Adventures in Vidding: What is that Mask thing, anyways?

[permetaform]

Okay, so I started the post on internal movement but then I realized that I needed to explain sound and music and...so that will take awhile. (then again, what's hilarious is that I learned more than I ever wanted to know about masks while writing this up...though I'm also getting some really evil vid ideas. heh, bloooood)

So here's the quickie guide to Masks instead. Please please please tell me where something's unclear so that I can edit this to make it better!


Masks: Basic Concept

Might also be known as "alpha channel", "matte", or "keying".

In it's simplest form, a "mask" is a black, white, and/or grey image that tells the computer where you want an image to be transparent and how much.

Examples:
White = Opaque,
Black = Completely transparent
40% gray = 40% transparent.

It works the same way in Adobe Photoshop masks. That way, you can affect how an image itself without manually changing the image itself.

This pic from the Adobe help files explains it best, I think:

Separated color channels (left), the alpha channel (center), and all color channels viewed together (right)

In the same way that all colors in print matter are created by spraying red, yellow, and blue ink in a very defined pattern, images on the computer are defined (ie. 'keyed') thru their specific color 'channel'. The alpha channel keys for transparency.

This is, in effect, how blue screen (green screen) works. They color key the alpha channel for blue, so the alpha channel tell the blue channel (or green channel; 'cause people don't wear green so often), "Hey, see that? You're imagining it, there's nothing over there."

Alpha Channel: Controls transparency. When you're messing around with opacity, you're really messing with the alpha channel as a whole. Masking affects specific parts of the alpha channel.

Mask: The alpha channel. Also, it's frequently used to mean the black/white image you are imprinting on the alpha channel; or in other words, what you're making the alpha channel to look like.

Matte: An *external* image or clip that defines transparency. Technically, a mask is internal to the image and a matte is another image that you use to define transparency, but I use 'mask' for both.

Keying: The process of defining transparency by color (color key) or brightness (luma key). Basically like a blue screen.

Implications

What this means?

Just as with Keyframes you control time, with Masks you control space.

You can blend two clips together, you can cut out a circle and put a clock on the wall, you can make someone walk in the background (remember those scenes in Yami no Matsuai where you have the punched out face and the image pan?), you can cut out a background and put in something else...

rivkat has a vid called "Running up a Hill", with a Clexy comparison of the two leads. She used the heads of Clark and Lex from that barn scene ("friendship of legends") or perhaps it was the LookOfLove from the the BoKent in hospital ep. Anyhoo, took the heads, cut out the background and added the moving kryptonian text from the cave wall.

*Brilliant* use of masks, and highly effective. And it created internal movement! ::beams::

By color keying specific areas you can change the color of JUST that area.

Is the blood in your clip not red enough? no matter how much you fiddle with the color controls? Use 'non-red key', and everything not-red are removed only from the area with the blood. Everything not keyed remains the original color.

Want to make colors brighter or to add an image in the shadows? The 'luma key' removes darkness.

Do you have a moving person you want to cut out? Or remove the background from a moving object? Use 'track matte' or 'difference matte'.

(more on the specific keying methods further down)

General Method for a non-moving mask

1) Get a frame of whatever you're vidding, And black out the parts you want gone...
(remember! grays and colors are options too...)
2) Apply the appropriate mask effect to the clip.
3) Set conditions for the mask in 'effects control'.

Specific Effects

Confusing ain't it? I hate how they just alphabetizes them, so very user un-friendly.

Let's break it down.

[non-moving masks]

Alpha Adjust - affects default settings. 'Ignore Alpha' removes transparency effects. 'Invert Alpha' reverses transparent and opaque areas.
Image Matte - makes stuff transparent depending on either alpha channel or brightness values. Easiest with grayscale; if you want to remove colors, color in what you want to remove. (ie. %30 green = 30% green removed)

Chroma key - color (or range of colors) that are transparent. Includes eyedropper tool.

RGB Difference key - simpler Chroma key, cannot blend the image or adjust transparency in grays
Blue Screen and Green Screen keys - what it sounds like. Removes true chroma blue and true chroma green (ie. radioactive blue/green)
Non Red key - removes blue-green and *also* lets you blend two clips. Good for blood. (the defringing option removes residual green or blue from the edges)

Luma key - creates transparency for darker values in the image (or for lighter values by setting Threshold low and Cutoff high).

Multiply - Drop mask onto track below your clip. Where mask is BRIGHT, the clip is transparent
Screen keys - Drop mask onto track below your clip. Where mask is DARK, the clip is transparent

Garbage Matte -

The microphone (left) is masked out by repositioning image handles in the Preview view of the Monitor window (center), creating a garbage matte that is then keyed and superimposed over a background (right).
So it cuts out random shit in a otherwise nice clip you want to remove. In effect, you change the boundaries of the clip without warping the image. Very neat. Combine with 'feather' or other effects to blend it better.

Remove Matte - If you're lucky and the clip happens to have a clip with a separate black/white channel (premultiplied) you can remove the colors and voila! Not only instant grayscale rendering, but it's also useful for track matte...

[moving masks]

Difference Matte -
(aka. "I want the moving object and nothing more!")
You need: source clip and mask image.

Drop the difference matte effect onto your video clip.

Where your clip and your mask image match, there's transparency, ie. the blue screen effect. Also, if you have a non-moving background and a moving person, you can *still* cut out the background.

Track Matte -
(aka. "My clip moves, my mask moves, I have so much shit moving!")
You need: source clip, a contrasting clip, AND mask image.

The mask image specifies where the contrasting clip is transparent. (Uppermost track, hide this track)

The contrasting clip moves. (track in the middle; is also where you drop the 'track mask' effect)

Your source clip is happy and does its own thing. (lowest track)

[setting up mask conditions]

Matte Alpha - affects alpha channel
Matte Luma - affects image's luminance values.
Reverse Key - removes the white areas instead of the black
Similarity - Broadens or reduces the range of color made transparent. Higher values = more range.
Smoothing - Specifies amount of anti-aliasing (softening) on the boundary between transparent and opaque.
Blend - Higher values = blend more.
Cutoff - Darkens or lightens shadows.
Threshold - Controls amount of shadows in the color range you keyed out. Higher values = more shadows.

Comments

[danielleleigh.livejournal.com]

dude, I'm not a vidder or anything but I have to say how damn cool it is that you've been posting these little instruction manuals. I sort of look at them and shake my head in amazement that vidders EVER produce anything and then thank god that they do.

[permetaform]

=) heh, I keep thinking that these tutorials are too intimidating and going back and editing them. Heck that initial Vidding for Newbies and grown so large I may as well just call them Vidding Upside Yo' Head.

...I think I need to make a simpler Newbie Vidder post. merp