Compressor: How to Deinterlace Video

There are two basic ways to record and display a video image:

• Progressive
• Interlaced

BACKGROUND

A progressive image records and displays every pixel in the image at the same time. Progressive images are notated with a lower-case “p” as part of the image description: 720p, 1080p. Progressive images are the preferred option for all web videos because they retain the highest resolution and image quality.

The image above is a detail from fast-moving shot of a young girl twirling on a merry-go-round. Notice how clean the edges are and how all hair detail is retained.

An interlaced image records and displays an image as rows of pixels where, first, all the odd-numbered rows are displayed, field 1, followed a fraction of a second later by all the even-numbered rows, field 2. These two fields are offset in time, generally 1/50th or 1/60th of a second. Interlaced images include both NTSC and PAL standard-definition video, as well as video formats notated with a lower-case “i”; for example, 1080i.

The image above is a detail from an interlaced image of the moving fingers of a speaker. Notice the thin, horizontal lines radiating off the moving fingers and the complete lack of detail? Those are interlace lines. On a TV set, they look fine. On the web, they look awful.

While all web videos everywhere are progressive, DVDs, most cable outlets and some broadcast networks display interlaced video for bandwidth reasons. Examples in the US include: CBS, NBC, PBS, National Geographic, Discovery and many others.

GENERAL EDITING ADVICE

It is extremely easy to convert progressive video to interlaced with no loss in image quality. However, it is much more difficult to convert interlaced to progressive and, when you do, there will always be a loss in image quality.

For this reason, regardless of your delivery format, you are always best advised to shoot progressive video wherever possible.

Regardless of the software you are using to edit, the rules for editing HD are the same: always set your project/sequence settings to match the format you need to deliver. If needed, your software will convert progressive to interlaced or interlaced to progressive during the process of editing.

HOW DEINTERLACING WORKS

There are four ways to deinterlace an image; all of which reduce image quality:

• Remove every other line (one field). This reduces the size of the image by 50% but retains overall image quality.
• Duplicate every other line (one field). This keeps the image size the same, but reduces vertical resolution 50%.
• Replace every other line (field 2) by blending the line above it with the line below it (from field 1). This keeps the image size the same but reduces vertical resolution. (This is analogous to frame blending.)
• Replace every other line (field 2) by calculating a new line of video from the line above and below it in field 1. (This is analogous to optical flow.)

As you can imagine, different options take different amounts of time (they are listed from fastest to slowest) and yield varying quality (they are listed from potentially lowest to highest quality).

Many times we need to create a project that was mastered as interlaced into progressive to post to the web. Here, you are wiser to do the conversion in Compressor rather than your editing software because Compressor offers potentially:

• More control
• Higher quality
• Faster speeds

Notice the word “potentially.” As with all video compression, if other compression settings – for example, bit rate – are not set properly, you won’t get good results.

HOW TO DEINTERLACE VIDEO IN COMPRESSOR

Here’s the interlaced example we’ll work with today. Notice the horizontal lines radiating out from his moving face. Notice also that his face has lost most of its texture and detail.

NOTE: Deinterlacing a still image is virtually perfect, as there is no difference caused by movement between the two frames. When judging the results of interlacing, always look for something that is moving in the frame.

While this may set automatically when you select MPEG-4 or QuickTime, be sure the Field Order menu is set to Progressive. This tells Compressor to output a progressive file.

The Quality settings determine image quality for:

• Resizing images
• Changing frame rates
• Converting interlaced footage to progressive

The default settings are OK for deinterlacing, but you may enhance quality by changing one setting: Retiming Quality.
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(Click image to see a larger, uncompressed version of this image.)

Here is a close-up from each frame, starting with the source image, with Retiming Quality set to Faster, Better, and Best. Faster and Better both compressed a six second clip in less than one second. Best took 55 seconds to compress the same six-second clip.

NOTE: The image above is scaled to 75%, click the image to see the uncompressed source images at 100%. Other than pasting the images into Photoshop, no image modification was done to any image.

As I look at this, I see a little improvement between Faster and Better, but not enough improvement – if any –  to justify using Best.

(Click image to see a larger, uncompressed version of this image.)

In the Quality menu are a few other settings that are useful when resizing images: Adaptive Details, Anti-aliasing, and Details Level. Here’s the same portion from each frame: Source image, Adaptive Details turned off, Adaptive Details turned on, Details level set to 20.

NOTE: The image above is scaled to 75%, click the image to see the uncompressed source images at 100%. Other than pasting the images into Photoshop, no image modification was done to any image.

Again, as I look at this, I don’t see any significant differences between these images. These settings are useful when resizing images, but not for deinterlacing.

SUMMARY

In the past, we had more controls over deinterlacing in Compressor. The latest version – 4.2 – simplifies the process, though I’m not sure for the better. If you are in a hurry, leave the Quality settings at their default.

If you want a marginal improvement in deinterlace quality, set Retiming Quality to Better, and leave Adaptive Details unchecked and Anti-alias and Details level sliders at 0.

Just remember, however, that you will always get better image quality if you shoot progressive than to convert interlaced video to progressive after the fact.

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