The first is radiating out from a central point, using one motif that gets larger as it grows out from the center. I've gone a bit wonky on the left hand lower corner, but that is easily sorted. The sections are divided by a double line of stitching. Likewise, I have deliberately tried not to match the side corners of the triangles with the triangles of the next section. If the corners meet up, the eye seems to be unable to decide whether to see the triangles as the positive - or the space in between as positive and the triangles as negative.
The next one again uses the idea of radiating out from an off-center center. I've done very irregular curves and filled some of the spaces with the every useful pebble motif. I quite like this one and it would be very easy to variations replacing the pebble motif with a different filler.