Font Characteristics

Font Format

The Font Engine 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, or is said to be PLAIN.

  • When the selected font does not have a graphical representation of the required character, the first character in font is drawn instead.

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

Pixel Transparency

The Font Renderer 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:

\[foreground * pixelValue / pmax + background * (pmax - pixelValue) / pmax\]

Language

Supported Languages

The Font Renderer manages the Unicode basic multilingual languages, whose characters are encoded on 16-bit, i.e. Unicodes from 0x0000 to 0xFFFF. It allows to render left-to-right or right-to-left writing systems: Latin (English, etc.), Arabic, Chinese, etc. are some supported languages. Note that the rendering is always performed left-to-right, even if the string are written right-to-left. There is no support for top-to-bottom writing systems. Some languages require diacritics and contextual letters; the Font Renderer manages simple rules in order to combine several characters.

Arabic Support

The Font Renderer manages the ARABIC font specificities: the diacritics and contextual letters.

To render an Arabic text, the Font Renderer requires several points:

  • To determinate if a character has to overlap the previous character, the Font Renderer uses a specific range of ARABIC characters: from 0xfe70 to 0xfefc. All other 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:

    ../_images/arabic.png

    the string characters must be : ‘\ufee2\ufedc\ufe91\u0020\ufe8e\ufe92\ufea3\ufeae\ufee3’. The Font Renderer 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 Renderer as-is. No support is performed by the Font Renderer to obtain a linear text.