Take a look at the hex map below.
Here we see the corner pixels near hex 0101. The blue pixels belong to hex 0101. The red pixels belong to it's neighboring hexes:
In the enlarged view below, we can see that, if the upper-left (blue-boxed) corner were to be considered as simultaneously part of each and every hex that it touched, then it would be a part of hexes 0000, 0100, and 0101:
And, we can also see below that, if the left (blue-boxed) corner were to be considered as simultaneously part of each and every hex that it touched, then it would be a part of hexes 0000, 0001, and 0101..
If our player happened to click directly on one of those two pixels, it would then be very difficult to try to determine which hex had actually been clicked.
Instead, my "Upper-Left" Convention simply declares that those two particular pixels belong to hex 0101; and only to hex 0101.
Now consider this:
This shows the area of the map near hex 0101, blown up ten times. Note the blue pixels which are labled "MP" which stands for "My Pixel", i.e. a Corner Pixel which belongs to "This Hex". The red pixels are labeled "NP" which stands for "Neighbor's Pixel", i.e. a Corner Pixel which belongs to a hex other than "This Hex":
M.D.J. 2022/08/27