I havn't find the way to delete the pins from the webpage yet, so I deleted them manually. Yet on library, a pixel's color cannot be decided, so I guessed it.
Minimap without pin while unknown spot on library is red
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How I deleted pins manually:
After zooming-in, pixels hidden by pins could be shown. This is the way to know the pixel's colors behind the pin.
First, I have to know which pixels belong to the pin: because the pin uses anti-aliasing function, I shouldn't directly edit the copied image, as some pixels looks like pin pixels but are actually not, and some pixels looks like map pixels but are actually pin pixels. So I zoomed-in the map, copied the image, pasted it into MS Paint, and filled map part around the pin into other colors, and finally made the left part, which is the pin, as monochrome.
If I just simply move these shapes onto the copied mini-map image, my workload would be too heavy. Thus, I made Pony Town's mini-map's size as 800x800 (so each spot's size is 4x4. If the mini-map's size is 700x700, then some spot would be 4x4 and some would be 3x3), then copied the image into MS Paint, then pasted monochrome pin's shape on the pin, then change the 800x800 image into 200x200.
Then I can change the pin's pixels into the mini-map's pixels by zooming-in and see these pixels. (Yet one pixel on library is still hidden after zooming-in (the maximized total mini-map’s size is 3000x3000), so it's cannot be known. Thus I guessed it through the real library's color. Now the library's roof is covered with snow, so I looked at >3515684. Yet it cannot be 100% correct. And this way is too slow, so in the future, the way editing the webpage's code to delete the pin on the webpage must be found.)
!large Change the pin's pixels into the mini-map's pixels by zooming-in and see these pixels; and the unknown pixel
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Update: The unknown pixel is now known
In “Settings”’ “Minimap” section, for PC's webpage, the default “Icon size” is shown as 1x, but for moblile phone, the number is higher. So I used Inspect (F12), then clicked “Toggle device emulation”, then chose “Dimensions: iPhone 14 Pro Max” (where default “Icon size” is shown as 6x), then set “Settings”’ “Minimap”’s “Icon size” as 1x.