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Scaling

Scaling is another key part of the preprocessing process, and it refers to resizing the cropped source rectangle to the proper output frame size—for example, going from a 720 by 480 (standard-definition) source frame to a 320 by 240 output frame for Web video (Figure 4.3).

Figure 4.3

Figure 4.3 These images demonstrate how a 720 by 480 source clip (left) can be scaled down to 320 by 240 (right) for Web delivery with no loss in quality.

Scaling up video to higher frame sizes (as shown in Figure 4.4) is not recommended, but in the case of user-generated content (such as that captured via mobile phone), it is sometimes necessary. Be prepared for poor results when you go the upscaling route.

Figure 4.4

Figure 4.4 Upscaling, unlike downscaling, is a bad idea—you can't add pixels that weren't in the image to begin with without compromising quality.

A key question when scaling video for Web playback is just how big should you resize to? There are all sorts of possibilities, but here are two general rules of thumb: the larger the frame size, the bigger the output file will need to be to maintain good quality, and the slower it will play on some machines. Of course, very small frame sizes will always be low quality, so you need to find a good balance that works for your projects. Here are some general guidelines, regardless of aspect ratio:

  • Widths of 640 pixels: This is a great high-quality encode if you don't need to worry about file size or playback speed (some older computers will have difficulty playing back files this large without dropping frames).
  • Widths of 400 to 480 pixels: Though smaller than SD, these frame sizes are still large enough for good viewing on modern computer displays. Widths of 400 to 480 will also fit in well with modern Web site designs.
  • Widths of 320 pixels: Ironically, just seven years ago, 320x was considered "large" for video files for the Web, but no more. This makes a small but viewable video file.
  • Widths of 300 pixels and smaller: On a computer screen, these will look more like moving postage stamps—not a rich video experience. These are more suited to mobile video, given the screen resolution and bandwidth.

For more details on general guidelines for frame sizes, see Table 4.1. These ratios assume correct pixel aspect ratio (PAR).

Table 4.1. Common Sub-SD Frame Sizes

Output

Full-Screen (4:3)

Wide-Screen (16:9)

Near-DVD quality

640x480

640x360

Large broadband

480x360

480x272

Medium broadband

400x300

400x224

Small broadband and large mobile

320x240

320x180

Web dial-up and medium mobile

240x180

240x134

Small mobile

160x120

160x90

Image Aspect Ratio Correction

It's not uncommon in modern postproduction to deliver letterboxed content (for example, a 16:9 image that has black bars above and below the image to fill a 4:3 screen). Some cameras actually shoot in letterbox, but more often the bars are added in postproduction to make an edit more cinematic (wide-screen being so heavily tied to film).

Some broadcast facilities will also ship finished content on tapes in anamorphic format—that is, wide-screen content that is horizontally squished to fit in 4:3. During playback, specialized hardware is used to restore the wide-screen aspect ratio. When working with content that is either letterboxed or anamorphic, it's important to correct the image appropriately. In the case of letterboxing, this probably means cropping out the black bars, and in the case of anamorphic, it means changing the aspect ratio from 4:3 to 16:9 to correct the image.

Pixel Aspect Ratio Correction

An important element of scaling is pixel aspect ratio correction. Generally speaking, production formats such as DV use nonsquare pixels. For example, you already know DV-NTSC is 720 by 480. Just by looking at the numbers, you would assume that the aspect ratio is 3:2. But that's not the case. DV-NTSC is either 4:3 or 16:9, depending on how it was produced. In 4:3 mode, the pixels are more narrow than square, and in 16:9 mode, they're more wide than square. Nonsquare pixels are also used for MPEG-2 for DVD and MPEG-1 for SVCD, with different shapes for MPEG-2 for SVCD.

Web video formats, however, use square pixels, which makes it easier to do the math. For example, 320 by 240 is 4:3 in square pixels.

Make sure you get the aspect ratio correction right when you convert to square-pixel sources from nonsquare-pixel sources by verifying that the output frame size matches the source aspect ratio. So if you use a 4:3 source, a 4:3 frame size such as 160 by 120, 320 by 240, 400 by 300, or 640 by 480 (all square pixel) are acceptable choices, even if the source frame size is 720 by 480, 720 by 486, 640 by 480, or 352 by 480 (all nonsquare pixel).

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