Here are some examples what stereo3d filter can do:

side by side half width left first

above bellow left first

-vf stereo3d=sbs2l:abl

Converted to different anaglyph formats:

Red blue gray/monochrome	Red green gray/monochrome
-vf stereod3d=sbs2l:arbg	-vf stereo3d=sbs2l:argg

Red cyan gray/monochrome	Red cyan color	Red cyan half color	Red cyan dubois
-vf stereo3d=sbs2l:arcg	-vf stereo3d=sbs2l:arcc	-vf stereo3d=sbs2l:arch	-vf stereo3d=sbs2l:arcd

Green magenta gray/monochrome	Green magenta color	Green magenta half color	Green magenta dubois
-vf stereo3d=sbs2l:agmg	-vf stereo3d=sbs2l:agmc	-vf stereo3d=sbs2l:agmh	-vf stereo3d=sbs2l:agmd

Yellow blue gray/monochrome	Yellow blue color	Yellow blue half color	Yellow blue dubois
-vf stereo3d=sbs2l:aybg	-vf stereo3d=sbs2l:aybc	-vf stereo3d=sbs2l:aybh	-vf stereo3d=sbs2l:aybd