Recording screencasts on a HiDPI display
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Before recording a screencast, choose the target platform. YouTube is a great way to get started because it serves videos at zero bandwidth cost. Each platform has a preferred aspect ratio and resolution for video uploads. YouTube’s default aspect ratio is 16:9. The pixel ratio of width to height in your final video should be 16:9. The following table shows recommended resolutions for YouTube:
| Name | Alternate name | Resolution |
|---|---|---|
| 4K | 2160p | 3840 by 2160 |
| Quad HD | 1440p | 2560 by 1440 |
| Full HD | 1080p | 1920 by 1080 |
| HD | 720p | 1280 by 720 |
You can refer to a video as Full HD if its aspect ratio is 16:9 with a resolution of at least 1920 by 1080. The resolution can be higher; for example, 2304 by 1296 is higher than Full HD but still falls short of Quad HD.
A coding screencast should be at least Full HD to ensure that the code is crisp and easy to read.
Scaled resolution vs. actual resolution
When you record on a HiDPI display, you must understand the difference between scaled and actual resolution. I record on the affordable LG Ultra HD 24UD58-B. Its physical-pixel resolution is 3840 by 2160. If macOS used this resolution directly, fonts would appear tiny and hard to read. Therefore, macOS uses a scaled resolution; for the LG Ultra HD the default scaled resolution is 1920 by 1080.
One scaled pixel corresponds to multiple physical pixels. On the LG Ultra HD with a 1920 by 1080 scaled resolution, each scaled pixel is represented by two physical pixels in width and two in height. The ratio between scaled and actual width/height pixels is called the device pixel ratio (DPR). In this example the DPR is 2, so one scaled pixel consists of four physical pixels, as shown in this graphic:
macOS always uses the default scaled resolution unless you choose a different one:
Depending on the scaled resolution you choose, you get a different DPR. DPR does not have to be an integer:
| Resolution | DPR | Description |
|---|---|---|
| 3840 by 2160 | 1 | Actual resolution (More Space) |
| 2560 by 1440 | 1.5 | Scaled Quad HD |
| 1920 by 1080 | 2 | Scaled Full HD (Default) |
Applications can use physical pixels instead of scaled pixels for parts of the screen, which is why photos and videos look crisp on HiDPI displays. It’s almost as if an application has more pixels up its sleeve that it can use when appropriate.
Why DPR matters for screen recording
In this article, egghead.io explains how to record a screencast. The article states that egghead.io serves videos in Full HD (1920 by 1080), yet recommends recording at 1280 by 720 (in HiDPI mode), which sounds contradictory.
Some screen recorders capture physical pixels instead of scaled pixels. They record 2560 by 1440 physical pixels when you record 1280 by 720 scaled pixels. The captured resolution is Quad HD, but you need a higher resolution to get 4K. Examples of screen recorders that capture physical pixels include QuickTime, ScreenFlow and Cleanshot X.
To serve 4K videos, you must record at 1920 by 1080 scaled resolution. With DPR 2, actual pixels for width and height double, resulting in 3840 by 2160 for the final video. On my LG Ultra HD, I have to record the entire screen.
When you choose a screen recorder, verify that it supports HiDPI displays and records actual pixels instead of scaled pixels.
Mind the aspect ratio on MacBooks
If you don’t have an external HiDPI display, you can record a screencast on your MacBook’s Retina display. On my MacBook Air M1 the aspect ratio is 16:10, not 16:9. As of macOS 12.4, there is no way to set a 16:9 resolution for the built-in display. Another surprise is that the DPR of the default scaled resolution is 2560/1440 = 16/9 = 1.7777778 (less than 2):
| Resolution | DPR | Description |
|---|---|---|
| 2560 by 1600 | 1 | Actual resolution |
| 1440 by 900 | 16/9 | Default scaled resolution |
To record a 16:9 screencast on a MacBook Air M1 with the default scaled resolution, you have two options:
- Record a 1280 by 720 portion: With a DPR of 16/9, the recording’s resolution would be 2275 by 1280, which is lower than what you get with the LG Ultra HD but still better than Full HD.
- Record a 1440 by 810 portion: This is the largest 16:9 area that fits on the MacBook Air M1 built-in display (using the default scaled resolution). With a DPR of 16/9, the recording’s resolution would be 2560 by 1440 (Quad HD, not 4K). Therefore, on a MacBook Air M1’s built-in display you cannot record a 4K screencast.
When you record a portion of your screen, you must place and size windows inside the recording area. I manage windows with Raycast’s window management extension, which makes placing windows on a desktop a breeze. However, window placement is relative to the entire desktop, so window managers are not suitable for placing windows precisely inside a recording area.
Record full screen 16:9 on a MacBook’s built-in display
Many screencast creators prefer recording the entire screen, as this Twitter thread shows. With my LG Ultra HD, it is no problem to record 16:9 full screen. But when you record on the built-in 16:10 MacBook display, macOS does not allow you to change the resolution to a 16:9 ratio. But you can always connect to an external display with a different resolution and mirror it to your built-in display.
BetterDummy can do this virtually: it lets you create a 16:9 dummy display and mirror it to your built-in display. You can then record 16:9 full screen on the mirrored dummy screen.