Shadow Help?

# 1 18-03-2011 , 09:19 PM

Subscriber
Join Date: May 2010

Posts: 8

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.
user added image

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

Thanks
Paul

Quote

# 2 19-03-2011 , 05:03 AM

Registered User
Join Date: Mar 2007

Location: TN (USA)

Posts: 1,889

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.

Don't be satisfied with what you can do but rather strive to do the things you can't do!
Exceed Expectations!

Last edited by Perfecto; 19-03-2011 at 05:09 AM.

Quote

# 3 19-03-2011 , 09:56 AM

The Nurb Herd
Join Date: Oct 2007

Location: London

Posts: 2,381

do you have any lights? ibl doesnt count.
jay made a friday tutorial on this shader. have a search for it.

Quote

# 4 19-03-2011 , 07:07 PM

Subscriber
Join Date: May 2010

Posts: 8

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

Quote

# 5 20-03-2011 , 07:20 PM

Subscriber
Join Date: May 2010

Posts: 8

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

Quote

# 6 20-03-2011 , 09:44 PM

The Nurb Herd
Join Date: Oct 2007

Location: London

Posts: 2,381

i'll take a look.

Quote

# 7 21-03-2011 , 02:32 PM

Registered User
Join Date: Mar 2007

Location: TN (USA)

Posts: 1,889

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.

Don't be satisfied with what you can do but rather strive to do the things you can't do!
Exceed Expectations!

Quote

# 8 21-03-2011 , 10:18 PM

Subscriber
Join Date: Nov 2002

Location: USA

Posts: 1,170

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.

Quote

# 9 21-03-2011 , 10:36 PM

The Nurb Herd
Join Date: Oct 2007

Location: London

Posts: 2,381

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.

Quote

# 10 22-03-2011 , 11:55 AM

Moderator
Join Date: May 2003

Location: London

Posts: 1,001

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.

FX supervisor - double negative

Quote

Shadow Help?

Posting Rules Forum Rules

Similar Threads

Topics

Free Courses

Full Courses

VFX News