Elena, you did a very nice job with the border, but you don't seem to be using the term, "sharpness" in a conventional way.
In message #3 of this thread, I posted a template for you that was 2250 x 1545 pixels in size and the boundary between the interior and black surround was sharp at the level of a single pixel. This is because the boundary in mine was determined by a threshold operation. One can not get a sharper transition than going from full black to full white in one pixel, as mine does.
In comparison, in your latest post (#11), you post an example of a decorative border that you developed. It's also sharp at the level of individual pixels, but, it's only 903 x 1059 pixels in size. In other words, comparing the long direction of both examples, your posted example has only about half the number of pixels as mine.
By the conventional definitions of edge sharpness, the two examples are either equally sharp (# of pixels to make a full transition), or mine is actually sharper (fraction of the total image length to make a full transition).
Perhaps you are trying to express some aspect of the image other than "sharpness". For example, perhaps you are concerned about the shape of the border or repeatability of the pattern. If either of those is the metric, then yours is clearly much nicer than mine, but neither of those criteria should ever be called "sharpness".
Tom M
