Shadows going through objects

[tptnshp]

Maybe some of you can assist me on this topic. Working in 3DS MAX 9 for rendering.

I have a scene which is supposed to result in an image that depicts an aerial view highlighting a proposed building that will be inserted into an existing parcel of land. The goal is to exclude the shadows of the existing buildings and expose only the shadows caused by the design proposal under worst case scenario (winter solar).

It is a very simple scene, all geometry is imported from rhinoceros. The existing buildings are rendered as a standard material with a light gray color. The proposed buildings (simple box masses) are rendered as red boxes.

All the geometry is imposed over a plane with a NYC zoning map applied to the diffuse channel of a standard material.

I started rendering this in scanline by creating two lights. The first light was a sunlight system and it would only be generating shadows of the proposed buildings. To accomplish this I turned off "cast shadows" on the existing buildings on the object level. In order to not illuminate the objects and only receive the shadows I had to make the light color black and the object shadow color white with a density of -1. This maintained illumination and therefore kept the shadows. I know that this would not work in VRAY because i tried it and well, it just didnt work.

In order to control the general scene lighting, I used a independent light positioned over the work with shadows turned off, and this seemed to work fine.

THE PROBLEM: Sorry for the long winded wind up, the shadows from the proposed building are going through the existing buildings. There seems to be a problem with the accuracy of the shadows, technically they should be stopped by obstructions but they are not, they act as if they are not affected by interfering geometry.

I am attaching a rendering of what I mean

I would appreciate any help any one can give me, especially if they know of a way to work this in VRAY using mattes but keeping the process simple with little to no photoshop work, and i would like to not have to fake this. Initially I thought the geometry was the offender but I modeled a box natively in 3DS MAX to interfere with the shadow as well and it does the same thing. Im totally stumped on this.

THANKS!!!!!

[Neilg]

I'm not sure it's possible with one render - by excluding the other boxes from casting shadows you also let the other shadow pass straight through them. It isnt a problem, it's just a byproduct of taking the other buildings out of the calculation - anything that blocks a shadow casts it, stop it casting and it stops it from blocking it too. You're already breaking the rules to get that far.

I had a quick play around and couldnt see any easy way to do it in single frame, although this would be very easy if you were to render in two passes.
There may be some convoluted material and multiple light setup that could do it, but it's not jumping out at me.

Here's the same result you're getting in scanline, but in vray anyway - if you decide to comprimise (plus it only requires one light).
It's done by excluding the other boxes from casting shadows but leaving illumination on, done through the vray light controls 'exclude' button.

[tptnshp]

I am trying to light the entire scene independently of the light that is causing the shadows. It seems that if an object is not casting shadows it acts as if it is invisible and allows the shadows of other objects to pass through it.

Is there no way to make an object not cast shadows and also block shadows that are against it? I guess youre saying there is not....

Also the light in the scene you posted is causing the scene to illuminate. Perhaps the only way is to render in passes.

[tptnshp]

can you explain to me how to do this in multiple passes,

the only thing I can think of is to render two shadow passes and then subtract the one from the other in photoshop and then place the subtracted pass over a diffuse pass

am i right here?