Methods for rendering and compositing a photo merge scene

[stayinwonderland]

I have a photo of a house that I'm tweaking with surrounding 3d elements. I've camera matched the photo in 3ds max and built a rough shape (the same as the house in the photo) and changed the model's alpha contribution to -1.0 in vray properties so I can put things on it and it will be invisible in the render but also obscure/occlude stuff that goes behind it.

 

Only thing is shadows. I'm currently growing ivy on it and it all renders fine but ovbiously they don't have shadows because the geometry they're growing on is invisible.

 

Any ideas how I could get shadows to appear and then maybe render those shadows out separately so I could comp them in photoshop?

 

cheers.

[Corey Beaulieu]

In the vray properties also check "shadows" and "affect alpha" and your shadows will be there.

[stayinwonderland]

Ah that's great. Now only problem is, my plan is to render the elements on a black background, save as a tga with alpha and bring it into photoshop to lay over the original photo of the house. So will the shadows get lost or just get saved to the tga alpha info?

[Scott Schroeder]

If you are using Vray you'll need to enable the shadows in the vray matte properties. Make sure you are also telling Vray to use it as a matte object and not just setting the alpha to -1. For separate shadows you can either render out separate passes or use one of the shadow render elements.

 

Or you can create a layer in the middle of your photo and your rendered ivy in photoshop, and just draw in blob shadows with the paint brush. Set the layer to multiply/overlay/soft light, etc to whatever looks good and adjust the layer's opacity.

[Corey Beaulieu]

they become part of the alpha.

 

sometimes it can be a good idea to get a multimatte or just a general matte for the object themselves so that you can use it to mask the object from the shadow.

[stayinwonderland]

Ah, going to have to take baby steps on this. So this is what I did:

 

1. turned on the 'shadows' and 'affect alpha' and, with my photo as the viewport/environment background, hit render and sure enough, the fake house is invislbe but receiving shadows.

2. for a real test render, I turned off the viewport/environment background and set it to black, cranked up the render quality and hit render. But this time, for some reason, I could see my invisible house - probably because it was receiving a load of shadows from the scene - but then this never happened in #1 above.

 

Weird that it would suddenly be renderable just because I turned off the environment background?

 

I just added a matte material to it and nothing changed.

[Corey Beaulieu]

It's not that it rendering, but that it's visible, right?

 

I've seen this before and have always reduced it to GI. Try changing the properties to not receive or cast GI. I say try because when this happens to me, I get annoyed and just render a matte. I don't spend much time trying to fix it.

[stayinwonderland]

Yeah it is tricky. I set it to not receive or cast GI and it pretty much disappeared but when i bring it into photoshop there's still fait traces of the geometry that's supposed to be invisible.

 

Also, the shadows are really solid black. If I uncheck 'affect alpha' it gives almost no shadow but maybe a faint 'something' which is somewhat preferable to solid shadows.

 

What's this 'render a matte' option technique then?

[Corey Beaulieu]

Essentially you can render a shadow pass and try that as a matte on its own or you can just use the G-Buffer ID on all of the objects in your scene. Then use the matte to clip the scene from the shadow and invert it to get the shadow from the scene. At least this way you can reduce the intensity of the shadow without effecting your scene.

 

There is likely a way to combine the shadow element with the matte, but without the pieces in front of me I can't be specific as to how.