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- Introducing Premiere Pro: the compositing program
- Understanding transparency and alpha channels
- Adjusting opacity
- Working with garbage mattes
- Luma keys and chroma keys
- The Ultra Keyer
- Nesting sequences
- Understanding and using blend modes
- Creating track mattes
Skill Level Intermediate
To properly understand what's going on when you're working with alpha channels inside of Premiere Pro. I think it's really important to get a clear sense of what an alpha channel actually is. I find that when I'm teaching this kind of stuff in the classroom, students very often have a mental block, in the way that I did when I first started out working with that special fourth channel of information. I think it helps if you begin by thinking about color channels in the same way that we think about audio channels. If you imagine when you're working with audio, you've got left and right channel sound.
And you get used to recording stereo sound that way and playing it back in stereo. But the truth is that the left speaker and the right speaker, those channels of sound information, don't have an orientation towards the left or right. It's just that they happen to be recorded from the left or right, and they have to be played back the same way. It's kind of the same with channels and video, and graphics. Now, I'm inside Photoshop here, and I've got an image that I've created that has a clean red, green and blue set of color information.
If I switch over to the Channels panel here, I can see right now I'm looking at the composite of all three. If I switch over so that I'm just viewing the red channel information. You can see I've got light picture information in this splodge on the left and dark picture information in these two other splodges, where the green and blue are. The rest of this picture is just simply absent, there's no pixel information, which means that it's transparent. In fact, this means that, because Photoshop documents they innately support transparency, they automatically have alpha channel information. The alpha channel for this part of the picture, the outer region here where there is no color information, that has an alpha of zero. If I switch over to viewing the green portion, you can see a similar thing. Let me just switch away from that Selection tool, and then you can see again for the blue.
So, just the blue has light information for that channel and dark for the other two channels. What you're seeing here is a gray scale representation of a color channel. It just so happens that Photo Shop knows, and Premier Pro knows and After Effects, and every editing system that you might ever have worked with. They all know that this channel that we call blue should be displayed in the color blue. The same thing for the red and the green. There our special effects that you can apply that would allow you to migrate the color information to one channel to another.
And I guess here's the magic that you need to understand to properly get what's going on with our channels. It isn't really a blue channel, it's a greyscale. If I look at my blue channel here, what I'm seeing is a light area that indicates a lot of that channel. And then two dark areas that indicate none of that channel is totally dark, and this is totally light. All of the color channels are actually black and white gray scales, if you view them in this way. In exactly the same way, that truly your left and right stereo recordings in audio are not left and right, they're actually mono.
All of the recordings that you make using mono stereo microphones, or even fully surround sound microphone setups are just a combination of multiple mono channels. And that's kind of what's happening here with our color channel information. If I go over to my Layers panel in Photoshop, I've got on my layer here, and if you're not familiar with Photoshop don't worry about this. This is not a workshop on how to use Photoshop but layer's in photoshop behave in very much the same way as tracks inside of Premiere Pro, but of course I'm working with a single image instead of whole clips.
Here in my Layers panel, I've got a mask that I've applied. Now, if I just turn this mask on you can see if you look at the thumbnail for that mask, that I've got a dark region that represents non-visible parts of my image and a light region, that line across the middle that is the visable part. This mask combined with my original image, tells Photo Shop to make some of the pixels invisible. I can just toggle this off and on.
I'm using the Shift key on my keyboard here. With the mask not applied, there's all the picture information. With the mask applied there's none of that picture information. So hopefully what this shows you is that the oppacity or transparency or visibility of each pixel in your media is a channel as well. You've got red, green, and blue, plus alpha. And the alpha is defining the overall visibility. Bear in mind, of course, that a 50% visible white Pixel is not the same as a 50% luminous pixel. 50% luminous is gray.
50% transparency at 100% luminous is 50% visible 100% luminous. Put that in front of black and it'll look gray. But put it in front of any other color, and it'll give you completely different results. Because you'll be combining the luminance with the background. Not all formats actually support Alpha. If I pop back into Premiere Pro now, you can see that I've got multiple pieces of media inside of my project. And if I click, for example, on this piece of media, this Clouds No Alpha piece, you can see in the little information display at the top of the Project panel. I've got clouds no alpha is the name .mp4, it's an H264 file the video resolution and the pixel aspect ratio, frame rate and duration but no mention of alpha.
If I select this other item, this is a QuickTime movie rapper that contains a PNG sequence and PNG supports alpha very nicely, thank you very much. You can see at the top of my \Project panel I have alpha listed. So you can quickly see, by looking at the Project panel, whether an item in your project has alpha or not. Regular Windows AVIs, for example, they're not going to support alpha. Regular H264 files, like this one, they don't support alpha either. Not all of the codecs supported by the QuickTime rapper format will support Alpha.
So you may want to do a little bit of research before you start generating files as I have done here in applications like After Effects. Now that I have my files here, if I for example, throw a piece of meat here into my sequence and put the No Alpha version in front, well I'm just going to get the clouds. If I remove that and put the version with alpha on, Premier Pro will honor it and I'll get my smokey room effect. If a file type has alpha, Premier Pro is probably going to support it, and now you know what that means.