If it’s just a shape and you have it’s coordinates then you’re going to want to calculate the points (unknown) that fall exactly between any two other points (known). This will be very easy for rectangular shapes.
Then you can attempt to determine the space inside the shape given an offset and possibly draw the line.
For this ‘H’ there are three rectangles you should discover algorithmically and with then you could draw that red interior.
P.S.
If you want do it for less uniform shapes you’ll have to generalize a big. Calculating which points are closest to each other on the same axis (x or y line).