Matte Object - Rendering Cars Separate Pass - HELP

[william.morris]

Hello All,

I have searched for weeks and read every thread going and I can not seem to find the answer to my problem.

I have rendered out an animation now I am trying to render the cars out in a separate pass, and the comp the cars over the top of my animation in aftereffects.

I have attached a few images to illistrate, I have selected everything apart from the cars and made it a 'Matte Object', '-1 Alpha Contribution' ticked 'Shadows' and 'affect alpha'

This renders my car out with its shadow but when i lay it over the top of my render the shadows are very harsh and do not match.

I have tried everything, white and black background.

Does anybody have any ideas???

Much Appreciation

Kind Regards

Will

[Morbid Angel]

Hi Will,

There are several issues with your setup. First you have to think about how you are splitting out the layers, in order to rebuild them back properly in comp you have to split them out properly from max. Right now you have a set (road) with sky. It gets shadows from other objects in the scene. Then you have a car against just the sky it appears. So the shadow is wrong in brightness already, since with alpha you need to render against black, not anything else.

What you have to do is think in layers out of max first. So render the set against black. Then render the sky. Render the car with the set as visible, but set to matte, alpha -1 but no shadow option (so its cuts out just the car). Then you need to render just the shadow / gi portion of the car, where the set is alpha -1, but casts no shadows (so it only receive shadows from car) and car is set to visible to camera as off.

Each of the render layers has to have render elements such as lighting, gi, diffuse filter. Since your car is coming in and out of shadow, you have to rebuild the layers together in comp the same way, otherwise the shadow / gi portion will never match. So for example when you have raw lighting you multiply the car's shadow over it and get a black shadow over other shadows, and that get added with gi. If you don't do that and simply layer any shadow over the shadow area, you will get double shadow and it will look wrong. The reason for the shadow to be a separate layer is because you will get edge issues no matter what, so best to have them separate.

Its a lot more work then one might think I hope I explained it well enough.

[william.morris]

Thank you for your great and quick reply.

I understand your email, I can see that there is no easy and straight forward way of doing. It has driven me crazy for days.

Its is probably easier to rendering them in with the whole scene, but I find my render times trebble to get the same level of GI and quality when using animation prespass, compare to rendering the same frame in single frame mode.

So I have now decided to render the Car, Road, kerbs, pavement, anything that the car will cast shadow onto, all part of the same render pass.

Then make everything else in the scene a Matte object and Matte for Reflections.

I will then overlay the cars and road render pass etc over the top in After effects. Did a little test seemed to work ok and hopefully save me render time.

Thanks Again for your Reply!