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The LVGL project library

A project log for Portable Foldable Scoreboard

A portable, scalable and foldable display, that can be used as a Scoreboard or Timekeeper in outdoor sport events, with minimal setup

LordGuillyLordGuilly 06/06/2021 at 09:360 Comments

The scoreboard graphic requirements are simple, as it's just a couple of digits. But I didn't want to reinvent the wheel, so started looking at the available libraries in the internet.

Basic requirements were

- simple primitives for displaying numbers

- availability for embedded systems, or easy to port if not

- small memory footprint.

It's not that I did a massive research, as I quickly found the LVGL project, which ticked most of the boxes. The website (https://lvgl.io/) described it as "GUI" oriented, and showed a lot of fancy things that I didn't need, but I decided to give it a go.

The library resource used for the digits is called label, and each one need its own style defined. There is a hierarchy of styles, and they are inherited, so with one style per label, the individual colours or effects can be achieved. 

A label can be altered/changed wtih an animation. It's a simple way to, for example, move in, move out, scale, etc. At the end of the animation, a finish callback is called. By starting a new animation on it, you can do more nice and ellaborated things

Once the basic setup is done, the library requires a periodic call to a task handler, lv_task_handler(). The period between calls needs to be reported by another API, lv_tick_inc(). The handler will call a "flush" callback if the graphic buffer changed (as a result of an animation, or an event).

The flush callback is optimized to report smaller sections of the screen, to save time and memory. But that's not possible with the WS2812 driver method. So the flush callback simply sets a flag that will trigger a full display update in a different function.

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