LOGO-L> hemisphere on a grid

[MHELHEFNY@FRCU.EUN.EG]

;Hello George Mills.
;Here is the code for a hemi-sphere or hemi-ellipsoid shell using 
;the grid procedure given in the saddle and the "sombrero" surfaces.
;Best Regards.
;Mhelhefny.
;------------------------------------------------------------------
to grid :a :b :step
pu gridx :a :b :step
pu gridz :a :b :step
end

to go ; sphere (or ellipsoid if you put :a = say 25)
turnon3d 
grid 15 15 first questionbox[input step (.5 - 1)][]
end

to gridx :a :b :step
for[x -:a :a :step][
   for[z -:b :b :step][
      setxyz :x*15 y*15 :z*15 pd]pu]
end

to gridz :a :b :step
for[z -:b :b :step][
   for[x -:a :a :step][
      setxyz :x*15 y*15 :z*15 pd]pu]
end

to turnon3d
perspective cs ht
setturtle -1
setxyz 900 900 900
setturtle 0 
end

to y
ifelse((:x* :x)/(:a* :a) + (:z* :z)/(:b* :b))>1 [op 0][
op 20* sqrt( 1-((:x* :x)/(:a* :a) + (:z* :z)/(:b* :b)))]
end
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