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How does Rewind compression work?
How does Rewind compression work?

Everything you need to know about Rewind's compression

Dan Siroker avatar
Written by Dan Siroker
Updated over a week ago

Storing all the recordings locally means compression is very important. We compress raw recording data up to 3,750x times so even with the smallest hard drive you can buy from Apple today, you can store years of recordings.

How much storage is Rewind likely to consume?

Looking at real-world usage for users who have used the product for at least three months we've found that:

  1. On average users use 14 GB per month.

  2. Some users use as low as 1 GB per month.

  3. Others use as high as 39 GB per month.

These differences are based on the factors described below. We hope to improve this over time. See "Will you continue to improve compression?"

What compression algorithm do you use?

While the details of our compression algorithm are a trade secret, we recognize that it seems too good to be true so we'll share one key insight: the amount of data that changes frame-to-frame when recording your screen is often remarkably redundant. In other words, not much changes. If we only store the information related to the changes, not all of the data, you can save a ton of space.

What factors impact storage size?

The key factors that impact storage size are:

  1. Resolution of the compressed data. For example, a 6K external display has significantly more pixels than a laptop.

  2. Frame rate. We currently capture frames every two seconds.

  3. Redundancy. The amount of redundant information frame-to-frame (see prior question) has a big impact on storage size. For example, if you record an hour using Rewind and nothing changes on your screen very little storage is needed. If instead you record an hour of a full-motion full-screen movie the amount of storage needed would be dramatically higher.

  4. Hours recorded. The more you record with Rewind, the more storage it will consume.

How did we measure our 3,750x compression ratio?

We have several representative test sets we have used to internally benchmark our compression. For example, one test set has 1,302 frames which is 10.5GB of raw recording data. We are able to compress that data down to 2.8MB. If you divide 10.5GB by 2.8MB you get a 3,750x compression ratio.

What controls do I have over how much data is stored?

If you go to "Preferences" and then "Storage" you can change how long recordings are retained. If you reduce the retention period, you will save more space. This will also limit how far back you can search:

Will you continue to improve compression?

We have a long roadmap of ideas for how to further improve compression and are working on a way for you to choose where to store your data. If this is important to you, reach out to [email protected]. We will prioritize these ideas based on your feedback. Please share your experience with us!

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