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Old 18-03-2011, 10:19 PM   #1
RobboUK
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Exclamation Shadow Help?

Hey guys, I have been struggling with this for about two days, I have modelled a car and I am now trying to composite it into a scene, the only problem is that I can't seem to get the shadows to work. I have used the useBackground shader but this leaves a black plane in the render and nothing in the alpha channel. I really dont get why because it has worked before when i have tried this, I am using IBL with mental ray and ray trace on.


Can you guys help at all, I can PM the scene file if that helps

Thanks
Paul
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Old 19-03-2011, 06:03 AM   #2
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There's a few things that come to mind.
1. Make sure your light is casting shadows.
2. Make sure that in your background shader attributes that shadow mask is set to 1, or something greater than 0.
3. If you are using ray trace shadows, make sure you are using mental ray renderer, or have raytrace turned on in the software render settings.

Edit: Actually, it sounds like you're wanting to render out a shadow pass.
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Last edited by Perfecto : 19-03-2011 at 06:09 AM.
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Old 19-03-2011, 10:56 AM   #3
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do you have any lights? ibl doesnt count.
jay made a friday tutorial on this shader. have a search for it.
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Old 19-03-2011, 08:07 PM   #4
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Thanks perfecto, i will try these when I am back at my computer, likely to be tomorrow, yeah honestdom, ive got a directional light aswell to make sure there were some shadows and when using a lambert texture there are shadows, will have a search for that tutorial

Thanks guys
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Old 20-03-2011, 08:20 PM   #5
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Perfecto I have tried doing what you have said and i still dont get any shadows at all, think if i sent you the file in a pm you could have a quick look?

Thanks
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Old 20-03-2011, 10:44 PM   #6
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i'll take a look.
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Old 21-03-2011, 03:32 PM   #7
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I'd be more than happy to take a look for you as well, but honestdom would be a better choice as I've never used Image Based Lighting before. I'll have to read the help files to understand IBL and how it works. I'll read up on it today.
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Old 21-03-2011, 11:18 PM   #8
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other alternative to use ibl would be a surface shader on the plane and use mib_fg_occlusion and turn the cars and image primary visibility off. not a real shadow since it is only ambient occlusion, but it is calculated with final gather. if you turn the cameras background to white you can multiply this shadowpass directly over your background and overlay your car.
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Old 21-03-2011, 11:36 PM   #9
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thats not really an alternative to using an ibl. its the same as doing an ambient occlusion render.
the thing about this scene is that the car is moving. so using final gather may cause some flickering issues that ambient occlusion would not.
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Old 22-03-2011, 12:55 PM   #10
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Hmmm,
Looks like you arn't using an image plane as your background.

Remember the use background shader will just query the current image plane as the basic color component for the surface with an alpha of 0 and then shoves everything else on top.

It doesnt do anythign with primary rays so if you have an image enviroment shoved on a card or sphere it will not get rendered.

A better way to use this shader would be to render a pass with primary visibilty turned off on everything except the use background shader which will mean you will get nothing but your shadow and reflection contributions as a seperate pass. Then just layer your car beauty on top and your BG image below in comp.

There is no real need to render everything at once, make use of render layers to preview the result before comp.
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