Font Renderer


The Font Renderer is included in the MicroUI module (see MicroUI) for the application side; and is included in the Display module (see Display) for the C side.

Functional Description

The Graphics Engine redirects all MicroUI font drawings to the internal software algorithms. There is no indirection to a set of Low Level API.

digraph {

   graph [

   node [

   edge [

   app [label="Painter API"]
   stack [label="Graphics Engine"]
   algo [label="Software Algorithms"]
   hard  [label="hardware"]

   app -> stack -> algo -> hard

External Resources

Memory Management

The Font Renderer is able to load some fonts located outside the CPU addresses’ space range. It uses the External Resource Loader.

When a font is located in such memory, the Font Renderer copies a very short part of the resource (the font file) into a RAM memory (into CPU addresses space range): the font header. This header stays located in RAM until MicroEJ Application is using the font. As soon as the MicroEJ Application uses another external font, new font replaces the old one. Then, on MicroEJ Application demand, the Font Renderer loads some extra information from the font into the RAM memory (the font meta data, the font pixels, etc.). This extra information is automatically unloaded from RAM when the Font Renderer no longer needs them.

This extra information is stored into a RAM section called .bss.microui.display.externalFontsHeap. Its size is automatically calculated according to the external fonts used by the firmware. However it is possible to change this value by setting the MicroEJ application property ej.microui.memory.externalfontsheap.size. This option is very useful when building a kernel: the kernel may anticipate the section size required by the features.


When this size is smaller than the size required by an external font, some characters may be not drawn.

Configuration File

Like internal resources, the Font Generator uses a configuration file (also called the “list file”) for describing fonts that need to be processed. The list file must be specified in the MicroEJ Application launcher (see Application Options). However, all the files in the application classpath with the suffix .fontsext.list are automatically parsed by the Font Generator tool.


This chapter describes the steps to open an external resource from the application:

  1. Add the font in the application project (most of the time in the source folder src/main/resources and in the package fonts).
  2. Create / open the configuration file (usually application.fontsext.list).
  3. Add the relative path of the font: see Fonts.
  4. Launch the application: the Font Generator converts the font in RAW format in the external resources folder ([application_output_folder]/externalResources).
  5. Deploy the external resources in the external memory (SDCard, flash, etc.).
  6. (optional) Update the implementation of the External Resources Loader.
  7. Build and link the application with the BSP.
  8. The application loads the external resource using Font.getFont(String).
  9. The font loader looks for the font and only reads the font header.
  10. (optional) The external resource is closed if the external resource is inside the CPU addresses’ space range.
  11. The application can use the font.
  12. The external resource is never closed: the font’s bytes are copied in RAM on demand (drawString, etc.).


The simulator (Front Panel) does not manage the external resources. All fonts listed in .fontsext.list files are generated in the external resources folder, and this folder is added to the simulator’s classpath.

Backward Compatibility

As explained here, the notion of Dynamic styles and the style UNDERLINED are not supported anymore by MicroUI 3. However, an external font may have been generated with an older version of the Font Generator; consequently, the generated file can hold the Dynamic style. The Font Renderer can load these old versions of fonts. However, there are some runtime limitations:

  • The Dynamic styles are ignored. The font is drawn without any dynamic algorithm.
  • The font style (the style returned by Font.isBold() and Font.isItalic()) is the Dynamic style. For instance, when a font holds the style bold as dynamic style and the style italic as built-in style, the font is considered as bold + italic; even if the style bold is not rendered.



The Font Renderer is part of the MicroUI module and Display module. You must install them in order to be able to use some fonts.


The MicroUI font APIs are available in the class ej.microui.display.Font.