How much disk does a screen memory use?
A transparent estimate for any app that remembers your screen, from your own habits — screen time, display, how often a picture is kept and how it’s stored. The method is explained below; change any assumption you disagree with.
How much disk would a screen memory use?
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This is a general estimate, not a measurement of DejaVu. We will publish DejaVu’s measured figures before launch.
How this estimate works
The model is deliberately simple, so you can check it:
pictures kept per day = screen time × 3,600 ÷ seconds between pictures × share of time the screen changes
size per day = pictures kept per day × size of one picture × 1.08 (text and search index)
- Share of time the screen changes. Apps that keep a picture only when the screen changes skip still moments. We use 30% for mostly reading, 55% for mixed use and 85% for lots of video and scrolling.
- Size of one picture, video method. For compressed video (HEVC) of screen content, we assume about 0.1 bit per pixel for each picture kept — roughly 28 KB for a 1080p frame — and scale it with the number of pixels you keep. That is our assumption, not a measurement, and conservative: video usually grows more slowly than the pixel count. As a sanity check, keeping full Retina resolution all day lands in the same range as the roughly 12–14 GB a month reported for Rewind, which also recorded audio.
- Size of one picture, screenshot method. Separate screenshots (JPEG) don’t benefit from the similarity between frames. Set the slider to the size of a typical screenshot, or measure one of yours: only its size is read, in your browser.
- Half resolution. Many apps keep pictures at a lower resolution than a Retina display, since text stays readable. Half resolution means a quarter of the pixels.
- Text and search index. Text takes little space compared with pictures; we add 8% for it and its index.
What the estimate leaves out
- Retention. Most apps let you set a limit or keep pictures for a shorter time than text. Your real usage is capped by that limit.
- Several displays. Multiply by the number of displays an app remembers.
- Audio. Apps that also record audio need more space. DejaVu never records audio.
How DejaVu keeps it light
DejaVu keeps a picture only when your screen actually changes, recognises videos and animations so it doesn’t keep every frame of them, and stores pictures as hardware-encoded HEVC video, sealed frame by frame. You choose a disk limit; past it, the oldest memories go first. The details are in How DejaVu decides what to keep. Our own measured figures will come before launch — not before.
Questions
How much space did Rewind use?
According to The Sweet Setup’s review, Rewind estimated about 14 GB a month; the reviewer measured roughly 12 GB a month over two months. Rewind also recorded audio, and usage varied a lot with screen time.
Is this how much DejaVu will use?
No. This is a general model, not a measurement of DejaVu. We will publish DejaVu’s figures, measured on a MacBook Air, before launch.
Why does video take so much less space than screenshots?
Video encoders store what changed between frames rather than every frame in full. On a screen, most pixels don’t change from one moment to the next, so consecutive frames cost very little.
Does this tool upload my screenshot when I measure one?
No. It reads only the file’s size, in your browser. The picture itself is never opened or sent.
Sources
- A First Look at Rewind.ai, The Sweet Setup
Public information as of the date above. If something has changed, write to contact@godejavu.app and we’ll correct it.