General Compositing Problem

[Interlife]

hello guys,
this is my first post here and i will appreciate it a lot if someone helps me ( i am in a tight deadline)
so here is the problem (it is not strictly a combustion question):
1- i am creating an Ad where there is a field of grass made in 3dsmax using Hair WSM and rendered using default scanline render.
2- composited on top of that a number of trees made in max and rendered with VRay and casting shadow on a matted-shadow plane(the same plane used to grow grass in step1 but obviously without the grass)
3-when composited using combustion the problem is that the shadow matte created in max (casted over a plane with no grass) will affect badly the grass field layer (see the pic)so it appears like there is no grass where the shadow is ...I know it is multipling the image of the shadow wich is planar(no grass ) with the grass underneath it.
how to solve this problem i know the major techniques of matte-shadow compositing but i cannnot find a suitable and proffesional way to solve this.
thanks for your time.

[im.thatoneguy]

You're going to want to create an edgematte for the shadow pass. Then in that edge matte cut the shadow alpha out using an adjusted-high contrast version of your grass layer.

That way the edge will blend from "Grassy" to grass with a reduced gain but not just softly with actual grass texture.

[bio]

Maybe you can try rendering just the matte for the shadow area, and then bringing it in any compositing software and color correct it to look like a shadow. This way you'll still have the original grass in the shadow area.