threshold: Threshold grayscale image

In imager: Image Processing Library Based on 'CImg'

Threshold grayscale image

Description

Thresholding corresponding to setting all values below a threshold to 0, all above to 1. If you call threshold with thr="auto" a threshold will be computed automatically using kmeans (ie., using a variant of Otsu's method). This works well if the pixel values have a clear bimodal distribution. If you call threshold with a string argument of the form "XX%" (e.g., "98%"), the threshold will be set at percentile XX. Computing quantiles or running kmeans is expensive for large images, so if approx == TRUE threshold will skip pixels if the total number of pixels is above 10,000. Note that thresholding a colour image will threshold all the colour channels jointly, which may not be the desired behaviour! Use iiply(im,"c",threshold) to find optimal values for each channel separately.

Usage

threshold(im, thr = "auto", approx = TRUE, adjust = 1)

Arguments

im	the image
thr	a threshold, either numeric, or "auto", or a string for quantiles
approx	Skip pixels when computing quantiles in large images (default TRUE)
adjust	use to adjust the automatic threshold: if the auto-threshold is at k, effective threshold will be at adjust*k (default 1)

Value

a pixset with the selected pixels

Author(s)

Simon Barthelme

Examples

im <- load.example("birds")
im.g <- grayscale(im)
threshold(im.g,"15%") %>% plot
threshold(im.g,"auto") %>% plot
threshold(im.g,.1) %>% plot
#If auto-threshold is too high, adjust downwards or upwards
#using "adjust"
threshold(im,adjust=.5) %>% plot
threshold(im,adjust=1.3) %>% plot

imager documentation built on May 29, 2024, 1:38 a.m.