Rounded eyelet (pattern)
This looks like a fairly complex pattern made up of larger, filled octagons joined together by smaller straight squares. In actual fact the pattern is easy to follow by completing a number of offset eyelets and then outlining each eyelet with a octagonal frame.
Start by completing a single diagonal stitch across one thread and down over three threads.
Add another seven diagonal stitches of the same length all going down into the same hole to complete an eyelet.
Structure of stitch
Works well for an even density and and darker areas.
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.151