Mask an object with an object?

[TheBadger]

Hi,

I want to know if I can mask an object with another object. So where the one appears the other cannot. I know that I read this cannot be done a long time ago, is that still the case?

The reason I want to do it this way, is because it would, in this case, be easier to tell some plants to not populate with my boardwalk, then to try and draw a mask the shape of the board walk. In this case the board walk is just wood laid on the ground (not raised off the ground with posts) and I would like to see some plants sticking up between the boards.
http://www.publicdomainpictures.net/pictures/10000/nahled/87-1275995807txWg.jpg

Have not finished the model yet, so would not be too much trouble to put legs on the boardwalk and just have it above the ground, but the way I am asking about is my preference.

Thanks.

[bigben]

One possibility might be an overhead ortho render, black sleepers on white surface, no shadows... then project the render through the camera as a mask for the plant distribution. That would only affect the placement of plants though, you might still have leaves/plants passing through the edges of the sleepers if they weren't very narrow

[PabloMack]

...black sleepers on white surface... edges of the sleepers if they weren't very narrow

For Badger's benefit, "sleepers" in Australianese translates into "rail ties" in Americanese. I immediately wanted to confirm that bigben is from down under. I guess this term "sleepers" applies to more than just rail roads? I wonder how this term came about.

[bigben]

We punish terrorists by making them hold up railway tracks???  Better than using ties confiscated by the fashion police???