How to draw a grid with FFmpeg

Draw a grid with cell 100x100 pixels, thickness 2 pixels, with color red and an opacity of 50%

drawgrid=width=100:height=100:thickness=2:color=red@0.5

Draw a white 3x3 grid with an opacity of 50%:

drawgrid=w=iw/3:h=ih/3:t=2:c=white@0.5