Recording a 5k display

Daniel GibbsNews and Updates

If you have one of the few known 5K displays (the most likely being an iMac), you may have tried to perform a 5K screen recording but not been particularly happy with the result. This article talks about some of the technical limitations around recording 5K displays and compares the size and frame rate of various ways of recording a 5K display.

In case you can’t be bothered reading the whole article, the takeaway is: for maximum performance (high frame rate), don’t record in retina resolution. In iShowU Instant you can do this by making sure that the Use retina resolution checkbox is unchecked on the Output tab in Advanced Mode.

Technical limits

As a baseline, let’s look at QuickTime Player. You can take basic screen recordings in QuickTime Player by selecting New Screen Recording from the File menu or by pressing ⌃⌘N. However, when you perform a full screen recording of a 5K display using this method, the resulting video is only 4096 x 2304! This is because the H.264 video encoder has a maximum size limit of 4096 x 2304 pixels; it simply can’t encode anything higher than that. Apple have chosen to handle this by limiting the output size of the recording to this size as well, regardless of the screen size.

In iShowU Instant, we do this a bit differently; if the recording size is greater than 4096 x 2292 (there are irrelevant technical details for the slight difference here), it will automatically switch to using JPEG for the video encoder instead of H.264. This means that if you record a 5K display (with retina recording turned on), you will get a 5K video. The downside is that JPEG-encoded recordings are much larger.

Testing

To illustrate the difference between different ways of recording 5K displays we’ve done some test recordings and have compared the file sizes and average frame rates of the results:

  • QuickTime Player “5K” – A full screen recording in QuickTime Player. This ends up being a 4096 x 2304 recording
  • iShowU Instant 5K – A full screen retina recording in iShowU Instant. This uses the JPEG encoder instead of H.264.
  • iShowU Instant <5K – A 4096 x 2292 recording in iShowU Instant. This is the largest H.264 recording possible with iShowU Instant.
  • iShowU Instant non-retina – A full screen non-retina recording in iShowU Instant. This still records the whole screen, but at a resolution of 2560 x 1440.

For each of the different recording methods we performed three different recordings:

  • Desktop – A still desktop with the mouse cursor moving around
  • Web Browser – A web browser scrolling through content on Google News
  • Minecraft – Full screen gameplay of the game Minecraft

Images of the tests used when recording a 5K display

Each recording was taken on a 5K iMac, was ~30 seconds long, and those taken in iShowU Instant had the codec set to Automatic and the quality set to the default of 90%.

Test results

For the Desktop and Web Browser tests, QuickTime Player came out on top in terms of frame rate for retina recordings with iShowU Instant’s recording of almost the same size coming pretty close. iShowU Instant’s 5K recording didn’t match these in terms of frame rate (and the file size was humongous) but was the only recording that was truly 5K.

Surprisingly in the Minecraft test, a fairly graphically intensive recording, both iShowU Instant’s 5K recording and <5K recording both had higher frame rates than the QuickTime Player recording.

The overall winner in terms of frame rate for all three tests was, of course, iShowU Instant’s non-retina recording. Recording the whole screen (2560 x 1440) at normal resolution resulted in realtime frame rates of almost 30 fps!

Desktop Web Browser Minecraft
QuickTime Player “5K”
Encoder: H.264
Size: 4096 x 2304 (retina)
File size: 81MB
Frame rate: 14.05 fps
File size: 76.3MB
Frame rate: 11.72 fps
File size: 128.1MB
Frame rate: 3.63 fps
iShowU Instant 5K
Encoder: JPEG
Size: 5120 x 2880 (retina)
File size: 754MB
Frame rate: 5.54 fps
File size: 465.3MB
Frame rate: 8.43 fps
File size: 615MB
Frame rate: 5.97 fps
iShowU Instant < 5K
Encoder: H.264
Size: 4096 x 2292 (retina)
File size: 49.6MB
Frame rate: 12.35 fps
File size: 29.5MB
Frame rate: 11.53 fps
File size: 62.5MB
Frame rate: 4.2 fps
iShowU Instant non-retina
Encoder: H.264
Size: 2560 x 1440
File size: 56.1MB
Frame rate: 26.76 fps
File size: 36.3MB
Frame rate: 23.38 fps
File size: 69.1MB
Frame rate: 29.44 fps

Disclaimer: The above tests were not done in any scientifically valid way; I did a single recording for each test, so take the results with a grain of salt.

Summary

If you’re looking for high frame rate recordings, consider recording a normal resolution (instead of retina); however, if you must have 5K, try iShowU Instant but be prepared for huge file sizes and lower frame rates.