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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.
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