Publishers of technology books, eBooks, and videos for creative people

Home > Articles > Digital Audio, Video > Adobe After Effects

Selections: The Key to Compositing in Adobe After Effects 7.0

A particle physicist works with atoms, bakers and bankers work with their own types of dough, and compositors work with selections—many different types of selections, each of them derived uniquely. In this chapter, you'll look at the foundation techniques that define how a layer merges with those behind it.
This chapter is from the book

I'm fixing a hole where the rain gets in And stops my mind from wandering Where it will go.

—John Lennon and Paul McCartney

A particle physicist works with atoms, bakers and bankers work with their own types of dough, and compositors work with selections—many different types of selections, each of them derived uniquely.

If compositing were simply a question of taking pristine, perfect foreground source A and overlaying it onto perfectly matching background plate B, there would be no compositor in the effects process; an editor could accomplish the job before lunchtime.

Instead, compositors break sequences of images apart and reassemble them, sometimes painstakingly, and usually including the element of motion. Often, it is one element, one frame, or one area of a shot that needs special attention. By the clever use of selections, a compositor can save fellow artists and camera operators, taking control of whatever part of the source footage is necessary.

In this chapter, we'll look at the foundation techniques that define how a layer merges with those behind it. Then throughout Section II, "Effects Compositing Essentials," and in particular in Chapters 6 and 7, we will focus on particular ways to refine selections, creating high-contrast mattes, and pulling keys from blue-screen footage.

The Many Ways to Create Selections

After Effects offers a number of ways to create selections, yet far fewer than exist in Photoshop. Why? To Photoshop's advantage, its selection tools must operate only with still images. This is a much simpler development problem, believe it or not, than creating selection tools for moving images, in which results must appear consistent across the changing array of frames that makes up a single clip.

So what are the After Effects methods for creating selections? Take a look.

Pull a Matte

You may think that pulling a matte refers to keying out the blue or green from an effects film shoot ( Figure 3.1 ). True enough, but there are other types of mattes too. Even more common than blue-screen keys are high-contrast, or hi-con, mattes. You create these by maximizing the contrast of a particular channel or area of the image. There are other types of mattes possible as well, such as the elusive difference matte. Chapter 6, "Color Keying," discusses pulling mattes in depth.


Figure 3.1 This split-screen image shows a blue-screen shoot (left) and the resulting matte. (All baseball images courtesy of Tim Fink Productions.)

Use an Existing Alpha Channel

Using an existing alpha channel sounds like a no-brainer, right? The source footage was created with an alpha channel (typically in a 3D animation program); just bring it in as is ( Figure 3.2 ). No worries, done deal, right?


Figure 3.2 This close-up of a computer-generated baseball is split into color (left) and alpha channels.

That's theoretically true, except that After Effects mostly conceals an issue that is quite explicit in other similar software packages (such as Apple's Shake): the interpretation of the alpha channel. Is the edge premultiplied or not, and what do you do in either case? The questions are raised not only when you import the initial image (as you do in After Effects) but also when applying effects to layers with an alpha.

How does After Effects get around this, and is it a good or a bad thing for compositors? For the answer, see the "Alpha Channels and Premultiplication" section later in this chapter.

Create a Mask (or Several)

A mask, by contrast, is a vector shape that determines the opaque and transparent areas of an image ( Figure 3.3 ). Masks are generally created by hand, although After Effects does include a provision to generate them automatically by examining the raster data of an image.


Figure 3.3 In this split-screen view, you can see the garbage matte mask that was added to remove areas of the stage that didn't have a blue screen.

The Layer > Auto-trace command is new and improved in version 7.0 of After Effects. With sufficient contrast on any channel you may find that it automatically creates detailed and accurate outlines. The downside of this approach is that, with all but the very simplest shots, it will create lots of overlapping outlines—dozens, typically—when you might want one or two. Combine this with changes over time and you can end up with far more than you can manage.

This chapter focuses on the basics of creating and combining masks. You can find a follow-up discussion in Chapter 7, "Rotoscoping and Paint," which focuses specifically on rotoscoping, the art of animating selections over time.

Use Blending Modes Instead

It's even possible to composite without selections at all, instead using blending modes (Add, Multiply, Screen) to combine color channels mathematically, pixel by pixel, in ways that mimic how light and shadow play out in the world ( Figure 3.4 ). You can also use selections combined with blending modes to get the best of both worlds.


Figure 3.4 Blending modes are the preferred way for compositing elements that are composed of light rather than matter, such as fire.

This chapter goes over the modes that are relevant to effects compositing and gets into the nitty-gritty of what these blending modes are actually doing as they combine pixel data.

Use an Effect

Certain effects can be used to create transparency selections or to refine the ones you have. Section II discusses these in detail. Many common effects, such as Levels and Curves, include control of the transparency (alpha) channel, while most of the effects in the Channel menu permit you to create or alter the alpha channel directly.

Combine Techniques

As with blending modes, there is no reason not to combine techniques, using a garbage matte to clean up a color key or a hi-con matte to enhance the effect of a blending mode. This is very common in advanced effects work, where no two shots are exactly the same and where a single frame or clip can require a variety of approaches.

So What's the Big Deal?

The real skills here are knowing which approach to apply for a given situation, knowing how to apply it, and knowing when to try something else, either to enhance or to replace what you already have. None of these techniques or approaches is particularly sophisticated by itself, but applying them properly requires a clear vision of what the shot needs and how to supply it.

Peachpit Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Peachpit and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Peachpit products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Adobe Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive:

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020