Pixel aspect ratio and frame aspect ratio



Pixel aspect ratio (PAR) is the ratio of width to height of one pixel in an image. Frame aspect ratio (sometimes called image aspect ratio or IAR) is the ratio of width to height of the image frame.

A 4:3 frame aspect ratio (left), and a wider 16:9 frame aspect ratio (right)

Most computer monitors use square pixels, but many video formats—including ITU-R 601 (D1) and DV—use non-square rectangular pixels.

Some video formats output the same frame aspect ratio but use a different pixel aspect ratio. For example, some NTSC digitizers produce a 4:3 frame aspect ratio, with square pixels (1.0 pixel aspect ratio), and a frame with pixel dimensions of 640x480. D1 NTSC produces the same 4:3 frame aspect ratio but uses nonsquare pixels (0.91 pixel aspect ratio) and a frame with pixel dimensions of 720x486. D1 pixels, which are always nonsquare, are vertically oriented in systems producing NTSC video and horizontally oriented in systems producing PAL video.

If you display nonsquare pixels on a square-pixel monitor without alteration, images and motion appear distorted; for example, circles distort into ellipses. However, when displayed on a video monitor, the images are correct. When you import D1 NTSC or DV source footage into After Effects, the image looks slightly wider than it does on a D1 or DV system. (D1 PAL footage looks slightly narrower.) The opposite occurs when you import anamorphic footage using D1/DV NTSC Widescreen or D1/DV PAL Widescreen. Widescreen video formats have a frame aspect ratio of 16:9.

Note: To preview non-square pixels on a computer monitor, click the Toggle Pixel Aspect Ratio Correction button  at the bottom of the Composition panel.
Square and nonsquare pixels

A.
Square pixels and 4:3 frame aspect ratio

B.
Nonsquare pixels and 4:3 frame aspect ratio

C.
Nonsquare pixels displayed on a square-pixel monitor

If a footage item uses nonsquare pixels, After Effects displays the pixel aspect ratio next to the thumbnail image for the footage item in the Project panel. You can change the pixel aspect ratio interpretation for individual footage items in the Interpret Footage dialog box. By ensuring that all footage items are interpreted correctly, you can combine footage items with different pixel aspect ratios in the same composition.

After Effects reads and writes pixel aspect ratios directly from QuickTime movies. For example, if you import a movie captured as widescreen (16:9 DV), After Effects automatically tags it correctly. Similarly, AVI and PSD files contain information that explicitly indicates the pixel aspect ratio of the images.

If a footage item does not contain information that explicitly indicates the pixel aspect ratio of the image, After Effects uses the pixel dimensions of the footage item frame to make a guess. When you import a footage item with either the D1 pixel dimensions of 720x486 or the DV pixel dimensions of 720x480, After Effects automatically interprets that footage item as D1/DV NTSC. When you import a footage item with the D1 or DV pixel dimensions of 720x576, After Effects automatically interprets that footage item as D1/DV PAL. However, you can make sure that all files are interpreted correctly by looking in the Project panel or the Interpret Footage dialog box.

Note: Make sure to reset the pixel aspect ratio to Square Pixels when you import a square-pixel file that happens to have a D1 or DV pixel dimensions—for example, a non-DV image that happens to have pixel dimensions of 720x480.

The pixel aspect ratio setting of the composition should match the pixel aspect ratio of the final output format. In most cases, you can simply choose a composition settings preset. In contrast, set the pixel aspect ratio for each footage item to the pixel aspect ratio of the original source footage.

Trish and Chris Meyer provide tips and tricks regarding pixel aspect ratio in two PDF documents on the Artbeats website:

Chris Pirazzi provides technical details about aspect ratios on his Lurker's Guide to Video website.

Upgrade pixel aspect ratios to correct values

After Effects CS3 and earlier used pixel aspect ratios for standard-definition video formats that ignore the concept of clean aperture. By not accounting for the fact that clean aperture differs from production aperture in standard-definition video, the pixel aspect ratios used by After Effects CS3 and earlier were slightly inaccurate. The incorrect pixel aspect ratios cause some images to appear subtly distorted.

Note: The clean aperture is the portion of the image that is free from artifacts and distortions that appear at the edges of an image. The production aperture is the entire image.

The BBC provides technical details and guidelines on the BBC website regarding dimensions and aspect ratios for PAL video, including an explanation of the discrepancy in pixel aspect ratios. The same concepts apply to NTSC video.

Chris Meyer explains why the corrected pixel aspect ratios are better and how some workflows are affected in the “New Pixel Aspect Ratios” video in the After Effects CS4 New Creative Techniques series on the Lynda.com website.

Pixel aspect ratio values in After Effects CS4 have been corrected as follows:

format

value in After Effects CS4

previous value

D1/DV NTSC

0.91

0.9

D1/DV NTSC Widescreen

1.21

1.2

D1/DV PAL

1.09

1.07

D1/DV PAL Widescreen

1.46

1.42

This discrepancy is limited to these older, standard-definition formats for which clean aperture differs from production aperture. This discrepancy doesn’t exist in newer formats.

New projects and compositions created in After Effects CS4 use the correct pixel aspect ratio values by default.

Projects and compositions created in After Effects CS3 or earlier are upgraded to use the correct pixel aspect ratios when these projects are opened in After Effects CS4.

Note: If you have a custom interpretation rules file, then you should update it with the correct pixel aspect ratio values.

If you use square-pixel footage items that are designed to fill the frame in a composition with non-square pixels, you may find that the change in pixel aspect ratios causes a difference in behavior between After Effects CS4 and earlier versions. For example, if you previously created 768x576 square-pixel footage items to use in a PAL D1/DV composition, you should now create those items with square-pixel dimensions of 788x576.

Composition settings presets for square-pixel equivalents of standard definition formats have changed as follows:

format

pixel dimensions in After Effects CS4

previous pixel dimensions

NTSC D1 square-pixel equivalent

720x534

720x540

NTSC D1 Widescreen square-pixel equivalent

872x486

864x486

PAL D1/DV square-pixel equivalent

788x576

768x576

PAL D1/DV Widescreen square-pixel equivalent

1050x576

1024x576

Change pixel aspect ratio interpretation for a footage item

  1. Select a footage item in the Project panel.
  2. Choose File > Interpret Footage > Main.
  3. Choose a ratio from the Pixel Aspect Ratio menu and click OK.

Change pixel aspect ratio for a composition

  1. Choose Composition > Composition Settings.
  2. Do one of the following:
    • Choose a composition settings preset from the Preset menu.

    • Choose a value from the Pixel Aspect Ratio menu.

Common pixel aspect ratios

 

Pixel aspect ratio

When to use

Square pixels

1.0

Footage has a 640x480 or 648x486 frame size, is 1920x1080 HD (not HDV or DVCPRO HD), is 1280x720 HD or HDV, or was exported from an application that doesn’t support nonsquare pixels. This setting can also be appropriate for footage that was transferred from film or for customized projects.

D1/DV NTSC

0.91

Footage has a 720x486 or 720x480 frame size, and the desired result is a 4:3 frame aspect ratio. This setting can also be appropriate for footage that was exported from an application that works with nonsquare pixels, such as a 3D animation application.

D1/DV NTSC Widescreen

1.21

Footage has a 720x486 or 720x480 frame size, and the desired result is a 16:9 frame aspect ratio.

D1/DV PAL

1.09

Footage has a 720x576 frame size, and the desired result is a 4:3 frame aspect ratio.

D1/DV PAL Widescreen

1.46

Footage has a 720x576 frame size, and the desired result is a 16:9 frame aspect ratio.

Anamorphic 2:1

2.0

Footage was shot using an anamorphic film lens, or it was anamorphically transferred from a film frame with a 2:1 aspect ratio.

HDV 1080/DVCPRO HD 720, HD Anamorphic 1080

1.33

Footage has a 1440x1080 or 960x720 frame size, and the desired result is a 16:9 frame aspect ratio.

DVCPRO HD 1080

1.5

Footage has a 1280x1080 frame size, and the desired result is a 16:9 frame aspect ratio.