You are right ... the scope squares are constant. But there should be no need to make the scope squares be a multiple of 8. You could just as easily use 10 or 12 pixels. Just compute before update a bit mask for the grid squares and then OR that into the bytes as you process them. At least that takes care of the vertical aspect of the squares.

For the horizontal portion all that is needed is to see that you are on a pixel row that coincides with the grid and OR on a dotted pattern. The pattern could be from a table in CODE memory that is indexed with the horizontal byte number along the row that you are updating.

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Michael Karas