PAR for the Course
Working with the new pixel aspect ratios in CS4.
By Chris and Trish Meyer | July 02, 2009
Clean Aperture Sizes
Now that we know the correct pixel aspect ratios, we can use them to determine what the Clean Aperture is for each frame size. Keep in mind that the Production Aperture sizes - the pixels you actually need to push through your production chain - have not changed; they're still 720x486, 720x480, or 720x576. What has changed is that we're now supposed to be aware of the Clean Aperture, which is the actual image we need to worry about presenting to the viewer.
When calculating these sizes (as well as the square pixel artwork sizes below), one more idiosyncrasy needs to be kept in mind: even numbers rule. Whenever possible, you want to either pad or crop by pairs of pixels or lines. If you have an odd number, and then divide it by 2 (for example, trying to center an image), you end up with a half pixel - which means additional antialiasing (and in turn, softening) will take place. Cutting an odd number of lines vertically also flips field order, which is usually a no-no. So the math often gets adjusted by the desire to keep pairs of pixels around the edges.
For NTSC, 486 lines x (4/3) x (11/10) = 712.8 pixels across for its Clean Aperture size. Fractional numbers are even worse than odd numbers, so this gets rounded down to 712, yielding a clean ap size of 712x486.
(Anyone remember early NTSC DV cameras that had 4 pixel wide black bars running down the left and right edges? Now you know where they came from: The difference between Production and Clean Aperture. The manufacturers probably figured that being digital, they no longer had to worry about edge ringing artifacts and the such. Plus, it no doubt saved on bandwidth...)
For PAL, 576 lines x (4/3) x (54/59)= 702.9. Using the same rounding logic as above, PAL's Clean Aperture size is 702x576.
NTSC: Production Aperture = 720x486; Clean Aperture = 712x486
PAL: Production Aperture = 720x576; Clean Aperture = 702x576
(Yes, the difference between Clean and Production Aperture also affects where the safe area guides should be placed. I discuss that here.)
Square Pixel Sizes
Once we have the Clean Aperture, we can determine the correct square pixel dimensions to create artwork in applications such as Photoshop, Flash, and Illustrator. And that's what the new pixel aspect ratios primarily affected: The recommended square pixel sizes for standard definition video graphics, with some slight collateral damage on how you move content between standard and high definition (more about that on the last page).
First thinking of the 4:3 Clean Aperture image, 712.8 x (3/4) = 534.6 lines for the height, rounding to 534. So if you wanted to create an image in Photoshop etc. that was just The Image - and nothing but The Image - you would create at a size of 712x534.
However, if you scaled the 534 horizontal dimension down to fit the 486 lines of an NTSC D1 composition (which is based on Production Aperture sizes, not Clean Aperture), the image would not fill out the width of the composition. You can either live with the slim empty bars down the side (usually, they'll be cropped by the TV's bezel, although 4:3 content "pillarboxed" into the center of a 16:9 screen will re-expose them), cover for them in some manner such as filling your background with black, or - the preferred solution - create a Production Aperture square pixel image of 720x534 (the new square pixel preset in CS4 - not 720x540). Just keep in mind that some of those pixels down the left and right are excess.
The new NTSC 4:3 square pixel size is 720x534, not 720x540.
The logic is similar: For a Clean Aperture image, 712.8 x (40/33) = 864 pixels wide. You can also get there by going 486 x (16/9) = 864; both yield a Clean Aperture widescreen square pixel size of 864x486. This indeed is the size many used previously (by the way, there was no need to use 960x540; it just wastes pixels), and you can still use it now - just keep in mind that it "only" fills the Clean Aperture, not the full Production Aperture.
If you hate empty pixels, for Production Aperture, 720 x (40/33) = 872.7 pixels. Rounding down to an even number yields 872x486 as the square pixel size to create widescreen NTSC artwork. Just again: Remember that 4 pixels on each side are excess.
Creating a new NTSC widescreen square pixel document in Photoshop CS4.
Starting again with the Clean Aperture size, 576 lines x (4/3) = 768 lines, yielding a 768x576 square pixel image - a familiar size from the past.
However, the PAL square pixel Production Aperture calculation takes a slight left turn (just when you thought you were getting the hang of this...). 720 x (59/54) = 786.7. You would think you would just round it down to 786 pixels wide. However, 786 (prod ap) - 768 (clean ap) = 18 pixels, or 9 excess pixels per side. And remember: we try to avoid odd numbers of pixels. Therefore, the BBC says instead we should round this up to 788x576 pixels, which is indeed what Adobe has done in the PAL square pixel presets in CS4.
Here's some more familiar numbers: 576 x (16/9) = 1024, which yields a 1024x576 square pixel image - a size many used for widescreen PAL production. (Just checking our math, 702.9 x (118/91) = 1024 which proves those weird numbers actually work.) The difference is, pre-CS4 we were jamming this correct Clean Aperture size into a Production Aperture composition, resulting in pixels unintentionally getting cut off downstream.
As for the correct PAL widescreen square pixel Production Aperture size, 720 x (118/81) = 1048.9. However, throwing one last curve ball, the BBC says we should round this up to 1050x576 pixels, which is again what Adobe has done in CS4.
Clean Aperture Square Pixel Sizes (the old standard):
- NTSC 4:3 - 712x534 (you could also use 720x540 and scale down by slightly more to fit)
- NTSC 16:9 - 864x486
- PAL 4:3 - 768x576
- PAL 16:9 - 1024x576
Production Aperture Square Pixel Sizes (used in CS4; safest for production):
- NTSC 4:3 - 720x534
- NTSC 16:9 - 872x486
- PAL 4:3 - 788x576
- PAL 16:9 - 1050x576
480 lines: Common Fiction; Uncommon Fact
More than one user has looked at DV's 720x480 frame size (compared to D1's 720x486), and started calculating a whole new set of numbers for square pixel artwork based on 480 being the height. As a result, they often come up with a new pixel aspect ratio for DV footage. This falls into the flat earth category: nice try, but wrong. In NTSC, DV is just D1 with 6 lines cropped off the height (in most cases, 4 from the top and 2 from the bottom) - they have the same pixel aspect ratio, and should otherwise be treated the same.
However, there is another 480 line format. The Advanced Televisions Systems Committee (ATSC for short) - the people who brought you the high def specs - also defined a 704x480 standard definition format. This format has the same pixel aspect ratio as NTSC D1 and DV (10/11 for 4:3 images; 40/33 for widescreen), and its Clean and Production Aperture sizes are the same (it's strictly digital - no need to compensate for edge artifacts introduced by analog processing). I have seen this frame size pop up in consumer DV recorders that allow their images to be transferred to and from computers, and have heard of its use by other MPEG streams. Although there is a chance that you might not ever encounter this format, now you know how to deal with it if you do: Just use the same PAR as for normal non-square NTSC footage.
next page: what does this all mean during a job?
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