Video
Non-square Pixels - 3
Window sizes
When you preview video while editing in on-screen windows, you want to see the images in the same proportions they will appear on a TV. Most editors allow you to choose the size of these windows, so you should size them in TV aspect ratio. But if you size them according to the video pixel sizes when working with DV (which is the default in many editors) they will be the wrong shape.
Doing this means the video images are re-scaled as they are displayed, which is processor-intensive, but the least intensive is to make sure we only scale in one dimension. Such scaling also minimises image degradation. The obvious thing is to keep a familiar number of lines and adjust the width, because we perceive the width as being "wrong". In fact it's easier to keep the width based on 720 and adjust the height.
Remembering that the TV image width is always 704, this gives a full-frame height of 528 (704:528 = 4:3). The window size for viewing the full DV frame should therefore be 720 x 528, as shown below..
For smaller windows we can use tidy fractional values, which again minimises the scaling effort and retains image quality. Halving both dimensions gives a 1/4-frame window of 360 x 264, etc. You may notice that all these sizes apply equally to 25 & 30 fps systems.
When does it matter?
It might not worry you that your images are a bit distorted when editing (making people look fatter in America and thinner in Europe). You know it will all come out right on the TV.
However, there are situations where things will not come out as you expect if you don't take account of the distortion. These include the following situations:
- Importing still images
- Effects involving circles and squares
- Effects involving frame rotation
Editors
A few video editing packages understand the concept of viewable aspect ratio as well as pixel-based frame size. These include Digital Origin's EditDV, and Avid's somewhat expensive Xpress DV. With editors such as these you don't have to worry about the pixel shape, because the editor will do all its calculations based on how the frames will be seen, and compensate for the rectangular pixels automatically.
However, the majority of editors still don't offer this feature, including perhaps the most popular two - Premiere and MediaStudio. When using these editors you have to compensate yourself if you want accurate results.
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