The author is suggesting you use the mean X value of all the pixels in the shape, instead of the mode x value.
I'm sure fancier image editing software does this, but your eyes are pretty good at it too because it's just dividing the surface area in half, or thinking about it as the "center of mass" as you put it: where could you attach a string to this shape such that it doesn't tip over one way or another?
The author is suggesting you use the mean X value of all the pixels in the shape, instead of the mode x value.
I'm sure fancier image editing software does this, but your eyes are pretty good at it too because it's just dividing the surface area in half, or thinking about it as the "center of mass" as you put it: where could you attach a string to this shape such that it doesn't tip over one way or another?