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alpha masks and chroma key

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Brandon Yates
alpha masks and chroma key
on Jun 8, 2009 at 10:23:05 pm

Hiiiiiii,

So I'm having a problem and can't seem to find a good tutorial on how to do this.

I have greenscreen footage in Premiere. Ihave it all keyed out so the green / background is black and only the image of the actor is there in the video. However when I export it to avi / targa and import it into Maya, instead of the background being invisible it is just black. How can I get the alpha to translate to Maya from Premiere?


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Michiel Duvekot
Re: alpha masks and chroma key
on Jun 9, 2009 at 12:23:49 am

avi files cannot store an alpha channel. Some forms of tga, but not all, van do this. Choose a format that does support an alpha channel.

Michiel
http://www.thnkr.com


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Brandon Yates
still not working ;/
on Jun 9, 2009 at 12:50:30 am

Thanks mucho for helping man. It's still not working though. I tried exporting the movie from Premiere as these so far.... .bmp .gif .tif .and previously .avi and .tga, everything premiere offers, but it's still coming up in Maya with the black background showing. Gosh dangit charlie brown!!! Help.....


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Adam Benson
Re: still not working ;/
on Jun 14, 2009 at 6:44:47 pm

Hey There,
The limit may be in Premiere's settings. I haven't worked with premiere in years, but I do know that as editing packages go, it is relatively limited. The file formats that support Alpha channels are MOV, TIFF, TGA, RLA, RPF, GIF (but, ew! No!), IFF and a few others. JPG, BMP, DPX and most others DO NOT hold Alpha channel information. What you'll notice about all of the ones that do is that they are image formats, and to export a timeline using any of these (except MOV) will give you an image sequence. In other words, you'll have a whole bunch of image files instead of one video file. This is actually preferable, especially when working with Maya. You'll most likely want to make an individual folder for each export, so that you don't have a clutter of files all in one area.
Now, the trick will be to get Premiere to export one of these file formats WITH the alpha channel. In most software packages it will be one of the export settings, and often it does it by default. It probably won't be on the primary dialogue box when you first start your export, but it should be on one of the "Settings" dialogue boxes WITHIN your export settings menu. For example, you choose EXPORT > IMAGE SEQUENCE and the dialogue window pops up and on there you choose TGA and then a SETTINGS button comes up. Click on that SETTINGS button and there may be a place that says ALPHA with a checkbox, OR there may be something that says RGB or RGBA. In that case choose RGBA because the A stands for Alpha. Another thing to try is within your compression settings. You may see a place to choose color space; it will say "MILLIONS OF COLORS" and "MILLIONS OF COLORS +" the plus (+) is basically where the CODEC stores its ALPHA data. I don't actually know (or think) that this is the actual Alpha channel, but the MILLIONS OF COLORS + does store alpha data. In TIFF and TGA settings it may also say "Save Transparency". If that's the case make sure that is checked.
Another option you have, if all else fails, would be to export your chroma key as a black and white image. I don't mean a grey scale image like an old black and white TV, I mean either it's black or its white. If it's transparent then it should be black, otherwise it should be pure white. Many chroma key applications give you the option to export the footage as a MATTE, which is what this would be. After you export that you can import that into Maya separately and use that file as an OPACITY MASK.
The last bit of advice I could give you would to be to try and just export like, 5 frames, so that you're not wasting all day exporting the entire sequence. Then just do some trial and error with those 5 frames. Change a setting, export it, try it out. Rinse and repeat.
Good luck

Adam Benson
Visual Effects Artist/Musician/Sound Engineer/Editor
http://www.SleepDeprivedProductions.com
http://www.SleepDeprivedFilms.com


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Brandon Yates
Re: still not working ;/
on Jun 14, 2009 at 9:19:55 pm

hey Adam, thanks and God Bless you for helping. I was feeling overwhelmed today after a few weeks of going at this. So i wanted to ask if that is the standard method for creating cgi in live action? I want to do the basic star wars / sin city etc... where you mix your animation creations with real life actors. Every time I learn a software I get thrown a curve ball in the training where they use another program to do this. Right now I'm neck deep in training for Maya , Premiere, Lightwave, aftereffects, blender, boujou and even more... I'm just wondering what is the standard method of making live action cgi, because I'm still not sure how to get my video footage and my cgi's to match up. Another problem is that it seems most of the training and demo's of maya, lightwave, etc. are for purely animation, and there is not any training for how to mix the animation with the film footage. Do you know of any good training videos for that?


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