MicroUI library defines a Low Level UI framework for embedded devices. This module allows the creation of basic Human-Machine-Interfaces (HMI), with output on a pixel-based screen.


MicroUI library is the entry point to perform some drawings on a display and to interact with user input events. This library contains only a minimal set of basic APIs. High-level libraries can be used to have more expressive power, such as MWT (Micro Widget Toolkit). In addition to this restricted set of APIs, the MicroUI implementation has been designed so that the EDC and BON footprint is minimal.

At application startup all MicroUI objects relative to the I/O devices are created and accessible. The following MicroUI methods allow you to access these objects:

MicroUI is not a standalone library. It requires a configuration step and several extensions to drive I/O devices (display, inputs, LEDs).

First, MicroUI requires a configuration step in order to create these internal objects before the call to the main() method. The chapter Static Initialization explains how to perform the configuration step.


This configuration step is the same for both embedded and simulated VEE Ports.

The embedded VEE Port requires some additional C libraries to drive the I/O devices. Each C library is dedicated to a specific kind of I/O device. A specific chapter is available to explain each kind of I/O device.

MicroUI C libraries
I/O devices Extension Name Chapter
Graphical / pixel-based display Display Display
Inputs (buttons, joystick, touch, pointers, etc.) Input Input

The simulation VEE Port uses a mock which simulates all I/O devices. Refer to the chapter Simulation.

Library ej.api.Drawing

This Foundation Library provides additional drawing APIs. This library is fully integrated in Display module.



The MicroUI implementation for MicroEJ uses one internal thread. This thread is created during the MicroUI initialization step, and is started by a call to MicroUI.start().


This thread has several roles:

  • It manages all display events (requestRender(), requestShow(), etc.).
  • It reads the I/O devices inputs and dispatches them into the event generators’ listeners. See input section: Input.
  • It allows to run some piece of code using the callSerially() method.


The thread is always running. The user has to count it to determine the number of concurrent threads the MicroEJ Core Engine can run (see Memory options in Standalone Application Options).


The thread cannot be stopped with a Java exception: the exceptions are always checked by the framework.

When an exception occurs in a user method called by the internal thread (for instance render()), the current UncaughtExceptionHandler receives the exception. When no exception handler is set, a default handler prints the stack trace.

Native Calls

The MicroUI implementation for MicroEJ uses native methods to perform some actions (read input devices events, perform drawings, turn on LEDs, etc.). The library implementation has been designed to not use blocking native methods (wait input devices, wait end of drawing, etc.) which can lock the full MicroEJ Core Engine execution.

The specification of the native methods is to perform the action as fast as possible. The action execution may be sequential or parallel because an action is able to use a third-party device (software or hardware). In this case, some callbacks are available to notify the end of this kind of parallel actions.

However some actions have to wait the end of a previous parallel action. By consequence the caller thread is blocked until the previous action is done; in other words, until the previous parallel action has called its callback. In this case, only the current Java thread is locked (because it cannot continue its execution until both actions are performed). All other Java threads can run, even a thread with a lower priority than current thread. If no thread has to be run, MicroEJ Core Engine goes in sleep mode until the native callback is called.


MicroUI provides several policies to use the antialiasing. These policies depend on several factors, including the kind of drawing and the display pixel rendering format. The main concept is that MicroUI does not allow you to draw something with a transparency level different from 255 (fully opaque). There are two exceptions: the images and the fonts.

For each pixel to draw, the antialiasing process blends the foreground color with a background color. This background color can be specified or not by the application:

  • specified: The background color is fixed by the application (GraphicsContext.setBackgroundColor()).
  • not specified: The background color is the original color of the destination pixel (a “read pixel” operation is performed for each pixel).


Drawing an image (a pre-generated image or an image decoded at runtime) which contains some transparency levels does not depend on the display pixel rendering format. During the image drawing, each pixel is converted into 32 bits by pixel format.

This pixel format contains 8 bits to store the transparency level (alpha). This byte is used to merge the foreground pixel (image transparent pixel) with the background pixel (buffer opaque pixel). The formula to obtain the pixel is:

\[{\alpha}Mult = ({\alpha}FG * {\alpha}BG) / 255\]
\[{\alpha}Out = {\alpha}FG + {\alpha}BG - {\alpha}Mult\]
\[COut = (CFG * {\alpha}FG + CBG * {\alpha}BG - CBG * {\alpha}Mult) / {\alpha}Out\]

The destination buffer is always opaque, so:

\[COut = (CFG * {\alpha}FG + CBG * (255 - {\alpha}Mult)) / 255\]


  • \({\alpha}\)FG is the alpha level of the foreground pixel (layer pixel),
  • \({\alpha}\)BG is the alpha level of the background pixel (working buffer pixel),
  • Cxx is a color component of a pixel (Red, Green or Blue),
  • \({\alpha}\)Out is the alpha level of the final pixel.


A font holds only a transparency level (alpha). This fixed alpha level is defined during the pre-generation of a font (see Fonts).

  • 1 means 2 levels are managed: fully opaque and fully transparent.
  • 2 means 4 levels are managed: fully opaque, fully transparent and 2 intermediate levels.
  • 4 means 16 levels are managed: fully opaque, fully transparent and 14 intermediate levels.
  • 8 means 256 levels are managed: fully opaque, fully transparent and 254 intermediate levels.


The antialiasing mode for the fonts concerns only the fonts with more than 1 bit per pixel (see Font Generator).


The MicroUI library is an additional module. In the VEE Port configuration file, check UI > MicroUI to install the library. When checked, the XML file microui/microui.xml is required during VEE Port creation in order to configure the module. This configuration step is used to extend the MicroUI library. Refer to the chapter Static Initialization for more information about the MicroUI Initialization step.


See MicroUI chapter in Application Developer Guide.