In a 4x4 square tile, it's easy to know which pixels are inside the tile's boundaries. I know immediately that the 4th vertical pixel is supposed to be drawn but the 5th isn't.
However, on a rhombus' shaped tile, those boundaries aren't that obvious since the picture that contains it is a rectangle. This means that, within that rectangle, there will be pixels that are supposed to be seen and pixels which aren't.
If I'm drawing an isometric tile, how would you suggest I get to know which pixels are within its boundaries and which aren't?
I thought about drawing border lines so that I can draw the texture inside but since the texture should be a power-of-two size, there's no middle pixel on each axis, so it won't be just a matter of connecting 4 dots. So I decided to connect each of the 2 middle pixels in each edge, with the closest pixel on the closest edge (1st picture on top). However, if I draw inside of them, the texture won't reach the edge, which I imagine will create gaps. If this is the case, should I draw the lines one pixel beyond those two middle pixels in each side to get the outside boundaries (2nd image on top)? Are these lines a reliable indicator of which pixels should be seen?