Backgrounds, Elements and Alphas (Oh My!)

Here at 12 Inch Design, our royalty free products consist of animated video backgrounds and animated video elements.  Now you probably have a good idea what an animated video background is.  It's usually a video file of a cool animation that you'll use in your productions.  They're good for using behind text and blending with your video. These video backgrounds are also known as 24 bit backgrounds.

What does 24 bit mean? It means that for each color channel, Red, Green, and Blue (or RGB) there are 8 bits (1's and 0's in binary, the language of all computers) that are used to represent the "amount" of color.  So a completely black frame of video would have an RGB value of 0,0,0 (and if you looked at the 8 bits of each color you'd see: 00000000,00000000,00000000.)  If you have a completely white RGB frame the opposite is true, you'd have an RGB value of 255,255,255 (with the bits looking like 11111111,11111111,11111111).

So 8+8+8 = 24 which is where we get the term 24 bit image.  24 bit images are usually just fine for representing a high quality "true color" image and they're small enough to be managable.  Anything lower than 24 bit (say, 16 bit) doesn't give you a great representation of an image and you'll see banding and artifacts.  Now, there are even formats that support more than 8 bits per color channel, but those are usually only used when doing high-end film work.  (And some of you high-end folks may want the YUV format to be mentioned, but let's not get off track here.)

Now, compressed images are still 24 bit images, they've just had a mathematical shortcut applied to them to make the file size smaller.  Some codecs (compression schemes) are better at shrinking and then expanding your image than others.  The PNG codec that we use in our files does an excellent job of keeping the quality of your image top-notch even though it's had some efficient compression applied.

So just to review, our video backgrounds are RGB 24 bit, PNG compressed QuickTime files. Got it? Good!

  
 

Video Elements and Alpha Channels

Now let's talk about video elements.  What are these?  Well, as far as they relate to our products, video elements are anything with an Alpha channel.  Lower Thirds, animated overlays, animated mattes, etc.  But what the heck is an Alpha channel?

The Alpha channel is yet another channel that can be added to an image (video frame or otherwise) that can control the transparency of an image, and it also uses 8 bits to do so.  So if you have a video file that has an Alpha channel included, it's called a 32 bit file.  That's because you now have your RGB channels plus another channel called the Alpha.  Put it all together and you have an RGBA image (Red,Green,Blue and Alpha 8+8+8+8 = 32).

Because your alpha has 8 bits to work with, you can have up to 256 "levels" of gray (0-255, you start with 0 not 1.) So your Alpha can be completely black (0), a shade of gray (say, 128) or completely white (255).  By using the Alpha channel, you can say which parts of your video or image you want to be see-through, solid, or semi-transparent.

Lets say you have a 32 bit (RGBA) video of an animated star.  In most paint programs and video editing packages, a black alpha is completely transparent, while a white alpha is completely solid.  So if you want to place the animated star video over some other video on your editing system's timeline, you'll need the the value of the alpha channel where the star is to be white, and the rest of the area around the star should be black in the Alpha (this setup will already be done for you in 32 bit pre-prepared elements such as ours.)  Wherever there is black in the Alpha, other video that is underneath will show through.  Like so:

  
  DV PNG Compare


 

So, something like a lower third that is placed over other video needs an Alpha channel to determine where the image should be solid and where other things should be able to show through.  Just think of the Alpha channel as the "transparency control" channel.

(By the way, 24 bit files are treated like they contain a solid white Alpha channel, even though there isn't an actual Alpha channel included.)

Note: The Avid system works the opposite way with Alphas; black is solid and white is transparent.  But the Avid gives you the option when you import a clip to invert the Alpha, so everything still works just fine.

This brings us to why we use PNG compressed QuickTime files for our video files.  You might think the QuickTime files are fine for Mac users but Windows users would rather work with AVI or WMV files.  We could use AVI or WMV for our backgrounds, because those formats support 24 bit files.  Alas, the only Windows native AVI format that supports a 32 bit file with Alpha channel (which we, and you, need for elements like lower thirds) is the uncompressed AVI file.  And those files are usually huge!  Again, just to be clear, WMV files can't contain an Alpha channel and the only kind of AVI that can have an Alpha is an Uncompressed AVI.

Frankly, most Windows based video editing systems that are at all professional support 24 and 32 bit QuickTime files these days, so it's not really that much of an issue.

The PNG compressed QuickTime files give us the ability to have an Alpha channel, excellent (basically uncompressed) quality, reasonable file sizes, and good compatibility on Mac and PCs, so that's why it's our format of choice.

  
 

AND LASTLY...

So what if the software you're using doesn't support QuickTime or it doesn't support a 32 bit QuickTime?  Then you're going to need to use a program that can convert a 24 bit Quicktime to an AVI or WMV file or a program that can convert a 32 bit QuickTime to an uncompressed AVI.  QuickTime Pro from Apple can do most of what you need.

Oh and in case you need to know, our 32 bit video elements and overlays should be imported as Pre-Multiplied Black Alphas.

Whoa! What does that mean?  Well you see, you can have a Straight alpha or a Pre-Multiplied alpha...but that's a lesson for another time.