Font Engine Core


The Font Engine Core module is a built-in module of the MicroUI module (see MicroUI) for the application side; and is a built=in module of the Display module (see Display) for the C side.

Functional Description

Font Generation

Font Generation

Process overview:

  1. User uses the Font Designer module to create a new font, and imports characters from system fonts (*.ttf files) and / or user images (*.png, *.jpg, *.bmp, etc.).
  2. Font Designer module saves the font as a MicroEJ Font (*.ejf file).
  3. The user defines, in a text file, the fonts to load.
  4. The Font Generator outputs a raw file for each font to convert (the raw format is display device-dependent).
  5. The raw files are embedded as (hidden) resources within the MicroEJ Application. The raw files’ data are linked into the FLASH memory.
  6. When the MicroEJ Application creates a MicroUI DisplayFont object which targets a pre-generated image, the Font Engine Core only has to link from the MicroUI DisplayFont object to the data in the FLASH memory. Therefore, the loading is very fast; only the font data from the FLASH memory is used: no copy of the image data is sent to RAM memory first.
  7. When the MicroUI DisplayFont is no longer needed, it is garbage-collected by the platform, which just deletes the useless link to the FLASH memory.

Font Engine

Font Format

The font engine module provides fonts that conform to the Unicode Standard. The .ejf files hold font properties:

  • Identifiers: Fonts hold at least one identifier that can be one of the predefined Unicode scripts or a user-specified identifier. The intention is that an identifier indicates that the font contains a specific set of character codes, but this is not enforced.

  • Font height and width, in pixels. A font has a fixed height. This height includes the white pixels at the top and bottom of each character, simulating line spacing in paragraphs. A monospace font is a font where all characters have the same width; for example, a ‘!’ representation has the same width as a ‘w’. In a proportional font, ‘w’ will be wider than a ‘!’. No width is specified for a proportional font.

    Font Height

    Font Height

  • Baseline, in pixels. All characters have the same baseline, which is an imaginary line on top of which the characters seem to stand. Characters can be partly under the line, for example ‘g’ or ‘}’. The number of pixels specified is the number of pixels above the baseline.

    Font baseline

    Font baseline

  • Space character size, in pixels. For proportional fonts, the Space character (0x20) is a specific character because it has no filled pixels, and so its width must be specified. For monospace, the space size is equal to the font width (and hence the same as all other characters).

  • Styles: A font holds either a combination of these styles: BOLD, ITALIC, UNDERLINED, or is said to be PLAIN.

  • Runtime filters: Some fonts may allow the font engine to apply a transformation (in other words, a filter) on characters before they are displayed in order to provide some visual effect on characters (BOLD, ITALIC, UNDERLINED). Unless specified, a font allows the font engine to apply any of its filters.

  • When the selected font does not have a graphical representation of the required character, a rectangle is displayed instead. For proportional fonts, the width is one third of the height of the font.

    Default Character

    Default Character

Font Selection

The font engine implements the [MUI] selection semantics, and also tries to select fonts for which styles are built in, instead of applying a runtime filter. The font is selected based on the following process:

  1. Select fonts that define the specified identifier.
  2. Select within the step1 fonts, those whose height is the closest to the specified height.
  3. Select within the step2 fonts, those with built-in styles that match the specified styles.
  4. If more than one font is selected by the steps above, select those fonts that have the most built-in styles. If there is still more than one font, one is selected arbitrarily.

Runtime Transformation: Filters

The user interface extension font engine provides three runtime filters that may apply if the (currently selected) font allows it. The filters are:

The Three Font Runtime Style Transformations (filters).
Name Description Rendering sample
ITALIC Pixels on upper rows are shifted right. The higher a pixel is relative to the base line, the more it is right-shifted. ../_images/font-filter-italic.svg
BOLD 1 pixel is added to the right of each original pixel. ../_images/font-filter-bold.svg
UNDERLINED A line is displayed two pixels below the baseline position. ../_images/font-filter-underline.svg

Multiple filters may apply at the same time, combining their transformations on the displayed characters.

Pixel Transparency

The font engine renders the font according the the value stored for each pixel. If the value is 0, the pixel is not rendered. If the value is the maximum value (for example the value 3 for 2 bits-per-pixel), the pixel is rendered using the current foreground color, completely overwriting the current value of the destination pixel. For other values, the pixel is rendered by blending the selected foreground color with the current color of the destination.

If n is the number of bits-per-pixel, then the maximum value of a pixel (pmax) is 2^n – 1. The value of each color component of the final pixel is equal to:

foreground \* pixelValue / pmax + background \* (pmax - pixelValue) / pmax + adjustment

where adjustment is an adjustment factor specified in the board support package of the platform.

Font Identifier

All fonts are loaded at MicroUI startup. Before loading a font, the Image Engine Core module first tries to attribute a unique identifier to the future loaded font. This identifier will be used to retrieve the font after the loading step, in order to draw it and to free it.

An identifier targets a font file (an ejf raw file), which can contain until eight DisplayFont inside. To prevent some C allocation at runtime, the number of identifiers is allocated at compile-time. By consequence, the available number of identifiers is limited. The MicroEJ launcher of the MicroEJ Application has to specify the number of identifiers (refer to the chapter Application Options (Target > Memory) to have more information where specify this number of identifiers.)


This number has to include the number of system fonts. A system font is a font file specified during the MicroUI static initialization step (see Static Initialization).

When the limit of identifiers is reached, the MicroUI library throws an error, and the non-loaded fonts are unusable.

Arabic Support

The font engine manages the ARABIC font specificities: the diacritics and contextual letters. Contrary to the LATIN fonts, some ARABIC characters can overlap another character. When a character must overlap the previous character in the text, the font engine repositions the X coordinate before rendering the new character (instead of placing the next character just after the previous one).

To render an Arabic text, the font engine requires several points:

  • To determinate if a character has to overlap the previous character, the font engine uses a specific range of ARABIC characters: from 0xfe70 to 0xfefc. All others characters (ARABIC or not) outside this range are considered classic and no overlap is performed. Note that several ARABIC characters are available outside this range, but the same characters (same representation) are available inside this range.

  • The application strings must use the UTF-8 encoding. Furthermore, in order to force the use of characters in the range 0xfe70 to 0xfefc, the string must be filled with the following syntax: ‘\ufee2\ufedc\ufe91\u0020\ufe8e\ufe92\ufea3\ufeae\ufee3’; where \uxxxx is the UTF-8 character encoding.

  • The application string and its rendering are always performed from left to right. However the string contents are managed by the application itself, and so can be filled from right to left. To write the text:


    the string characters must be : ‘\ufee2\ufedc\ufe91\u0020\ufe8e\ufe92\ufea3\ufeae\ufee3’. The font engine will first render the character ‘\ufee2’, then ‘\ufedc,’ and so on.

  • Each character in the font (in the ejf file) must have a rendering compatible with the character position. The character will be rendered by the font engine as-is. No support is performed by the font engine to obtain a linear text.

External Resources

The Font Engine Core 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 Engine Core 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 during the full MicroEJ Application time. Then, on MicroEJ Application demand, the Font Engine Core 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 Engine Core no longer needs them.



The Font Engine Core modules are 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.