Resizing For Computer Viewing

To explain the basics of square pixel resizing its best to start with computer based viewing. Depending on what sort of standalone player and TV you might be viewing on there are several variables that just don't apply to watching on your computer. We'll deal with those in the following sections. For now we'll stay away from them to cover some basic topics.

Aspect Ratio and Error

One of the keys to proper resizing is keeping the proper AR. While this seems like an easy enough task at first glance, its complicated by the MPEG-4 requirement that both horizontal and vertical resolution be multiples of 16 (mod16). This will sometimes require that the Aspect Ratio be slightly incorrect or that borders be added to the left and right sides - sometimes both. For example, if your movie has an AR of 2.35:1 (with the black borders from the 1.78:1 DVD frame removed), and your destination is a 800x600 computer display, you would start by dividing 800 by 2.35 resulting in a vertical resolution of 340 (rounded down from 340.4). You'd then need to add a border (letterbox) totalling 12 lines to the top and bottom to reach a mod16 resolution for the encoder. Your video player should be able to add the additional top and bottom borders in fullscreen mode, so there's no point in wasting bits encoding them.

Notice the letterbox on the top and bottom and border on the left

About Approximations

In addition to MPEG encoding itself, all scaling (resizing) is lossy. Any process that can't be reversed to obtain the original signal (picture) exactly is considered lossy. Think of them as approximation or estimation of the image. In general you want to have the fewest lossy processes in between the original source and your TV. There is loss involved in capturing the original film or video and scaling it to DVD resolution. There's further loss in the MPEG-2 encoding for DVD, and you'll be introducing yet another generation of loss when encoding to MPEG-4. In addition there's loss involved in scaling video to send across a S-video or composite video connection to your TV, and even from an upscaling DVD player or HDTV's built in scaler. Although this seems like it would be too much to maintain a quality image throughout, if you approach it with all the facts you can achieve surprisingly good results.

These images are from the same frame. The one on the left is the original from DVD. The other has been scaled down and back up.

Decisions, Decisions, Decisions

So what factors should you consider for resizing? Well the most obvious is the resolution of your display, but it's not the first. Before we get to that let's ask the most basic question. What's the reason for resizing? If there's no compelling reason for a lossy process like resizing don't do it. Period. So what constitutes a compelling reason? There were two that were paramount in the early days of hobbyist encoding; file size and computer scaling. Before DVD players began including MPEG-4 playback capabilities nearly all MPEG-4 video was watched on a computer monitor. Even for TV viewing a computer had to generate the signal. This allowed the use of lower resolution than the original DVD source with little or no perceptible quality loss. As the standard media for storing encoded movies was CD-R, with a relatively low capacity for high bitrate video, low resolutions, accompanied by low bitrates were essential. With DVD players supporting MPEG-4 playback and Home Theater PCs (HTPCs) with several hundred Gigabytes, or sometimes even Terabytes of storage becoming more popular all the time, storage isn't necessarily as compelling an argument as it once was.

So Why Resize At All?

Rather than media limits, quality concerns should be the paramount issue for most situations. Consider your playback equipment. If you're still watching an analog TV you might want to encode at a lower resolution both to avoid your player scaling the image and to save a little bitrate. As we'll discuss in the next section, you may even find that your player cuts off several lines from each side of the image, which you may want to reclaim by resizing and adding borders. If your goal is to author to either HD DVD or AVCHD (Blu-ray) you could resize to a HD resolution like 1280x720, but both formats support standard definition video, and all next-gen players can upsample for you. The only convincing argument I can see for encoding SD video at a higher resolution is to obtain higher quality than your player's hardware can manage. For computer viewing, unless you have a very old computer, you should be able to watch at 1024x768 resolution in excellent quality using free postprocessing tools available both in media player software and some codecs (like ffdshow).

Resizing For a Particular Screen

Sometimes you need to resize for a particular application, such as fullscreen analog TV or a portable video player. In cases like these where the display resolution is lower than the original DVD you may want to lower the resolution accordingly. If you resize to fit the display you may end up with a file that won't look good on a higher definition display like a HDTV.

When Not To Resize

As a general rule, if you don't have a specific reason to resize to square pixels you shouldn't. For HDTV viewing you should be able to get excellent results from an upsampling player, making it a waste of bits (and encoding time) to do this before encoding. PAL video uses a wider PAR than NTSC, making it something of a waste to use square pixels for PAL video.

Next: Resizing Step by Step