Ok, working, but not exactly as should - darker should be near edges, not center as is now
Hmm ,there is a little bug:
Delphi-Quellcode:
else if distance <= outer_radius then
// decreasing Brightness from 100% downto 0%
// result := (distance - inner_radius) / (outer_radius - inner_radius) // wrong
result := (outer_radius - distance) / (outer_radius - inner_radius)