(x)
# extract its largest object
img_extract_largest(x) %>% img_show()

# now leave the image in its original dark-on-light aspect
x <- img_read(system.file("extdata", "blob.jpg", package="morphr"))
img_show(x)
# and extraction still works because the image is automatically inverted
img_extract_largest(x) %>% img_show()
# NB: use quiet=TRUE to suppress the message

# if we want to be sure to pick up the object itself and not the legend,
# we can start by chopping the legend away
x %>% img_chop(b=31) %>% img_extract_largest(quiet=TRUE) %>% img_show()

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