A fabric-based item may include fabric formed from intertwined strands of material. The strands of material may include extruded strands. Strand extrusion equipment may have electrically adjustable sources such as one or more sources of different polymers, dyes, particles, wire, and other elements to be incorporated into an extruded strand. The properties of the strands such as strand stiffness, strand diameter, conductivity, magnetic permeability, opacity, color, thermal conductivity, sand strength, may be varied along their lengths. Fabric formed from the strands may have different areas with different properties. Markers may be formed from particles at particular locations along the lengths of the strands, may be optical marker structures formed from circumferential rings of ink or other visible material on the strands, or may be other markers that can be sensed using electrical sensing, magnetic sensing, optical sensing, or other types of sensing when forming fabric from the strands.