That's not half bad actually. Dense 3d reconstruction is matching pixels from one view with pixels from another view. If the areas are smooth, low texture, then 3d information is difficult to reconstruct. The inside of the grooves looks pretty uniform. Shiny also makes it worse.
You might get a better model if you can find a way to coat the knob in something that produces visible patterns on the low texture areas. Sometimes spray paint can help here to reduce gloss. Might also try taking some sand paper to the knob and lighty sanding everything, including the inside of the grooves, to give it texture. Not enough to change the shape.
Otherwise, a different 3d reconstruction technique would work better (project light, line laser).
Topic: Struggling to scan small plastic knob (Read 6803 times)
