View Full Version : Shadow Transparency and Greenscreening Help

04 April 2005, 07:22 PM
Hi I'm new to compositing and theres one thing that I really want to learn how to do, I feel that I'm close but there are two things holding me back.

Anyway I'm using 3d studio max 7 and I'm just making an object in the viewport with a green floor and a green background which I'm going to use to greenscreen in front of a picture I took. Well when I use my lights right I get a shadow on the green floor which I would like to be there in the final composite. However when I render the image and import it into Combustion and use the discrete keyer I get rid of all the green but I have two problems I really want fixed.

The first problem is is that the shadow now has a green tint to it much darker than the green floor and background form before, and also the shadow isn't transparent showing the ground through it which I want to happen. My though was that in max I need to somehow enable an alpha channel or something on the shadow so when i import it it will automatically show through depending on the strenght of the gray shadow.

I've been working and working on this problem for quite some time and I just dont know how I can fix it... If anyone can point me in the right direction and kind of tell me how I can do this I would be most greatful.

04 April 2005, 10:20 PM
No, no, no, no!

If I had a dollar for every time I saw someone doing this, I wouldn't be very rich, but I'd be about $10 richer....

Greenscreen is used for generating an alpha channel when one is not immediately available. This is why it is great for live-action shooting (at least, I have yet to see a camera that can capture an alpha channel...)

However, if you're rendering something, you should be able to generate the alpha channel through the render. The render itself should be done against a black background, with a seperate alpha channel. I'd suggest doing the render of the object as a seperate render from the shadow - for the shadow all you need is a black and white image that defines where the shadow falls - then you can adjust how the brightness changes on the ground where the shadow image is white compared to where it is black.

Hope this has been some help - and apologies for the start of this post... I've just seen this mistake being made so many times before.

04 April 2005, 11:16 PM
Okay I rendered the object in 3d max first with a blue/green background no shadows whatsoever. But I just dont understand the shadow part. Before I just turned everything green and had no shadows meaning I still had floors and walls to know where my object is going but they were green. but now If I understand you correctly I made the walls black meaning that is going to be the stuff taken out in the alpha map, but now I cant cast any shadows because all the walls and floors are black which doesnt give me a good sence of variation in my shadow (sorry for the runon sentence). I other words I have no shadow at all because it has been blended into the wall and floor.

Ahh I hate this. Isn't there a way where max can just knock out the floors and walls and just leave a shadow that I can export as an alpha map?

Thanks for your reply It helped some I see what I have to do but the shadows is my biggest problem if you have any input I'd appreciate it again


04 April 2005, 06:34 AM
Forget anything about green and blue. The only green or blue you should be rendering out should be as part of the model itself.

When you do your main render, you want to hide the walls/floor so that you're just rendering the main object. To get the shadow, you want to make your main object invisible (but to still cast shadows) and make all of your other surfaces white - you'll then end up with a white image, with black where the shadows should be.

(You'll have to ask in the 3DSMax forum for specifics on how to do any of these)

04 April 2005, 02:51 PM
Just give the walls and floors a matte material, then render to tga and then you can composite your image in aftereffects. That should be about it. There are some useful tutorials with max that explain how this stuff works. It lets you practice with a pooltable and a fake ball. If you follow that tut. you should be able to do what you want :)

04 April 2005, 08:17 PM
Right on Guys thanks a lot I did what you said about rendering the image first and then the shadows and it worked out perfectly. I just have to keep practicing until I can get all the shadows the way i want it.


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