Breakdown Explanation of Matte

[Andrew Kramer]

A matte is generally a greyscal image that is representative of a layers alpha. for example anything that is black turn transparent and white opaque, and grey partially transparent. using a track matte luma will sample the layer above a layer in the timeline.

A mask is creating vector shapes that can be add and subtract alpha from a given layer.

masks are usally applied to a layer and a matte is a separate layer from the one intended to be altered.

a garbage matte can sometimes refer to a rough mask applied to say the edges of a video where a black bar or light stand may be.

[Steve Roberts]

A matte is a layer that affects the opacity (or silhouette) of another layer.
It needs that other layer in order to be called a matte.
It’s like a husband isn’t a husband without a wife. He’s a man. A matte layer isn’t a matte without the layer below.
Any layer can be a matte layer affecting the layer below. A piece of footage can be a luma matte layer, where the brights and darks of the footage hide or reveal the layer below.
But for a layer to be an alpha matte, it has to have a shape or variations in opacity — it doesn’t do much if it is a full-screen piece of footage.
Remember: a matte layer affects the opacity of the layer below. In AE, anyway.
One way to give the matte layer shape is to cut out parts of it with a mask.

A mask is a shape applied to a layer, either by drawing directly on the layer or copying a path from Illustrator.
The mask resides on that layer.
It doesn’t need other layers to exist — just the one layer it resides on.
Twirl down the properties for a masked layer and you’ll see the word “mask”.
A layer can have a lot of masks applied to it.

Rotoscoping is the drawing of masks.
It doesn’t have to be frame-by-frame. You can separate an object into different masks, and animate each mask differently. If part of the object doesn’t change shape, you can just animate the mask shape by changing its position, for example. You can also look at the motion of an object, then draw a mask at its extremes of position or shape, setting keys at those points, then letting AE interpolate the mask shape between those points. Study the motion before you start drawing masks.
Why work harder than you have to? You only have to draw masks on every frame if the object’s shape is organic or amorphous, or has no stiff parts. Like a flowing cape, maybe.
Garbage masks are simple masks drawn over greenscreen footage to cut out the stuff in frame or at the edges of frame, so you don’t have to try to key out that stuff. Just mask out the garbage.

I suppose a garbage matte would be a layer that has a garbage mask drawn on it, and it affects the opacity of the greenscreen layer below, but that seems like too much work. I’d just draw a garbage mask on the greenscreen layer.

Does that help?

[Chris Smith]

Roto generally shouldn’t be done frame by frame. It defeats the benefit of having the computer interpolate for you. There’s definately an art to roto. You should check the Cow archives for ‘roto’ as there are many tips. Two primary notes:

1. Break up your subject into multiple masks. For example if you are rotoing a person, use a seperate mask for the head, each leg, torso, etc..

2. Draw a shape on the first frame. Go to the last frame. Modify the shape to fit. now HALF the two keys. Meaning go to a point halfway between the others and reshape. Then go to the halfway points between those keys, reshape. Etc. The more keys you add the more the motion locks up. This method decreases your amount of work and decreases your chance of roto chatter.

Side note, I would use the B-Spline option in AE (“Rotospline”) instead of bezier.

Chris Smith
https://www.sugarfilmproduction.com

[Steve Roberts]

[Chris Smith] “Meaning go to a point halfway between the others and reshape”

Chris, I agree with your advice of letting AE do the interpolation, but as much as I think of you as a brother, I have to disagree with you on the above quote. Halving assumes that the motion is regular. If, for example, the object moves very quickly and jerkily (?) for one second, then slow and smooth for the next five, the roto will have too few KFs in the first second and too many in the next five if the KFs are placed at regular half-points.

That’s why I recommended that the user study the shape change (I said “motion”, actually) of the object, and place KFs where the shape hits extremes. An example would be when a pitcher winds up (extreme) then lets it fly (extreme). I think the user should watch the motion, then step through the footage until the object reaches an extreme shape, then starts to retreat — then step back to the extreme and reshape the mask, setting a KF. Say it gets bigger, then smaller — a KF could be placed when the object is biggest.

Of course, every roto job has its own quirks, and halving would work best for a lot of jobs. I’m just sayin’ … if the motion has extremes and is irregular, halving might not get the best result.

Of course, if I misunderstood you, then I’m a bozo. 🙂

Steve

[Chris Smith]

You are absolutely right Steve. Out of lazyness I was giving an extremely simple example to counteract the idea of frame by frame. I did a roto just today where I start like I describe using the halfway method until I “Box in” the motion detail that isn’t linear. At which point I bookend specific motions. I think there are a few methods and knowing when to switch between them is a great thing.

The halfway method I learned from the great Matt Silverman from the Commotion training tapes.

Chris Smith
https://www.sugarfilmproduction.com

[Steve Roberts]

Oh sure. Trump me with Silverman, why dontcha.

Halving, then boxing in …? Great method. I relent. 🙂

[Espnetboy3]

Thanks alot guys. One more quicky. How can you set AE to track your mask? File-Interpolate?

[Steve Roberts]

I’m not sure what you mean.

AE can track points in footage, but that’s probably not what you mean.
You can make one layer follow another layer’s position and rotation by parenting.
You can make one layer match another layer’s position by pickwhipping one position to another with expressions.
But you can’t make a mask follow anything. You can only animate its shape parameter by moving the mask vertices around on the layer or transforming the mask. You have to make the layer on which the mask sits move by one of the above methods.

Without jumping to a solution, what do you want to achieve?

[Chris Smith]

Unlike about every other program, AE won’t let you track your mask directly. But you can put a mask on a layer and track the position of a layer to move the whole mask. This layer can be used as an alpha matte on another layer.

Chris Smith
https://www.sugarfilmproduction.com

[Espnetboy3]

Chris that’s what i mean. Most programs you set your mask keyframe at the begining and then at the end and wherever you want in the middle like you were saying and then the program will track(roto) the object you masked depeding on color difference and such. Being there is no way to do this means you must go frame by frame with you mask in AE. Any other programs out there that are strong roto tools that arent fully expensive compositing software packages like shake? Sillhouette?