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Understanding the alpha channels

From: After Effects CS6 Essential Training

Video: Understanding the alpha channels

When you render graphics with transparency, you'll probably want to render the graphic with an Alpha channel. Now in the previous video, we did just that. We rendered a QuickTime that had an Alpha channel. So to see what that Alpha channel looks like, let's just select the Mill Pre Comp video that we see up here in our Project panel and drag it down to its own composition in the bottom of the Project panel. Now double-click the Mill Pre Comp 2 just in case it's not open on your system. And here you can see we have just the QuickTime and only the QuickTime.

Understanding the alpha channels

When you render graphics with transparency, you'll probably want to render the graphic with an Alpha channel. Now in the previous video, we did just that. We rendered a QuickTime that had an Alpha channel. So to see what that Alpha channel looks like, let's just select the Mill Pre Comp video that we see up here in our Project panel and drag it down to its own composition in the bottom of the Project panel. Now double-click the Mill Pre Comp 2 just in case it's not open on your system. And here you can see we have just the QuickTime and only the QuickTime.

If I turn the Visibility off, you can see. That's it. In the comp window here at the bottom, you can see I have this three-circle button. If I click on that, notice I can choose what channel I'd like to see, Red, Green, Blue, or Alpha. If I choose the Alpha channel, this will show me whatever is transparent. So whatever is white will allow the graphic to be seen and whatever is black will be transparent. If we select the QuickTime file up in the Project panel and look in the top of the Project panel, you'll notice it says Millions of Colors+ Premultiplied, and Animation.

That's letting us know that it's the Animation codec and it has an Alpha channel that is interpreted at Millions of Colors+ a Premultiplied Alpha channel. To properly see if the Alpha channel is A, interpreted properly, and B, just looks good, let's scroll up to 100% magnification, and I'm going to press the Spacebar and just click and drag to move up in my window here, and just make sure that our Resolution is set to Full. Just so we can see that contrast, let's jump back to the RGB channel by clicking the button in the bottom of the comp window, and then go up under Layer and choose New Solid.

You can choose any bright color that will contrast. I'm going to choose this bright purple. So if we click OK, I can drag that New Solid down to the background layer. And you can see, when you're looking at Alpha channels, you want to look bright at the edges to make sure that the colors of your graphics aren't changing too much. Notice how the white windmills, the edges are just turning gray and then purple. I don't have any funky, strange discoloration. Well if the Alpha channel is interpreted improperly, we would see that.

So let's just go into our Project panel and right-click on the Mill Pre Comp QuickTime. In here you can go to Interpret Footage and under Main, we could look at how the QuickTime Alpha channel is imported. At the top, in the Alpha section, let's go ahead and choose Straight instead of Premultiplied, and then press Enter on your keypad to say OK, and you notice, see, I have these black edges. I know you're thinking to yourself, okay, great! I know there are Premultiplied and there are Straight, but really what's the difference? Well basically the two different Alpha channels are created slightly differently.

A Premultiplied channel means that every single channel, here if we click this button in the bottom in the comp window, the Red channel, the Green channel, the Blue channel, and the Alpha channel will all have transparency data, and it's a combination of all those channels to make the transparency. Premultiplied channels are the most widely supported channels out there. So if you're delivering a QuickTime with an Alpha channel to someone and you have no idea where its end use is going to be, like what applications are going to import it in, you want to render it as Premultiplied.

If you're dealing with extraordinarily bright colors or colors that need to be very accurate, you want to render your Alpha channels with straight Alpha channels, because straight Alpha channels are more color-accurate because none of the transparency data is stored in any of the color channels. 100% of the transparency is stored in the Alpha channel. Now to change where those settings are, you need to do that in the Render Queue. So I'm just going to queue this up to render one more time making sure that we turn off the visibility of the background layer.

And with Pre Comp 2 selected, let's go up under Composition > Add to Render Queue. Now in the Render Queue, you want to look at the Output Module. If you click on the word Lossless, at the top, you can see I could choose my Format. I'm going to choose QuickTime. You can choose Windows Media or what have you. But under Video Output, here with the Channels RGB, let's click on that and choose RGB + Alpha. Now once I go to Alpha, I can make that change under the Depth, here it says Millions of Colors, under the Color we could change from Premultiplied (Matted) to Straight (Unmatted).

Now if I were to render this, and click OK, when I import this, this version I would want to render or interpret with a straight Alpha channel. Ninety-nine times out of a hundred, if you render it out of After Effects and import it back into After Effects, After Effects will automatically guess the proper Alpha channel. So when it comes to rendering graphics with Alpha channels, you want to make sure to pay attention to the specific kind of Alpha channel, whether it'd be Premultiplied or Straight.

Show transcript

This video is part of

Image for After Effects CS6 Essential Training
After Effects CS6 Essential Training

78 video lessons · 54529 viewers

Ian Robinson
Author

 
Expand all | Collapse all
  1. 1m 8s
    1. What is After Effects?
      1m 8s
  2. 2m 53s
    1. Welcome
      1m 40s
    2. Using the exercise files
      1m 13s
  3. 1h 8m
    1. The six foundations of AE
      5m 3s
    2. Introducing the interface and the workspace
      7m 51s
    3. Understanding compositions
      8m 48s
    4. Getting comfortable with layers
      7m 33s
    5. Getting started with animation and keyframes
      8m 30s
    6. Understanding effects
      3m 26s
    7. Moving in 3D space
      7m 41s
    8. Rendering your first animation
      8m 20s
    9. Specifying preferences and cache settings
      5m 44s
    10. Staying organized
      5m 15s
  4. 38m 6s
    1. Creating compositions
      7m 19s
    2. Importing footage and compositions
      7m 54s
    3. Preparing compositions for animation
      8m 7s
    4. Introducing renderers
      3m 15s
    5. Understanding precomposing
      7m 16s
    6. Relinking missing footage
      4m 15s
  5. 59m 58s
    1. Defining layers
      6m 23s
    2. Creating type
      5m 58s
    3. Creating layer solids and shapes with masks
      7m 55s
    4. Building shape layers
      6m 17s
    5. Understanding switches and blend modes
      8m 26s
    6. Crafting custom shapes and masks
      6m 18s
    7. Creating variable-width feathered masks
      5m 1s
    8. Rotoscoping with the Roto Brush
      8m 20s
    9. Refining with the Roto Brush
      5m 20s
  6. 1h 8m
    1. Understanding keyframes
      6m 1s
    2. Adding and adjusting keyframes
      9m 54s
    3. Interpolating keyframes
      8m 5s
    4. Adjusting keyframes in the Graph Editor
      7m 17s
    5. Understanding positional keyframes
      7m 0s
    6. Controlling animation with parenting and the pick whip
      9m 57s
    7. Understanding animation paths
      6m 27s
    8. Timing to audio
      4m 41s
    9. Trimming and sliding edits
      5m 31s
    10. Swapping images
      4m 1s
  7. 29m 7s
    1. Layering multiple effects
      9m 13s
    2. Generating graphic effects with adjustment layers
      7m 28s
    3. Building backgrounds with effects
      6m 50s
    4. Creating animated strokes
      5m 36s
  8. 40m 15s
    1. Introducing cameras
      10m 3s
    2. Working with 3D layers
      6m 37s
    3. Positioning layers
      6m 13s
    4. Adding lights and working with Material Options
      9m 21s
    5. Using 3D precompositions
      2m 5s
    6. Adjusting depth of field
      5m 56s
  9. 28m 31s
    1. Caching and prerendering
      6m 33s
    2. Understanding the alpha channels
      5m 18s
    3. Using the Render Queue
      4m 34s
    4. Rendering with Adobe Media Encoder
      7m 15s
    5. Archiving finished projects
      4m 51s
  10. 44m 27s
    1. Creating type animators
      12m 16s
    2. Animating type in 3D space
      6m 35s
    3. Adding and animating type on a path
      8m 45s
    4. Composing 3D type
      8m 41s
    5. Animating shape layers
      8m 10s
  11. 32m 45s
    1. Creating stylized video
      6m 47s
    2. Retiming video footage
      9m 31s
    3. Retouching with the Rubber Stamp tool
      10m 19s
    4. Smoothing shaky camera footage
      6m 8s
  12. 14m 19s
    1. Understanding keying
      3m 19s
    2. Creating a garbage mask
      4m 27s
    3. Getting started with Keylight
      6m 33s
  13. 15m 56s
    1. Importing Photoshop documents
      6m 11s
    2. Importing Illustrator files
      4m 24s
    3. Working With Premiere Pro projects
      5m 21s
  14. 1h 15m
    1. Adjusting ray-tracing quality
      8m 19s
    2. Tracking footage
      8m 15s
    3. Extruding shapes
      8m 39s
    4. Bending layers
      8m 38s
    5. Adjusting ray-traced lighting and materials
      9m 22s
    6. Adding environment maps
      4m 58s
    7. Beginning compositing
      8m 52s
    8. Creating render passes
      10m 17s
    9. Building a final composite
      8m 14s
  15. 1m 8s
    1. What's next
      1m 8s

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