Working With Aspect Ratios in TMPGEnc DVD Author 3 with DivX Authoring

TMPGEnc DVD Author 3 with DivX Authoring allows you to work and create in both 4:3 and 16:9 aspect ratios, but what does it mean and how can you take advantage of it?

The Aspect Ratio

The aspect ratio defines the proportional relation between the height and width of a video image. For example, in the case of a 4:3 image, if the picture height is 1 unit then the width is equal to 1 x 4:3 of the height, which turns to be approximately 1.33333... units.

Picture Aspect Ratio Versus Pixel Aspect Ratio

When talking about digital video the matter may become quite complicated by the fact there are two different ways to define the image proportion: using the Picture aspect ratio, and the Pixel aspect ratio.

Picture aspect ratio is the simplest. Like the aspect ratio described previously it defines the height and width proportion of the image. As you may know, a video picture is composed of a finite number of elements called pixels. The number of horizontal and vertical pixels defines the image resolution which is up to 720 by 480 in NTSC and 720 by 576 in PAL standard. When defining the image proportion by the Picture aspect ratio, the proportion of the image does not change whatever the number of horizontal and vertical pixels. Using this method in digital video, you leave the video decoder to do all the grunt-work to display a well-proportioned image by stretching the pixels if necessary. To sum-up, Picture aspect ratio is resolution-independent.

Pixel aspect ratio defines the horizontal and vertical size proportion of each pixel composing the image instead of defining the whole image proportion. For a given Pixel aspect ratio, the image proportion changes according to the number of horizontal and vertical pixels. Pixel aspect ratio is resolution-dependent, and this explains why it is sometimes difficult to convert from one resolution to another without stretching the image.

How To Sort Your Sources And Get the Best Results

First of all, like a good detective, know your sources. In the Clip editor/Clip properties/Clip settings you can see and change the Aspect ratio of the source file. The key for obtaining a correct picture output is to set the correct aspect ratio of the source. In most cases, if you work with MPEG sources, you can assume the displayed aspect ratio is the correct one. However this may not be the case when using AVI sources taken from the Internet or a DV camcorder. For AVI, the aspect ratio is Pixel 1:1, meaning the pixel element is square. In case of an AVI from a DV camcorder, verify the aspect ratio matches the camcorder recording settings. In case you shot in 16:9, you may have to change the aspect ratio from 4:3 to 16:9.

Choose the Right Track Settings

When making a DVD-Video project, the absolute rule is within each track all the clips must possess the same characteristics. Since TMPGEnc DVD Author 3 can transform the source files' content to follow this rule, having source files of different aspect ratios may create unexpected results such as having a black border appearing on the sides of the image. The solution is to group 4:3 and 16:9 aspect ratio clips in respective tracks. TMPGEnc DVD Author 3 bases the track settings on the characteristics of the first clip in the track. However, you can override this setting by changing the Source/Track settings/Video/Aspect ratio option (see screenshot to the right).

Take-Away Points to Remember:

• Know your souces' aspect ratios; is it 4:3 or 16:9?
• Make sure the aspect ratio for a source file is correctly set in the Clip Editor/Clip Properties/Clip Settings window.
• All the clips within each track must possess the same aspect ratio to avoid black bars being added to clips with different aspect ratios.
• TMPGEnc DVD Author 3 with DivX Authoring will base the track settings on the characteristics of the first clip in the track.
• Thus, you should always try to group 4:3 and 16:9 aspect ratio clips in their own respective tracks.
• To override aspect ratio settings for a track, go to the Aspect Ratio option in the Video tab of the Track Settings found in the Source Stage.

We hope you enjoyed this edition of TMPG Labs! See you next time!