topic Re: Rendering as a green screen or alpha channel?? in 3ds Max Shading, Lighting and Rendering Forum

Rendering as a green screen or alpha channel??

Anonymous — Mon, 13 Jan 2014 00:29:34 GMT

Hi, in 3ds max 2012, i want to render my model in front of a greenscreen . I then hope to import this in the form of png sequence to adobe after effects to be keyed out. For me, i usually do this through the environment settings and alter the background colour to green. Many people have told me to use alpha channel or something, but i don't understand that technique. Does it create a green/ blue screen?

Well, after i rendered it out, this is what happened. The floor on which the t-rex is standing is green, while the other parts are white. However, in the render screen, everything apart from the t-rex is green. The problems i encountered when i try to key it out in after effects is that since the back ground is white, it automatically becomes half transparent. But i still have to key out the green floor on which the t-rex is standing. Thus, after keying, the background (trees) become a different colour. Please have a look at the image.

* If there is another way to add a green screen other than through the environment settings such as alpha channel, please provide some advice/ tips/ link/ tuts. thanks!!!!

Re: Rendering as a green screen or alpha channel??

Steve_Curley — Mon, 13 Jan 2014 10:47:19 GMT

An Alpha channel is simply a mask - where it is White you will see the object, where it is Black you will see the background (or more precisely whatever is "behind" the object). Alpha can be embedded in the image (RGBA) or created as a separate black/white image. Probably best to render against a black background.

Greenscreen is normally used for live action and is probably overkill for what you're trying to do, and it adds an (unnecessary) extra step into the compositing process.

Re: Rendering as a green screen or alpha channel??

Anonymous — Mon, 13 Jan 2014 11:15:08 GMT

Thank you for your time. Investigating this as we speak. However, if I wish to save the rendered scene as png sequence, how do I enable "alpha channel" ?

Re: Rendering as a green screen or alpha channel??

Anonymous — Mon, 13 Jan 2014 11:27:12 GMT

I think I finally understood through the process of trial and error.
When I attempt to render with "alpha channel" the background becomes white, although I have the background environemt is grey in default.

However, when I turn it off in the render set up something called " info", the background becomes grey. As a result, this cannot be keyed out. XD

Re: Rendering as a green screen or alpha channel??

Steve_Curley — Mon, 13 Jan 2014 11:46:45 GMT

Rendering > Environment. The background needs to be black as should the Ambient color - note that this is NOT the viewport background. If that doesn't fix it, reset Max, create a single object and render to a single image. If that works you need to determine what is different between that new scene and your original scene. If it doesn't then you must have some default settings wrong, so save that simple scene, zip it and attach it so someone can take a look at it.

Re: Rendering as a green screen or alpha channel??

BenBisares — Tue, 14 Jan 2014 06:33:43 GMT

You really should use the alpha channel over the green screen technique. The alpha channel allows you to very precisely render the background transparent whereas with the green screen you'll have a hard time getting a nice clean edge due to the anti-aliasing with the green background. You might have to frequently adjust your keying parameters in your video editing program to make it look nice throughout the animation. More importantly if you have reflections on any of your objects the green background can appear on the surface of your object which will be a major issue (you’ll get transparency on the object).

Basically saving as a format that supports alpha channels (TGA, TIFF, PNG) should allow you to view the background or contents of the track underneath in your video editing program (Premiere, After Effects, etc..). For some programs you may need to enable something for it to enable the alpha channel.