post trees ghosted over scene

[s_gru]

hi folks. i'm just wondering a good approach for rendering out trees between the camera and the subject, such that i can have the trees on their own layer for composite later on. i'd like to "ghost" the trees so they don't all cover up the building. and in fact, i can ghost them more in the middle of the frame and towards the edges of the frame i can fade into solid.

my thoughts are - put the trees on a separate layer, hide that layer, render the scene. then hide everything except the tree layer and render just the trees against a black background. then just put the tree layer on top of the main render and mask off as needed.

i looked at using muti-mat? the RGB matt, but i noticed i don't get an alpha for that (say if all my trees of concern are red).

so just wondering if there's something else in vray i haven't thought of.

ps - using vray GPU

thanks,

[hermit.crab]

Well, using a Multimatte as a mask for the trees (and the underlying background) should do the trick. There are a couple of ways to create masks (i.e. Integer masks - VRayRenderID, VRayObjectID, and VRayMtlID or Cryptomatte), however, Multimatte is perfectly suitable for this scenario. I presume you're compositing within V-Ray and are using the "MultiMatte mask" (RMB>New Layer); there you can select which of the three (R,G,B) channels to use as a mask.

[s_gru]

i was not compositing within vray. i've been using vray a while now, but have not investigated that. sounds like i should because the video i just watched on it show a ton of stuff that could be a big time saver for me.