CGTalk - Compositing shadows

ankuragarwal

05-30-2005, 08:38 AM

I am very new to After Effects and trying to teach myself by reading its computer manual and experimenting with it.
I am having problems with compositing shadows. I created a scene in Maya, in which a girl is standing on a floor which I want to remove in AE.The girl's shadow is falling on the floor. When I key out the floor in AE (I have already a layer below the girl layer for the background, I want that the floor be keyed out but the shadow remain so that the shadow falls over the background, and the girl looks like she was rendered in that background), problems occur with the shadow. Since the shadow itself partakes some of the colour of the floor, it obviously is keyed out somewhat.
So, then I render the floor separately with the shadow on it, and the girl separately. But now, when I key the floor out, the shadow remains as it is but the problem is that the shadow does not look black but somewhat of the colour of the floor that I have keyed out.Spill suppression also didn't work, as far as I know using it, since it made similar colours faded out in the entire composition.

So, next what i did was that instead of giving a as unique colour as possible to the floor, I gave it a very transparent and very reflective material in Maya and rendered that floor with the shadow on it, and rendered again the girl separately.I saved all images as Targa, with alpha mask on.
In AE,when I raise the alpha levels threshold value, so that all the pixels of the same amount of semi-transparency as the floor be output as completely transparent, the floor vanishes from the image.But I still don't see the shadow in the composited image, whose alpha value is totally opaque and hence isn't affected.

Am I possibly doing something wrong, and if yes, what? Are there simpler and/or better ways to do what I am doing (there must be!!!)

Thanks for taking out the time for reading this post and understanding my problem. My profuse thanks for any forthcoming replies.

Ayreon

05-30-2005, 10:50 PM

One thing you could try is using the background image/footage that you are composing the girl on as a front projection texture. This way the shadow will be projected onto a surface with the same color values as the background image. I hope I made this a bit clear. I don't use Maya myself, so I don't now if it is called front projection in Maya.

Good luck!
Ayreon

Mylenium

05-31-2005, 05:45 AM

Why do you use keying? All you need to do is build an appropriate shadow density based shader in Maya that is only opaque where the shadow is.

Mylenium

mdurwin

05-31-2005, 08:19 PM

Too bad you're using Maya. There is a way to cast a shadow on an invisible plane that composites awesomely!I don't know about Maya but Lightwave works simiarly to what Ayreon proposes. I've had the same issue rendering volumetric particles in AE and trying to composite in Final Cut. It's touchy.

XanderFX

06-01-2005, 12:33 AM

In maya just apply the use background shader on your ground/wall planes and make sure the geometry's render stat is set to not cast shadows. You can find this option by selecting your geometry hitting ctrl+a and then going down to the render stats dropdown and turning of casts shadows.

In the Use Background shader settings you will want to adjust the reflectivity to match what you are compositing into or turn it off completely. If you want even more control over the shadow, such as blurring and adjusting the transparency level in AE, put your girl into one render layer and your floor into another render layer and turn on render layers in your render globals make sure the default render layer is turned off and you turn on output to subdirectories. This will give you 2 images for each frame one of the girl and one of her shadow this way you can really match your shadows to the existing shadows in the live action plate.

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