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

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

This chapter is from the book

Compositing: Science and Nature

What exactly is happening in a simple A over B composite? You're just laying one image over another, right? Nothing to it—was obvious from the first time you used Photoshop, right?

For most people, the basic compositing operation is a completely intuitive process that is rarely questioned, but to deal with the crucial part of an A over B composite—the edge detail of layer A—it helps to know what is going on, not only in the world of software but also in the real world as well. The two worlds do not operate the same, but it is the job of the digital world to re-create, as faithfully as possible, what is happening in the natural world.

Bitmap Alpha

A bitmap selection channel is one in which each pixel is either fully opaque or fully transparent. This is the type of selection generated by the Magic Wand tool in Photoshop. You can feature or blur the resulting edge, but the default selection has no semitransparent pixels.

This type of selection has its place—say, isolating pixels of one particular color range to change them—but it does not reflect how nature works. An edge made up of pixels which are either fully opaque or invisible cannot describe a curve or angle smoothly, and even a straight line with no edge transparency looks unnatural in an image whose goal is realism ( Figure 3.5 ).


Figure 3.5 Using 400% magnification, you can see clearly how ugly a curved or angled shape is if it can be described by only bitmap pixels, pixels that are either fully off or fully on.

Feathered Alpha

But although it's easy enough to see that a bitmap edge does not occur in nature, it's hard to imagine that a feathered alpha does occur—hard objects don't have semi-transparent edges, do they? Of course not. Look at the edge of this page, or anything in the foreground of your field of vision. Do you see semitransparent edge areas? No.

Not exactly, anyhow—but on the other hand, what you see isn't razor sharp in its hardness, either. It so happens that semi-transparent edge pixels are the best digital approximation we have for overlapping edges in nature ( Figure 3.6 ) because they solve two problems in translating the world of objects to the world of pixels:

  • They come closer to describing organic curves.
  • They approximate the physics of light as it bends around objects.

Figure 3.6 Ahh, that's better. With a proper alpha channel and a one-pixel feather adjustment, even at 400%, the edge does a much better job of describing a smooth, soft curve.

Say what? The first point is easy enough to see and appreciate, but the second one is a doozy, an observation that can be traced back a century to Einstein's annus mirabilus, in which he demonstrated how mass and light are interrelated. Light is bent—slightly—by any amount of mass that it passes. The greater the mass, the higher the amount of bending, right up to the extreme of black holes from which light cannot escape. With most objects, your eye sees a slight, subtle effect of the background light actually bending around the foreground object.

You can study this effect close-up in a digital photo with no compositing whatsoever ( Figures 3.7a and b ). In the digital image, you can see areas at the edge of objects that become a wash of color combining the foreground and background. This is caused by the influence of the mass of that object itself on the light (color) coming from behind it.


Figures 3.7a and b Consider a simple digital photo that is not composited (a). When you look closely, you can see that there is a natural softness at the edges of the sign despite that it is in sharp focus and hard-edged. The effect is even more evident close up, if you compare the sharpness within the sign itself to the softness of the edge (b). (Images courtesy of Nathan Moody.)


But we're by no means finished discovering surprises with basic image combination; the way that Opacity edits work in After Effects often takes people by surprise as well, although they can work with it for years without coming face to face with what is counterintuitive about it.

Say you have two identical layers, no alpha/transparency information for either layer. Set each layer to 50% Opacity, and the result should look exactly like either one of the layers at 100%, right?

Wrong ( Figure 3.8 )! If you've ever heard of Zeno's Paradox, it describes a phenomenon that is something like how opacity is calculated in After Effects, but for good reason (where as Zeno's actual paradox is pure folly).


Figure 3.8 A series of white solids overlaid with Normal blending at 50% over a black solid at 100%. Each layer only adds 50% to the delta (difference) between current opacity and 100%, so only the center begins to approach pure white (but registers 91.25%). This is consistent with Photoshop, but not with many other compositing applications.

A lead developer on the After Effects team once described the program's opacity calculations as follows: Imagine you have a light which is 1, and place a 50% transparent filter (say, a sheet of vellum) in front of it. Half the total light is permitted through the vellum (0.5 * 1 = 0.5). Put another 50% transparent sheet of vellum on top of that. Now half of half the light shows through (0.5 * 0.5 = 0.25). You can theoretically repeat ad infinitum without reaching 0% light transmission, at least in a pure digital environment.

Hence, and in some tangential relationship to Zeno's Paradox, After Effects mimics how transparency behaves in the real world. This is not how opacity settings are handled in many alternative compositing applications, and so it often takes users of such programs as Shake by surprise, but the operation is by design.

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