Contours of uniform 2D objects with contourc

Hi,

Using the function contour on gray-level images, I notice a strange feature about contours of a specific level with uniform connex components.

I want to test it for level 127 on a synthetic image, so I make athis illustrative test sample:

I = [255*ones(9,3),127*ones(9,3),zeros(9,3)];

Quoting the documentation: "To draw a single contour of level i, use contour(Z,[i i])."

So I call

contour(I,[127 127]);

The display shows only the right boundary, not the left one. When I type

contour(I,[127 128]);

then I have the two boundaries (with different colors).

I don't understand why is that so; I can't figure out with reading the doc. Does anybody know why is that so?

[Benoît THOUY]

Finally I understand that contour(I,[lvl lvl]) separates zones where point heights are strictly lower than lvl from zones where point heights are greater or equal than lvl.