Advantages of linear (alpha) keying
We can say that linear (alpha) keying systems are considered to have the highest level of quality, as they allow the mask to be explicitly specified and thus give you the full range of colors and transparency of a signal over another. As such, you can create transparent, feathered shadows on your composition will look great.
An “Alpha” or “Linear” key sends a separate video signal to the keying device that contains the key information. This video signal just looks like a grayscale image and defines exactly the transparency of the way the fill signal is supposed to be overlayed on the background.
The two SDI signals would look like this:
When working with Chroma key, the quality of the final product can be affected, especially if we wanted to use texts without having a solid background because of the curved and angular shape of many letter forms, it is a challenge to superimpose type over a video.
We've all seen letters suffering from edge crawl, where the border between the type and the underlying video seems damaged. In this case, the alpha channel's ability to control transparency provides the perfect solution for the best possible keying of the graphic.
In that case, our text will go from looking like the left graphic to looking like the right graphic.
And the same goes for all shapes, not only for letters, especially if the textures have very thin lines or very rough shapes. The linear keying will allow us to have the best possible quality.
On the other hand, if you want to use transparencies in your graphics, you will have to lean on linear keying because for Chroma keying this is something very difficult to achieve and depending on the equipment you are working with in some cases it is impossible. Also, if you want to have a graphic that combines transparencies with solids, like the graphic shown here, it becomes a task that cannot be achieved
Another issue that makes Chroma keying for graphics a complicated task is that to achieve it we have to exclude certain colors, the most used options to do this type of keying are blue, green and magenta.
But that means that you are limited in that you cannot use the key-color anywhere within the graphic that you want to show up on the screen because it will appear transparent when keyed out.
Flowics Graphics allows you to control different regions in the graphics and that will help you to have more than one graphic working in a video output simultaneously but if you are using Chroma keying you have to take into account that all graphics should exclude the color range of your background.
This graphic for example contains practically all the color ranges that we could use to do Chroma keying.
All of these problems are avoided by using Linear Keying because this method does not exclude any color during the process.
Finally one of the biggest problems with chrominance keyers is the fact that you are using the same signal for the Fill and the system internally generates the Key. This becomes a problem when you are dissolving between two different graphics.
This can result in that for a moment you have both signals and the dissolving looks more like a noise than a transparency, just as we mentioned, transparencies are a complicated subject in this type of keying and therefore as the graph disappears and we will see the next one we will have some green in the middle that generates a "ghosting" type effect. Similar to what can be seen in the graphic: