View Full Version : FLV with Alpha Channel

06-28-2003, 02:07 PM
Hey there,

Anyone know if it's possible to preserve the alpha channel when going from After Effects to Squeeze so that the flv has an alpha?

I'm trying to get a subject against a transparent background.


07-29-2003, 01:47 PM
hey idpexec,

just wondering if you had found any usefull info on your post re: exporting alpha channels. i too am in the same sort of predicament. apparently this software www.wildform.com/tutorials/transparent/#1 makes it possible to export swf's with transparent backgrounds. does anyone have any knowledge at all on importing alpha channels from video to flash.

thanks for your time, jevans

05-13-2004, 10:43 AM
Same question:

Can flash import a video alpha layer or do you have to use a png sequence or what?

Any help out there?

05-13-2004, 11:50 AM
I don't believe the Sorenson Spark codec allows for alpha transparency -- so for FLV I believe you're out of luck. What are your project intentions more specifically? You can use a PNG sequence, as charlieFlash mentions, or you could pre-composite if it's feasible in your project. pre-compositing will save you quite a bit of filesize either way, as you're dropping a whole 8-bit channel every frame.

If I'm wrong about FLV I'd love to hear, but I've read nowhere that FLV has support for 8-bit alpha.

05-13-2004, 12:12 PM
Thanks, this confirms what I suspected. Nowhere could I find info that Flash supports alpha channel video imports.

I was wondering because I don't have a ton of video experience (read: I rent movies from Blockbuster and thats about it) and I wanted to have a video of a narrator with a transparent bg that I could then load into flash on top of my other content and have the narrator 'interact' with the flash piece. I suppose exporting a png sequence from Premiere or Director would support selecting a bg color in my video clip to be the alpha channel?

05-13-2004, 01:17 PM
I don't use Premiere, but what you'll be looking for is a Chroma Key filter of some sort. Set the key colour to the background, and adjust the spill & tolerance until the key looks good. When you render from premiere there should be an option to do a frame sequence, and PNG is your best choice of formats -- just make sure it's 24bit with alpha channel enabled.

Be careful with doing chroma shots in video -- as I'm sure you're doing. There are a few things you want to pay careful attention to

1) MiniDV/DV standard is 4:1:1. What those numbers stand for are -
4: Luma (brightness)
1:1 Chroma (color)
and it reffers to how often each is 'sampled' or 'recorded'. So for every four frames, chroma is only sampled once in DV! That's the primary reason for a 'smeary' sort of look compared to film cameras that shoot 4:4:4 (or expensive high-def cameras).

What does this mean for your chroma key shot? It means that if there is any fast motion it will cause all sorts of problems with your key. To minimize this, try to keep your narrator in the center of your frame and avoid any quick gestures. If you really want him to flail his arms like a madman, try doing it in slow motion and speeding it up afterwards : ) There is software available to help reduce DV color contamination (my favourite is The Magic Bullet from http://www.theorphanage.com/ ), but I'm sure you don't want to dig that deep yet.

2) lighting! I'm sure you don't have a professional lighting kit, but you don't need one ^_^ If you have a few bucks go to home depot and pick up some tungsten or halogen work lamps of a few different wattages (if they have colour/temperature ratings, choose the lower temperature because it's a warmer light), some transparent amber plastic (I think they should have something like that), and whatever cheap hardware you can find to give you flexible positioning. Also if they have some light-diffusion material that will be very useful (traditionally chroma key lighting is done with soft-boxes).

What you want as far as lighting a chroma key shot are three things
a) Even lighting on your background
b) No shadows cast on your background
c) No blue being reflected off the backround onto your subject's shoulders/body (it will creep onto the edges of his body, causing for a difficult key. Spill control in software helps, but it will only do so much!)

To achieve this position your lights low and wide -- if you notice blue spilling onto your subject, try putting the transparent amber material (called a 'gel', but I'm not sure they'll have actual gels at home depot) in front of your lights. Keep in mind that you want the background as bright as possible without causing 'spill'.

3) The screen itself! Chroma key material is actually rather expensive, as with most things in the overpriced movie-making world. You're not going for a world-class key, but again...spending a few bucks will give you a much better result : ) If you have something flat that you can paint, buy some matte green paint (ask the paint person what the flattest dullest base they have is. you want the paint to be DEAD flat). Blue will work as well, but if your subject has dark skin it will cause problems. Green is almost always your better bet. You want the brighest and most saturated tint they have. In RGB terms you want 0x00FF00. UBER-GREEN : ) Paint a few coats and try to make sure it's as even as possible.

4) Shoot as close-up as you can -- if you're only going to be using him from midriff up, shoot as close as possible and leave as little headroom as you can in the shot. Scaling him down afterwards will reduce chroma key artifacts and make the key look better.

Well, there's your basic chroma key primer! I know it's probably more than you wanted, but I used to be the director of visual effects for a post house and I miss it sometimes... so let me relive my glory days just this once ^_^ If you have any questions, let me know! I'd love to help : )

05-14-2004, 07:50 AM
:eek: Wow that is great info! I am having the video professionally done for the final product (it's got to look good!), but for doing our in-house testing this info will help tremendously. Maybe I can now swap 'trade secrets' with the video crew and look all professional and stuff, HA.

Seriously, thanks I'll put this to good use.