Add extra properties to regionprops in skimage

I'm using the regionprops function from the scikit-image (or skimage) package to compute region features of a segmented image using the SLIC superpixel algorithm from the same package.

I need additional features than those computed in the fucntion, mainly : standrad deviation, skewness, kurtosis.

I modified the source code of _regionprops.py using the other features as template in order to include those properties :

    @property
    def sd_intensity(self):
        return np.std(self.intensity_image[self.image])

    @property
    def skew_intensity(self):
        return skew(self.intensity_image[self.image])

I know this is bad practice, and not a long term solution because my code won't be able to run on another machine or if i update skimage.

I discovered that the function skimage.measure.regionprops() has a extra_properties=None parameter, which according to the doc:

Add extra property computation functions that are not included with skimage.

My question is : Can I get a working example with np.std ? I don't really know how to use this parameter.

Thanks

extra_properties just takes a list of functions with region mask and intensity image as arguments. Here's a quick example:

from skimage import data, util
from skimage.measure import label, regionprops
import numpy as np
img = util.img_as_ubyte(data.coins()) > 110
label_img = label(img, connectivity=img.ndim)

def sd_intensity(regionmask, intensity_image):
        return np.std(intensity_image[regionmask])

def skew_intensity(regionmask, intensity_image):
        return skew(intensity_image[regionmask])
    
props = regionprops(label_img, intensity_image=img,
                    extra_properties=(sd_intensity, skew_intensity))

You can now access your extra properties using your function names

props[0].sd_intensity
>>> 0.4847985617008998

EDIT 08/28/2021, updated the example to actually compute region local statistics as pointed out by @CrisLuengo and @JDWarner (thank you guys)