Finding Compression Artifacts

compression artifacts
Previously, I’ve mentioned that the bit rate you choose for your video encoding can add artifacts to your footage. Dramatic changes in scenes and dissolves are some of the ways that can further cause problems.

But how do you know where the actual problems are? How do you see them? What’s the real difference if you encode at 10 Mbps or 2 Mbps?

There’s a way you can get your edit software to help point out where the big differences are. By overlaying the compressed version of your show on top of the original show and changing the compositing method, you’ll instantly see the differences.

Here’s how to do it:

  1. Compress a version of your show. Start with a very low bit rate so you can see the difference. (If your show is very long, just work with a section of it.) Import that compressed video.
  2. Create a new track above all your current tracks and place the compressed video on that new track. Make sure the compressed video is in sync with the original show. To check sync, park the playhead at a position of movement and turn the layer on and off to see if you notice any differences. If you don’t, they’re in sync. If you see a difference, slide the compressed clip one way or another until they line up.
  3. Now, for the magic. In the effects settings for the top clip, change the composite mode from “normal” to “difference.” This subtracts the pixel values between the two clips.

What you’ll end up with is a primarily black screen (where the pixels are the same), dotted with bursts of white pixels highlighting pixel differences. If you use a high bit rate, you may not see many white particles dancing across the screen, but with a low bit rate you may see a lot.

Here’s an example file with very few compression artifacts:

It was originally encoded at 10 Mbps. Make sure you’re in a darker environment when trying to view this or you might not see the artifacts.

And here’s one originally encoded at 1 Mbps:

The goal is to give you a hint of where to look in the compressed file. Once you see where to look, turn the composite mode back to normal and watch your show. Then you can decide whether the bit rate is a good compromise.

Michael Guncheon is an accomplished editor who has cut a wide range of projects, including music videos for Prince, a documentary on Genesis, and numerous commercials and corporate pieces. A partner at HDMG, a Minneapolis video production and post-production company, Guncheon has written several books on DSLRs and is the author of the Kodak Digital Photo Guide. He has presented his talk on shooting with HDSLRs at Twin Cities Public Television, WGBH in Boston, PBS in New York, the Hollywood Post Alliance and at the annual SMPTE conference in Hollywood.

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