Review: Onyx Boox Tab Ultra C

Here you will find my first blogpost about new eink tablet that I have recently purchased.

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The screen is 10.3“ Eink Kaleido 3 which offers 300 PPI for black color and 150 PPI for colors. This makes total resolution equivalent of 2480x1860 for blacks and 1240x930 for colors. It is capable of producing 16 shades of gray. This resolution of black combined with passive color filter provides up 4096 total combinations of color. This screen combined with dedicated GPU provides much better eink experience than old e-readers I’ve tried before. It is so fast that I am able to write this article inside Termux in doom Emacs. But it is not as fast or smooth as traditional displays. There is also quite noticeable latency, but it is usable.




The tablet is equipped with 6300 mAh Li-ion. But even with this big battery the tablet doesn’t last as much as previous e-ink devices which could last months on single charge. With this tablet you usually use around 7% per hour so you can expect whole day of work without problem. It is a e-ink tablet after all.

Screen refresh modes

Tab Ultra C provides 5 refresh modes for it’s eink screen. The full quality is shown in HD mode and REGAL mode (not available in system applications) but they flash the screen quite a bit and they are not fast. For more interactive work there are 3 modes: BALANCED, FAST, ULTRAFAST. These modes are fast enough for emergency video watching, scrolling on the web or writing blogpost on it.

For the faster refresh rate you pay with image degradation (It feels like it is only few shades of color) and more ghosting, but you don’t get full black rectangle flashes on your screen. In my case I use mostly ultrafast mode for anything interactive when I don’t need that much image quality (although even in ultrafast the quality is high). For reading or writing with stylus I choose HD or REGAL modes since I don’t need to refresh the screen so much anyway (for books where I turn quickly I might use balanced).

Ghosting (HD/REGAL)

The difference between HD and REGAL mode is that REGAL mode is proprietary function and algorithms that are optimized for the colored eink displays to minimize ghosting. From Onyx:

Regal Mode: As a refresh solution with less ghosting and better display quality especially optimized for color E Ink models, it is ideal for page-turn between color illustrations such as in comics and manga apps.

I was usually reading manga in Tachiyomi with balanced settings. When turning it to HD mode I notice bad ghosting for color (and even a bit of ghosting for grays), with the REGAL mode there is no ghosting visible. With the balanced mode the ghosting is not as bad as in HD but it is visible, although with the effect of dithering it is not as much intrusive as HD mode. When using swipe for full refresh in HD mode the ghosting is gone too.

First strike bug

What I found really bad is the notorious first strike bug that many users have already reported but haven’t been fixed. When writing with stylus in the faster refresh modes (balanced, fast, ultrafast) the first strike is missing first 1 second of writing on the screen. The application is working just okay – when you do full refresh or just move the screen it redraws the first strike perfectly. In HD mode it is not as noticeable but it is still there. My theory is that it is caused due to the delay of activating the drawing overlay for instant previews. It would be really great to see this bug fixed.

Note taking

Native applications

Using the native application for notes or the noting capabilities in pdfs and epubs feels nearly perfect. There are 5 brushes available with 5 gray levels and 11 colors.


Taking notes is simple and it is low-latency. When using more layers of colors there might be a little glitches but everything goes back to normal on refresh/redraw. There is option for syncing with onyx cloud or automatically export to either vector or bitmap pdf. Alternatively there is also manual exporting to png or .note (proprietary native onyx format for notes). There are even various modes for eraser (bitmap, strokes, laso). It really feels like drawing on high-end device (or paper).

Non-native applications

If you want to draw on the tablet then the native applications are not enough. I have tried krita and sketchbook. They work fine but with considerable latency and limited refresh rate due to eink and no optimizations.

There are 3 more applications that claim that are optimized for eink: OneNote, Evernote and WPS office. The optimization consists of overlay which only draws like default ball-pen and on movement or redraw it redraws the screen with rendering from the app. This was if you are using a huge colorful highlighter you will firstly just see small black line and with redraw it will expand to it’s final image. In my opinion, not very usable. A little pity.

Calibration of stylus

If your device doesn’t register your stylus correctly there is a setting to calibrate it. Note that if you are using more styluses, you can have only one calibration profile. Mine pen was kinda fine with factory settings, but I have wanted more precision when writing so I tried many times to calibrate it properly. It is better but it still doesn’t feel perfect.

With the purchase of the tablet they allow users to register for onyx cloud and get 10 GB account. In my case the region was Europe and hence the instead of so I will be describing my experience with that.

Exporting notes to pdf

When I am exporting pdf online hand-written document with text areas the text overflows the page itself which is not supposed to happen, exporting it on the tablet itself works. Working with text areas is in the current state problematic due to poor word wrapping and inconsistencies between the export and what image shown.

SCRCPY and Weylus

Since the tablet runs Android 11 we can do screen sharing. I’ve first found the project Weylus which allows using TUC as normal Wacom graphical tablet. Overall great idea but I wasn’t able to use it reliably. I suspect the older hardware.

Then I have came across scrcpy which turns the problem around. The writing is done on the tablet and scrcpy shares the tablet’s screen to laptop which it can be used as webcam. This way I don’t need to consider the latency between laptop and tablet as writing shows immediately on my tablet. The only downside is that I am limited to the tablet applications.

Screenshots for this blogpost were took on my laptop via scrcpy. s


I really enjoy reading books on this tablet. I have been primarily using the onyx ereader for my reading although I have used for some epubs KoReader from f-droid. The main reason for using KoReader was bug in the new v2 epub engine which couldn’t open my epubs for one series, other books were just fine in eReader. For manga I have installed Tachiyomi which works well (but I haven’t read much manga lately anyway).


Magnetic keyboard cover

With the tablet I have also bought the magnetic keyboard cover from Onyx. I have thought that the case would provide some degree of protection to this fragile eink device, but sadly that is not really the case. Since the whole case is magnetic, there are not many options how it even would protect it. What is does provide is hard cover for the screen (but that is not all there is to protection).

What it does very well is provide laptop experience for the eink device. The keyboard is very pleasant for it’s small size. I was surprised how well I could write on it (coming from laptop keyboard). I have no problem reaching 90 WPM even with the screen latency. It is quite highly priced, but comparing the keyboard case to the basic one I think it is more value for buck.

What is a little sad about the cover is that it cannot be used like protective case - the keyboard gets in the way. Also when you just lay it on the desk the magnets holding the tablet disturb the stylus making the writing experience a little wobbly (but I would say usable).


What I really like is that with Android 11 I am not so much dependent on just the Onyx ecosystem. What I would like is the ability to backup the .note automatically so I would be able to sync it with Syncthing (with some Linux software being able to render them, that would be awesome).

There are still things that need some polishing and features that would be really helpful like shortcut to pdf or notebook as icon on the homepage. Currently this not available and the closest thing is recent file sin Library or in Notes.

Overall I am really happy about this little tablet. Something niche for Klimi. With the addition of Termux, It really is full laptop/tablet experience while keeping the eink novelty of distraction free world.

For comparison how colors look good in sun this is the screenshot with similar content as the first image:


Update [2024-04-11 Thu]:

Stylus eraser

Recently I noticed that my stylus eraser does not align with the rest of the body of the stylus. I tried unscrewing it and it went without much force.


Figure 1: Unscrewed eraser with 2 turns

With a few turns I could disassemble the stylus and reveal another stylus! This one works as eraser even if I pull it out of the body. I was wondering how it could work, but this whole mechanism surprised me.


Figure 2: Disassembled stylus eraser

Date: 2023-07-28 Fri 00:00