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Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels

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CreativeCOW presents Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels -- Adobe After Effects Tutorial


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┬ęSimon Ubsdell. All rights reserved.


When most people hear the words "alpha channels", they think "transparency", but that's not exactly accurate. The truth is more complex, and a quite bit more interesting! Join longtime VFX artist, editor, software developer, and business owner Simon Ubsdell for Part 3 of the best look behind the technology of compositing that you've ever seen, packed with practical advice for applying the secrets of alpha channels that's simply not possible before understanding these underlying principles. No matter which applications you're using for editing, compositing, or visual effects, this one is a must-see!



Welcome the third tutorial in this series on the basics of compositing theory in which we're going to be focusing on one thing: the alpha channel, and it's a subject where understanding the underlying maths is really of huge benefit. Getting to grips with the numbers opens up a perspective on this problem that you simply can't arrive at any other way.

So far in this series we've looked at ways of blending two images together but this time we're looking at putting one image on top of another. This operation is what's commonly known as either Normal or Over.

One major topic that I want to flag up that we'll be covering is premultiplication, because I know it's something that a lot of people find tricky to understand. And the most intriguing discovery we're going to make is that although alpha is usually talked about as defining transparency, that's not an accurate description of what's actually happening -- and the reality is quite a bit more interesting.

There's s certain kind of magic to compositing in general, and alpha channels in particular, going back to the video hardware keys of early broadcast switchers, but even before that, to the whole business of how you do it on film, which of course provides the original model for suppressing the background, isolating the foreground, and adding them together.

Computers have somehow made it all a little bit less sexy at least in terms of the user experience, because they introduce levels of abstraction that obscure the interesting stuff happening below the surface.

Perhaps we all used to understand this kind of compositing a bit more clearly in a bygone time, because we were closer to physical processes. I suppose that's part of why I am interested in unpacking the maths, to show a bit more of the underlying principles that then make those principles easier to apply.













Simon Ubsdell
Simon Ubsdell
Hi, I'm Simon Ubsdell, Creative Director of TOKYO PRODUCTIONS, a UK-based boutique creative shop specializing in movie trailers, sales promos and TV Spots for the independent film sector both in the UK and across Europe.

I've been a film and video editor for over 30 years as well as being involved in motion graphics, sound design and mixing, music composition, visual effects and compositing, 3D modelling and animation, and colour grading, not to mention writing, directing and producing, and most recently, software development.

I am also a developer of plug-ins for the video post-production market having released a range of successful and acclaimed products both under the Tokyo brand and as Hawaiki with Robert Mackintosh.

Comments

Re: Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels
by Michael McCune
Thanks, Simon. Your tutorials are more rewarding with each viewing.

Also found the method to add a slider control to the Custom Tool interface, making experimentation with variable values in blend modes much easier. I suppose this is the method you use.

With a Custom Tool selected in the Flow pane and, for example, intending to create a slider for a variable, say, Number In 1,
  • First: in the Flow pane select the Spline tab;

  • Then in the Show All Controls tab on the left side, select the Custom Tool to be modified, say, CustomTool1.

  • Then right click and select Edit Controls from the drop-down menu; it opens a dialog box for the control's parameters;

  • Then select the ID field and use the drop-down menu to select the control to be modified, perhaps, Number In 1;

  • Then in the Name field change the name as might be helpful;

  • Then in the Page field select the (tab) page on which the control will be placed, perhaps, Channels;

  • Then in the Input Control field select the control type, perhaps, Slider. Note the range value, probably 0-1.


  • The new control will be placed in the selected tab, for example, at the bottom of the Channels tab.

    Thanks again, Mike
    @Michael McCune
    by Simon Ubsdell
    There is no need to go into the Spline tab. You can access Edit Controls but right-clicking on any tool within the Flow.

    Simon Ubsdell
    tokyo productions
    hawaiki
    Re: Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels
    by Simon Ubsdell
    Hi Michael,

    Thanks for the kind comments.

    I did an in-depth look at the Custom Tool in this tutorial from last year, which I hope answers most of your questions:







    To get the green and blue expression fields to match the red expression, you can simply right-click where it says Green/Blue Expression and select "Expression" from the list. That causes an expression to be activated. You can then drag the Plus sign up over where it says Red Expression and they will now be linked. It's the same process as pick-whip in After Effects.

    For more on Expressions generally, see this tutorial where I cover the basics in some depth:







    Simon Ubsdell
    tokyo productions
    hawaiki
    Re: Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels
    by Michael McCune
    Simon; -- if you are listening--- A wonderful series of Fusion tutorials!!!

    Have been through this tutorial and others in this series of yours and grasp more with each review. There is a lot to learn. One is rewarded again and again because there is so much there and so carefully explained.

    One bit that you clarified has driven me crazy before, namely, the fact that various file formats, such as TIFF, Targa and PNG, have premultiplication dealt with in inconsistent ways. Sometimes that format flag is read by the app and sometimes not. Arg!!!

    QUESTIONS: your tutorial shows the custom dialog with tweaks for automatically reflecting a change in one channel in other selected channels. I don't see that in the manual and may have missed it in your tutorial.

    How to set these fields up to reflect values in another field???

    Also helpful is your added value adjustment slider which is shown just under the LUT label in the Custom dialog box. This allows a quick and convenient way to vary a number field value without having to use multiple clicks to access that value directly.

    Please explain how you placed that slider in the Custom dialog box.

    BTW, have you ever thought of putting your series up on Lynda.com??? I ask this knowing all too well that the fine print in such business arrangements may mean that one loses significant control over the product. Or, maybe not.

    Thanks again for a wonderfully helpful and informative series, Simon!
    Mike
    Re: Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels
    by Michael McCune
    More to be learned here about Resolve and Fusion than anywhere else on the web that I know of.

    So appreciated.

    Mike
    Re: Tutorial: Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels
    by Jeremy Garchow
    This is a fantastic series, Simon. Thanks for putting in all the hard work!
    Re: Tutorial: Compositing Secrets Everyone Can Use: Pt. 3 - Alpha Channels
    by Simon Ubsdell
    Thanks, Jeremy.

    It's been a fun project to put together.

    Simon Ubsdell
    tokyo productions
    hawaiki


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