How would I create a material that only recieve shadows but also makes use of

# 1 18-04-2011 , 11:04 AM

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How would I create a material that only recieve shadows but also makes use of

bump maps.

I know I can use a background material but as far as I know this doesn't have a bump map slot in the attributes.

Basically I want to comp a 3D character into some live footage but I want to be able to use the live footage as a texture too, to create colour and bump on the shadow catching objects, floor/walls ect. So that the shadows that the 3d character creates react more realistically with their surroundings.

I thought that if I could render out these objects so that they receive shadows but no other light properties like specular/diffuse. Then I could simply render the objects out and place them in the live footage without any difference in their appearance compared to the original footage.

Anyone have any good advice.

Pete B

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# 2 18-04-2011 , 12:19 PM

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sounds like you need to build proxy geometry of the set.

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# 3 18-04-2011 , 12:29 PM

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Bump just works on the diffuse/ spec component by bending the normals. It wouldnt work with shadows anyway.

You can plug a displacement map in to do this, the render time will be high but just bake out the shadows if you can to avoid recalculation.

FX supervisor - double negative

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# 4 18-04-2011 , 05:48 PM

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Honestdom, I've built proxy object, just wanted to create a realistic shader for the shadows to fall on.

Lauriepriest.

I can understand this with regards to image based shadows, but also with raytraced?

unfortunately this is a camera tracked scene so can't cache the shadows.

Thanks for the tip. So can I plug a displacement into the background shader?

Pete B

Last edited by Bubblegummonster; 18-04-2011 at 05:52 PM.

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# 5 18-04-2011 , 06:41 PM

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its hard to say without seeing the scene but i can't imagine you need to do this. It will increase your render time loads. how many frames are you rendering? i say just work on it more in comp maybe.

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# 6 19-04-2011 , 11:59 AM

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I agree, I have a very fast machine but this does seem like overkill.

I basically just want to create shadows that aren't just flat. The scene is a 3d robot that's walking on an un-made road so the surface is pretty uneven. I just wanted the shadows to bend over the surface matching exactly where things like pebbles and pot holes were. I guess I could model each piece but that just seems crazy when a bump/displacement map would be so much easier and more accurate.

The scene is around 350 frames long.

Cheers again for your help.

Pete B

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# 7 19-04-2011 , 01:04 PM

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perhaps just model the larger things.
what did you track it in? if its boujou you could regenerate the point cloud after the solve, generate geo from that, and export that into maya as a starting point for the road... or just use that as the proxy geo. if the track is quite solid you could get some reasonably accurate undulations in the road from this.

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# 8 19-04-2011 , 01:05 PM

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You can still bake your shadows per frame so it will work fine.

Displacement will work fine for this, it doesnt matter what kind of shadows your using, bump maps will do nothing with your shadows. Raytracing requires geometry to calculate, bump maps just create normals based on a height map that it generates from the texture, a normal is just a vector direction.

With raytracing, no geometry means no calculation, scatter some random objects on the floor with dynamics, or use a rough displacement, you can always convert your displacement onto a high res mesh to save the time of it calculating every frame.

The closest thing to what you are describing is steep parallax mapping,

You can use a parallax mapping shader which can do self shadowing this will query the current sample on a surface and look up linked lights to decide to self shadow or not. Again this works best for facades and it cannot calculate other objects since it just queries its own height field to calculate the shadows.

FX supervisor - double negative

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# 9 19-04-2011 , 03:19 PM

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any more info on parallax mapping? not sure i get it without seeing an example. sounds interesting tho.

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# 10 25-04-2011 , 09:40 AM

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Hi guys thanks for the advice.

In the end I just went with a simple shadow catcher and added a few pieces of geo to break up the shadow at key areas.

Honestdom, I'm guessing this is similar to parallax normal mapping which if I remember correctly is a normal map, but duplicated with an offset so that when the viewer rotates the view the first map moves at a different speed to the second thus creating some very convincing depth. I know the tech is a lot more complicated then that. We had a coder looking into this a few years ago for one of the games we were working on but it's not very cost effective for a real time render.

Thanks again guys

Pete B

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# 11 30-04-2011 , 10:23 AM

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Thanks for being so helpful.

I thought I'd post the end result here so you can at least see what I was trying to achieve.

YouTube - Exclusive test footage of awesome new robot.

Thanks again

Pete B

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