Unmult, premultiplied, and straight

 
Two resource roundups were posted on PVC this week: Alpha channels: premultiplied vs straight and “Unmult” with and without a plug-in, on Unmult, Xmult, and channel and matte effects.

More this week in After Effects

Here are some AE items new this week that were not already mentioned here:

• Channel effects tutorials on 'luma to alpha' (part1, part 2) from Felt Tips on Vimeo.

• Learning the After Effects Graph Editor by John Kostrzewski on Fuel Your Motionography (more in AE Help, in CMG, and from Mylenium).

• AE Quick Tips #6 Super Smooth Curve from Felt Tips, which also explains the Graph Editor (see Ease and Wiz also, especially Ease and Wizz: alternative keyframe interpolations for After Effects by Chris Meyer).

• Update (pictured) from Mike Cardeiro of his script Trajectory, which lets you align layers in a comp evenly in 3D between 2 nulls, with delay controls for animation.

• ActivateDisabledExpressions, a script by Lloyd Alvarez, will go through selected layers, an entire comp or several selected comps and activate any disabled expressions.

• DuplicateLayers-n-UpdateExpressions, another script by Lloyd Alvarez, will duplicate the selected layers and if any of the layers have expressions referencing any of the layers being duplicated it will update the expressions to point to the newly duplicated layers (watch the demo).

• Update to The LoopMaker script by Lloyd Alvarez, which "loops any layers, footage or comps with one click™."

• Several tutorials and articles on AEtuts.

• OpenEXR workflow in Maya and After Effects CS5 by Guru Vaidya

• A New Trapcode Particular Preset: “Sparkler” – Including Motion Sketch Tutorial by Nick Campbell.

• AE Tutorial – Rusty checkerplate from Shortformvideo

• Both parts of Automatic Animation from Motionworks are up (Auto-Keyframe mode, Motion Sketch, Wiggler and Wiggle expressions, creating motion paths from Illustrator path, Shape Layer Wiggle, Text Wiggly Selector, and various automated effects).

• Dallas AEUG member John Begg put together a few expressions and "imagery to create an easy-to-use CC Sphere project [rendering a planet Earth]. The expressions are hooked to a Comp Light so it appears that moving the light around the scene changes the lighting on the sphere." For similar projects, see AE 3D Earths with & without 'Knowing' lava and Fly around CC Sphered layer in After Effects.

Alpha channels: premultiplied vs straight BACK

After Effects Help explains Alpha channel interpretation: premultiplied or straight, and Chris & Trish Meyer have an extended discusion in Managing Transparency Part 1 and Managing Transparency Part 2 at Artbeats.

Plus there a several video tutorials on alpha channels in AE if you want other reviews:

Luxx has a new tutorial oriented to AE and Cinema 4D users, Straight vs Premultiplied: Understanding Alpha Channels,


In Straight Alpha Channels, Richard Harrington explains the difference between straight & premultiplied alpha channels, as well as when and how to use straight alpha channels in an episode of Photoshop for Video.




In Straight Vs. Premultiplied, Aharon Rabinowitz for Creative Cow helps you understand the differences between Straight and Premultiplied video when working with After Effects alone, as well as between AE and other programs.




Update: Ben Rollason posted new After Effects tutorial videos on Creative Cow: Lens Flare in AE and Premultiplication Explained and Subtract and Divide Blending Modes in AE CS4.

Alpha channels: premultiplied vs straight (retired)

Alpha channels are grayscale channels in addition to the color channels of a file. They define transparency and often delineate objects or selections as a matte or stencil. Alpha channels can come pre-matted ("shaped") or as straight, that is, anti-aliased when composited.

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This post was updated on Pro Video Coalition as Alpha channels: premultiplied vs straight.