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Filed under: Post Production •Web Video •

PAR for the Course

Working Practices

That was a lot of math to slog through. I imagine what you really want to know is how this affects you in your everyday work. It boils down to this:

• If you’re working strictly in high definition, relax - nothing has changed.
• Standard definition comps and sequences have not changed in size; video captures will be the same size too - so no new numbers to learn there either. (The only thing that has changed is that we now know some of the pixels are merely padding - not active image. We still get all of the pixels to play with.)
• Non square pixel footage will round trip in and out of CS4 applications without change; there is no worry about the result “no longer working” in non-CS4 applications. Your video captures and old renders will also still fill out comps in AE CS4 and sequences in Premiere CS4 without issue - so again, relax.
• If you are creating square pixel artwork destined for a non square pixel standard definition project in a Creative Suite 4 application, you need to make a decision: create at Clean Aperture sizes (knowing all of the pixels you see will be considered part of the final image), or create at the new Production Aperture sizes (knowing that a few pixels along the left and right are just padding - not “real” image). I’d say do the latter, as it will cause less confusion, cover for more cases (such as the left and right edges of 4:3 content being revealed on a 16:9 screen or video projector, in web video, etc.) and require fewer additional decisions downstream (such as how to pad out the difference between Clean and Production Aperture). Note that all of the CS4 presets for square pixel artwork use the Production Aperture size.
• If you are handed square pixel artwork created at the old sizes, you need to make a decision: do you want to scale it to fit the Clean Aperture, or the Production Aperture? Scaling to the clean ap size means scaling it so that the artwork’s height fits the comp or sequence’s height, at the cost of some blank pixels to the left and right. Although there is a certain purity of purpose in aiming just for the Clean Aperture size (after all, that was probably what the image’s original creator really intended), as noted before you can’t rely on the left and right edges being cropped upon viewing - so you need to come up with a strategy for covering those pixels between Clean and Production aperture. Therefore, it is probably better that you scale the artwork so that it fills the width of the target comp or sequence, and live with cropping a few pixels off the top and bottom. (Don’t do a “scale to fit” - this will result in the height and width being scaled differently, re-introducing a slight distortion into the final image!)

The biggest decision comes when sizing legacy square pixel assets into new CS4 compositions or sequences. If you scale to match the height, you will see the entire image, but there will be some blank bars on the left and right that need to be padded out (above). If you scale to match the width, the entire Production Aperture frame will be covered, but a few pixels will be sliced off the top and bottom of your image (below).

I mentioned earlier that there are some curve balls involved when using high def footage. In HD, the Production and Clean Apertures are the same: There are no extra pixels to play with or worry about, unlike the standard definition case. If you are fitting an HD image into a widescreen SD frame or vice versa, you have the exact same issues as when fitting legacy square pixel artwork into an SD video frame: do you pad the sides, or crop the top and bottom?

If you scale a 1920x1080 HD image by 45.0% to fit into an SD widescreen comp in CS4, you are scaling to fit the clean ap of the HD image to the clean ap of the SD image - with the consequence of blank pixels on the left and right, as shown above. Scale by 45.42%, and you’ll lose some pixels off the top and bottom, but have no blank areas to explain to someone less patient (and informed) than you.

It’s Complicated, But It’s Not Voodoo

Yes, that’s a lot to think about, but at the end of the day the changes are far smaller than many users assumed when they heard about CS4’s new PARs. Most of the time, you won’t notice any changes, and your work will merely be more accurate now. Make some decisions now on how to make square pixel artwork and how to handle legacy square pixel content in a non-square comp or timeline (in both cases, I’d work at Production Aperture size, and live with a few pixels getting cropped off), stick with them, and get on with your life. If a special case comes up, at least now you know the actual underpinnings and real math to help you determine what the best solution will be.

If this is still too much to digest, but you have to use After Effects CS4, I wrote a quick note on how to use the old aspect ratios on old projects in AE CS4. But the sooner you can adapt to the right way of doing things, the better. After all, it’s only been a decade since the correct ratios were agreed upon in the first place! I wouldn’t be surprised if other companies eventually followed Adobe’s lead and started using the correct values in their own applications.

Further References

Here is a series of links you can follow if you want to study these technical issues in more detail:

• LurkerTech - a good all-around reference site on technical video issues (DVMP is a similar site with a more PAL-centric view)
• IceFloe Dispatch 19 - Apple’s Rosetta Stone on how to encode digital video
• The BBC’s Guide to Picture Size for delivering content
• The ATSC spec for digital television in North America
• The University of California Berkeley has a fairly recent copy of the hoary old ITU-R BT.601 specification online. Grab it while you can; this is the document from which all of this madness derives

 

The content contained in the CMG Blogs and CMG Keyframes posts on ProVideoCoalition are copyright Crish Design, except where otherwise attributed.

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Does this mean all frame aspect ratios for webvideos encoded from PAL or NTSC video (which i always encode with a square par) are no longer 1.33 or 1.78 and will become 1.37 and 1.82?

Posted by .(JavaScript must be enabled to view this email address)  on  07/09  at  09:00 AM

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Only if you don’t crop off the difference between production and clean aperture. (Personally, I think video should be cropped down to the action safe area before scaling down to web size; after all, this is all the active image area that the creator was expecting the viewer to see.)

Posted by Chris Meyer  on  07/09  at  09:23 AM

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Clean aperture for DV-NTSC is 704 x 480, at 10:11 pixel aspect ratio.  It is not 712.8 x 480.

Clean aperture for DV-PAL is 702 54/59 x 576, at 59:54 pixel aspect ratio.  (Premiere Pro truncates the fractional pixel: 702 x 576, at 128:117 pixel aspect ratio.)

Posted by .(JavaScript must be enabled to view this email address)  on  08/15  at  10:15 AM

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“Clean aperture for DV-NTSC is 704 x 480, at 10:11 pixel aspect ratio.  It is not 712.8 x 480.”

I beg to differ, but am completely sympathetic as to where the confusion comes from.

NTSC DV is a special case: it’s NTSC D1 with 6 horizontal lines missing, and it should be treated as such. Thus it has clean ap size of 712.8 x 480. And this isn’t my speculation or pontification; this is “The Word” from people a lot smarter and more respected than me.

However, there are other formats - such as ATSC digital standard definition - which are defined as having a true 480 lines. _These_ formats have different math, and 704x480 is the correct clean ap size for them. (So, I guess if you want to redefine what DV is for - not a budget version of D1, but instead a format onto itself for acquiring SD ATSC material - then you can think of DV-NTSC’s clean ap as being 704 x 480. It’s revisionist history, but works as a workflow. As long as you treat the underlying PAR as 10/11.)

I know - messy. Remember that DV was never intended to be professional production format; it was supposed to be the consumer replacement for Hi-8. We weren’t supposed to have to deal with all the ugliness that exists under its hood. But, here we are.

Posted by Chris Meyer  on  12/31  at  03:25 PM

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BTW, I’ve just read that FCP 7 now uses the new/correct PARs as well:

http://blogs.adobe.com/toddkopriva/2010/01/final-cut-pro-7-using-correcte.html

So, less tripping while round tripping! (Although it wasn’t as big of a problem as many feared in the first place.)

Posted by Chris Meyer  on  01/15  at  09:58 AM

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