How to read this
Returns to scale only look along rays, comparing with . Concavity compares any two bundles, also on different rays. Here every isoquant is a straight line, so is quasi-concave, and returns to scale are nowhere increasing.
- Look at the red curtain: above the segment from to the surface sags below the chord. Not concave.
- Press Make it homothetic: the isoquants become parallel, the red curtain disappears, and concavity returns.
- Face the chord gives the clearest view; drag to turn the plot.
Technology
Two bundles on different rays
Output over the input plane
and with their rays on the floor and the output along each ray on the surface. The thick curve is above the segment, the chord joins and . Thin lines: isoquants, on the surface and on the floor.