Working with pixel aspect ratios
Video: Working with pixel aspect ratiosMost of the compositions in this lesson were created at a size that's suitable for standard definition video and when we talk about standard definition we talk about an image aspect ratio that's four units wide to three units tall. If you were to pull out a calculator that would be four divided by three equals an image aspect ratio of 1.333. However, you might have noticed in most of the compositions we created and footage we used don't match these number. Go back to After Effects, to Composition > New Composition and look at Standard Presets like NTSC, D1 or DV.
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In this installment of After Effects Apprentice, Chris Meyer focuses on ways to edit and enhance layers in After Effects. Through a series of Quizzler challenges and Idea Corner examples, Chris shares alternative ways to employ modes, sequencing, and adjustment layers, while special sidebar movies cover the subjects of creating seamless loops, animating effects points, understanding pixel aspect ratios, and employing Brainstorm to explore the variety of different looks that effects can create. The course also covers tricks for enhancing boring footage and tips for converting scans into moving sequences. Exercise files are included with the course.
The After Effects Apprentice videos on lynda.com were created by Trish and Chris Meyer and are designed to be used on their own and as a companion to their book After Effects Apprentice. We are honored to host these tutorials in the lynda.com Online Training Library®.
- Sliding and trimming
- Slip-editing and insert-editing layers
- Employing blending modes to enhance layers and composites
- Applying, modifying, and saving animation presets and layer styles
- Using adjustment layers to affect multiple layers
- Experimenting with effects using Brainstorm
- Understanding pixel aspect ratios
Working with pixel aspect ratios
Most of the compositions in this lesson were created at a size that's suitable for standard definition video and when we talk about standard definition we talk about an image aspect ratio that's four units wide to three units tall. If you were to pull out a calculator that would be four divided by three equals an image aspect ratio of 1.333. However, you might have noticed in most of the compositions we created and footage we used don't match these number. Go back to After Effects, to Composition > New Composition and look at Standard Presets like NTSC, D1 or DV.
these have dimensions like a 720 x 480 or in the case of the D1, 720 x 486. The PAL standard has even different numbers, such as 720 x 576. And if we were to haul out our calculator again and do clear, 720 divided by 486, we'd find that we're nowhere close to 1.33 Image Aspect Ratio. We have something strange here. 1.48 with a bunch of numbers afterwards.
What's going on here? Well, it's video's fault. Most computers draw each pixel, each picture element, square. As wide as it is tall. However for various technical reasons most video formats have a pixel aspect ratio that is not square. A pixel may be wider or skinnier than it is tall. Let me give you some examples what this looks like. Here's some stock footage based on a nice circular pattern. I brought up a high definition version of this footage, which has square pixels just like our computer does, and you'll see inside the Project panel it even says 1.00.
That's the pixel aspect ratio for this footage. However, if I bring up the NTSC version you'll see that the circle's a bit wider than it should be and if I look at its information in the Project panel I see the number. It's 0.91. What that is saying is to draw this properly After Effects should scale the width of the shot by 91% to make it look correct again, to make it look like the high def shot does. We'll look at a couple of other clips. I'll open up the PAL version of the same clip and now you see it's got the opposite problem. It's skinny rather than wide, and if I look at its pixel aspect ratio in the Project panel I see it's 1.09. After Effects is telling me this clip will ultimately be displayed about 9% wider than I am seeing it on my computer.
But things get even crazier with widescreen footage, particularly Standard Def widescreen. I mean here is an NTSC widescreen image. You can see now those circles are really squished down and has a very distorted pixel aspect ratio of 1.21 and it needs to be display 21% wider to look right, and then PAL widescreen footage is even crazier. To display it properly, it needs to be stretched by 46%. At this point you're probably going, "This is a lot of math and you expect me to do this on every video job?" No, After Effects will do it for you as long as you set everything up correctly.
The first thing you need to do is make sure each footage item you import to a project has the correct pixel aspect ratio for that clip. After Effects is actually pretty good at either detecting or guessing this pixel aspect ratios. It occasionally has trouble with widescreen. It depends on how that footage was created. Whether or not After Effect can automatically detect that aspect ratio correctly. If you ever have footage that has a wrong aspect ratio,select the Project panel, open up Interpret Footage for that clip, and then change the Pixel Aspect Ratio pop-up for that footage to match the format of that footage.
It is really, really important that you don't lie to After Effects in this dialog. You don't say "I want to see it square, give me square pixels." No, that's not how the source was created. If it was widescreen video stored on let's say DV tape, you need to pick DV Widescreen and let After Effects do to the math to make things look correct. Click OK. When you create a new composition you have a similar pop-up. Most of the comps you create will be at your Output Format size. For example, if I was going to go to D1 I would choose the D1 preset, it will automatically enter the right dimensions and the right pixel aspect ratio to compensate for me.
You'll see that many of the other formats have their own aspect ratios including some of the high def formats, like HDV also has a very distorted pixel aspect ratio. But that's okay. If you're using these presets After Effects will enter the correct numbers for you, don't change it, and it will do the composition automatically. I go back to my Footage panel for now. While you're working in After Effects you may go, I really don't like looking at distorted footage like this. This is a little too weird for me to design properly. Well you have a couple choices.
one is, After Effects will often let you create square pixel equivalents of these different common video format sizes where you can create using nice normal square pixels where everything looks normal, After Effects will do the math underneath the hood to stretch footage out to fit, then you need render it in a way that squishes it back down to the size that format requires. The other option, which we normally avoid, to be honest, is this little Toggle Pixel Aspect Ratio Correction button. It's at the bottom of every Footage, Comp and Layer panel.
When you enable it After Effects will look at the pixel aspect ratio for that footage and perform that stretch just for display purposes to show you how that clip should look when finally displayed. Now the problem with this option is that at its default settings it does not use a very good algorithm to stretch the footage; it just uses nearest neighbor. That's why you see up there a ragged edge around my circle here, which used to be perfectly smooth. This is the reason I've always avoided this feature, to be honest.
However, as if After Effects CS5 they added a new feature. Under Preferences > Previews there is now a Viewer Quality Setting, and if you set Zoom Quality to More Accurate and click OK, it will now Anti-alias footage, as does this Display Pixel Aspect Ratio correction. It will slow down your previews a little bit. If you if you're working on a slower computer and you find this is really bothering you, you can indeed set Preference > Previews > Viewer Quality zoom to More Accurate Except RAM Preview.
That'll speed it up at the cost of image quality while you are performing one of these previews. However if I were you, I'd set this to More Accurate and not go down unless you absolutely need to. Okay, another reason to use this Display Correction for the Pixel Aspect Ratio is if the client is looking over your shoulder. For example, if I was to open up this Standard Def, D1 piece of footage, your client may look at themselves in the video and say "I don't look like that. My face is not that wide." That's because, in this case, D1 footage is artificially stretched in the computer and will be squished skinnier when it comes time to playback on a video system.
However, if this is freaking your client out, that's the time to go ahead and hit the switch and say that's how you'd really looked on the set. Don't worry. This is how it's going to look at the end of the day when you view it back on a TV. If you've created your composition, use one of presets and if it has non-square pixels and you left this pop-up alone After Effects will render your footage correctly. You don't need to worry about it anymore. However, if you chose one of these Square Pixel versions, you need to make a decision when it comes render time because the format itself does not take square pixels. It expects non-square pixels.
One is to ask the client will they take a square pixel version of the render. Perhaps whatever system they are loading it into will automatically compensate or maybe it's going to the Web where all the pixels are supposed to be square and you'll be doing them a favor by delivering it as square pixels. If you need to deliver it with pixels not square, check what the actual width for the video format is supposed to be, in this case 720, go ahead and work in your Square Pixel comp, but when it comes time to render, Make Movie, go to the Output Module Settings, enable Resize, you can turn off its Lock Aspect, and change the Width back down to the that size required by your video format.
This way, when After Effects renders, it will squish or stretch the final composition as necessary to fit that video format. I'll click OK. The issue of non-square pixels can indeed be very confusing. Fortunately, After Effects does all the hard work for you. The main thing is let it do its job. Don't try to outsmart it by creating elements that are, say, 720 x 46 and then calling them square pixels because After Effects will then treat them wrong. Set up everything in the first place and then you could ignore it for the rest of the job.
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