porting over the stuff I did in stackoverflow_examples to fix gradient_builtin. oddly, it works in stackoverflow_examples but still gives the strange shadow here. hmmm.
this is a crazy heisenbug. no problems in gradY in any case. gradX does the shadow thing here. gradX doesnt do the shadow thing in my stackoverflow example.
still heisenbug... shadowy gradX. removed return statement.
starting from the heisenbug-free stackoverflow version
using totally same code as in my stackoverflow example. just changed input img. heisenbug.
aha yesss, square Lena img works great. 640x480 car has the shadows.
lena