We present work on in-situ shape detection of filament material in the FLM range, using optical detection of the diameter from multiple angles in near real time. From the shadow images of the filament, the area is approximated by a parametric fit of an ellipse. This shape measurement can be measured and transmitted to a printer 25 times per second and includes an elliptical shape factor. The firmware can then modulate the deposition speed in order to adjust the volume flow to the planned target value. The possible applications include Fused Layer Manufacturing (FLM) printers and filament extrusion as the basis for a closed control loop. In order to achieve the necessary performance quality, a resolution of better than 8 µm is required, which we were able to prove in a prototype setup. Optimization steps that will later be necessary to achieve a diameter resolution of 5 µm or better have not yet been included.