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Final Cut Plugins

Matte Primer

Mattes, Masks and Alpha-Channels

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The Lyric Matte Filters kit provides a sophisticated set of tools for working with Mattes in Final Cut. This section provides a short primer on mattes and the associated concepts of masks and alpha-channels.

Mattes are used to mask out areas of an image or video frame, generally for one of two reasons:

  1. So that the masked areas are transparent or semi-transparent when layered over another video or image. This allows you to build up a final video by layering one or more clips in stacked video tracks, each with its own mask, and is known as Compositing. A common special case of compositing is blue-screening, also known as green-screening or chroma-keying, and Final Cut already has a set of Keying filters just for this purpose. These keying filters work the same way as Lyric's matte filters do and make use of the matte and alpha-channel techniques described in this primer.
     
  2. So that the effect of a filter can be limited to the areas of an image defined by the matte. For example, you might want to apply a graduated tint across the frame or a noise-softening blur to the stationary areas in a clip or a saturation bump to just the low-saturation areas, and so on. This is a very common techique in digital photo processing and can be employed successfully with video to achieve controlled image-correction and other stylistic or special effects.

This last use is actually not commonly-seen in Final Cut, most of the filters are not directly controllable by mattes, though you can achieve the effect by layering two copies of the same clip, applying the desired filters to the top clip and and use a matte to composite selected filtered areas over the bottom clip. This can be tricky to set up, so all the Lyric effect filters can be directly controlled by mattes, and the kit includes versions of several of the standard Final Cut filters enhanced to allow direct matte control.

Mattes, Masks and Alpha-channels

So, what exactly are mattes, masks & alpha-channels? Internally, Final Cut uses a so-called alpha-channel to actually control the areas of transparency in a video clip. This is a component in the internal representation of a video, described in detail in the Video Channels section below. It is essentially an additional gray-scale image that sits alongside the original image and defines the transparency of corresponding areas in the original - where the alpha-channel image is dark, the image is transparent, where it is light, the image is opaque (visible), with the level of opacity corresponding to the brightness level in the alpha-channel.

Final Cut provides a number of ways to supply an alpha-channel for a video. One way is to actually have the alpha-channel embedded in the video media file, and it is not uncommon for 3D or 2D animation renderings to include an alpha-channel that bounds the objects in the rendering. Another way is to apply a video Matte filter. These filters will usually create a new alpha-channel or modify the existing one. Final Cut comes with a small set of Matte filters and, as mentioned above, all the Key filters also work by generating or modifying the clip's alpha-channel. The Lyric Matte filters complement the built-in set and provide a number of new ways to create and modify alpha-channels.

A third way to supply an alpha-channel is to use a Mask Clip. This is a separate video clip containing either an embedded alpha-channel or a gray-scale video to be used as the alpha-channel (or even a Matte filter than generates an alpha-channel). The Mask Clip is placed on a video track above the clip to be masked and its clip Composite Mode is set to Travel Matte - Luma (if the mask is in the video itself) or Travel Matte - Alpha (if the mask is in that video's alpha-channel).

Of source, since video is comprised of changing frames, it is usually the case that the alpha-channel image frames are changing in concert to mask the appropriate areas of interest as the video plays. For this reason, video mattes & masks are often referred to as Travelling Mattes.

Video Channels

Let's quickly explore the concept of video channels to see where alpha-channels fit in. Most digital video and photos are stored and processed as a set of channels, such as RGB (red, green, blue) or YUV (luma, chroma U, chroma V). Technically, these channels comprise the dimensions of a color-space, which defines how the channels are mixed together to form the final video image. Different color-spaces have different characteristics, and Final Cut commonly uses either RGB or YUV and can handily flip back-and-forth between them. RGB is probably the most common channel-encoding in computer image processing and YUV is the most common for digitial video. The two schemes are depicted below.

For additional information, there's also an excellent article on mattes and masks by Kevin Monahan in the premiere issue of EditWell, the online Final Cut newsletter, available here.

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RGB

YUV

YUV takes advantage of the fact that human eyesite is much more sensitive to brightness detail than color detail, and so it stores all the brightness information in one channel (Y) and usually stores it with much more accuracy than the color channels.

For our purposes, it doesn't much matter which channel-encoding is used, both are 3 channels wide and mix down to (essentially) identical video images.

The alpha-channel is just a 4th channel added into the mix, though it doesn't affect the color, rather it is a gray-scale channel that is used to define transparency. If a pixel in the alpha-channel is white, the corresponding pixel in the video image is opaque (visible); it the alpha-channel pixel is black the corresponding pixel is completely transparent (invisible), and shades of gray in between specify graduated levels of opacity. So, in the example below, the white areas in the alpha-channel define the area of the flower petals and so only the petals are opaque in the final video.

Alpha-Channel

Note that it is common-practice to show the transparent areas of an image as a grey checkerboard - it is one of the selectable backgrounds in the Final Cut image view menu in the Canvas & Viewer windows.

The above example shows the alpha-channel being supplied by a Color Matte filter, though it could just as well have come from a Mask Clip in a higher video track set to Travel-Luma or Travel-Alpha composite mode, or from an embedded alpha-channel in the video media file.

As mentioned above, the alpha-channel is a grey-scale image whose level of brightness sets the level of opacity. In the next example, the gradation from white to black around the oval matte causes the image to gradually become less opaque.

Note that the R, G & B channel images are actually stored as gray-scale; they're shown colored here for descriptive purposes.

Lyric's Matte Filters also provide the ability to generate compound mattes by mixing multiple mattes in various ways. In the next example, two mattes have been added to the flower video and the Matte Mixing mode in the second matte has been set to Add to Existing Alpha.

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This causes the visible areas in both mattes to be combined. You can also intersect and subtract mattes and there's a general Matte Operations filter that can be used to perform these operations on other mattes, masks or alpha-channels.

As with stacked video image filters in which the filtered image at each stage is passed down as input to the next filter, stacking multiple matte filters causes the new or modified alpha-channel to be passed down as input to the next filter, allowing the alpha-channel compositing shown in this example.

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