Original subtitles

In this clip we're going to be looking at analyzing the alpha channel.

Now,

in our previous clip we were looking at analyzing the greenscreen and that

was to see if it was evenly lit and any special areas that we may want to pay

attention to so that was perhaps wispy hair or motion blur but now let's take

a look at the alpha channel itself.

You may recall the hard matte and the hard matte,

of course,

is 100% black and white and it's 100% black and white so that we can

ensure this particular character is not see through.

So here,

our character and the table cloth is opaque however look

what it's doing to our edges up here.

It's not looking so great.

We don't have that nice fall off that we get with the soft matte so

we will have to refine it but the good news is is that it's keeping

all the areas that should be opaque, opaque.

Another few terms for hard matte, of course,

are core matte and inner or inside matte and those

terms can be used interchangeably.

So now, we'll take a look at our soft matte.

Our soft matte has lots of detail in our alpha channel,

in fact, in some areas it has too much detail so let's take a look.

Down here we have approximately 10%, 1% here, and 4% here.

As a general rule of thumb 5% is the maximum that you

should have in areas beyond your objects.

It is the most forgiving.

Anything above 5% may cause some issues so let's take a look.

So here,

we're taking a look at the areas down here where it was approximately 10%.

So let's take a look.

This is before and this is after and you can see it's

starting to gray out this checkerboard, not so much up here,

we can't see it at all up here, it's minimal,

I wouldn't worry about it too much but we can see we're at our

borderline here where we want to push it.

So generally speaking, don't push it past that 5%.

There is exceptions to the rule but we want to retain our background

information and not blur it our or obstruct it.

So let's take a look at a more extreme example.

Here we have increased the alpha values to approximately 25% and again,

let's look at that.

A and B.

Again,

it's making it quite dark so it is quite aggressive and

definitely something that we want to address.

Another term for the soft matte, of course, is a fringe matte.

I like to think of it as the fringe on clothing or fringe as in a hair fringe.

So lets take a look at combining the two of them together.

This has been combined for us and we have some nice soft edges here.

Of course these values are fine.

They are less than 5% and we have a nice,

even core or a hard matte within our image.

So again, on top of the background, let's take a look and it's looking not bad.

So again,

we've gotten rid of those hard edges that we saw before in the hard matte.

It's nice and soft and our image is opaque where it needs to be opaque.

So,

as a quick overview of what we learned in this clip alpha channels that are

outside of our image or object should be reduced if not removed entirely but if

you do need them there try to keep them below 5%.

Some other terms for a hard matte are core matte or inner or inside matte

and another term for a soft matte is a fringe matte.