Hi Marco and Henrik,

Both approaches are fantastic to me! I really want to study both of your codings.

Again, thank you very much!

Eiji

Hi all,

removing a complex background can be a difficult task; therefore Marco's nice solution can hardly be shorter. If one concentrates on the fact that due to snow the background is basically gray and the trees stay (at least a little bit) green, things get much easier (though I am not sure that this approach answers the original question):

greenQ[p : {r_, g_, b_}] := With[{mean = Mean[p]}, If[g > 1.03 mean, 1, 0]]
mask = Erosion[Dilation[ImageApply[greenQ, img], 4], 4]

this gives a "mask" for green trees. Lets check:

Regards -- Henrik