Invisible trees

[Maciej]

Any idea how to place the invisible tree to create shadow on the facede?

I tried to mark the tree as an object and assign alpha channel to it as I did it with ground plane, but unfortunately, it doesn't work wit any plant.

My way to achieve it was to produce a png file, considering no tree overlaps the rendered object, and than get rid of the plant using PS (see attached)

The point is the existing tree is where it is. So I need the shadow on the wall, but no new tree - any idea how to do it ?

[mike makki]

photoshop.....

[George Ioannidis]

GIMP

[George Ioannidis]

Seriously now. It could be a great addition to nXt to have the ability to enable or disable particular rays during the rendering process. For instance, in Blender Cycles, it is quite easy thing to do. You just tag the tree to cast shadow (shadow ray) but you disable all the other rays calculation.

[Jelena]

on large non-flat surfaces like here, i sometimes use a inverted alpha-map of a tree, put it on a surface and turn this in direction of sun...

[Maciej]

So do I, very aproximately, Jelena.

However, it would be nice just to use a nXt tree,  to achieve this aim direct, wouldn't it ?

Roy,  any chance in the future? 

[Kevin Lockwood]

Here's a couple ways I go after this problem.

I just tested tagging a tree as "Alpha channel" (sorry, no renders to attach). It sort of works. Nxt plants don't seem to work, but a 3d model can. I tried inserting a textured mesh and tagging, but no luck either. I finally I tried a 3d acad model of a tree, tagged it and rendered. Saving it out to Tiff with alpha omits the tree, but leaves a whole. Now, do a second render without the tree and overlay in photoshop. 

Another way if the tree model wont mask fully (in some tests only the trunk masked, leaves did not) is to save the render to nxtimage, save the material mask to a file and use the selection tool to grab just the tree textures. Use that selection in the same manner as the 2 images above.

I've got a bad cold and my head feels like its full of jello. If I feel better, I'll try to post samples later.