UEFI News and Commentary

Saturday, January 23, 2010

UEFI HII (Part 8): Proportional Fonts

The UEFI Specification provids services and storage for manipulating bitmap fonts as part of the Human Interface Infrastructure (HII). Last week, we looked at the Simple Font, which describes a means of storing a fixed-width font for Unicode character values. Each character in the Simple Font is either in an 8 x 19 (narrow) pixel or 16 x 19 (wide) pixel character cell.

Proportional fonts extend bitmapped font support in several significant ways:
  1. There can be multiple fonts, each described by its name, height and style (bold, italics, underline, etc.).
  2. Each character cell in the font can be a different width.
So, for UEFI, a font is a logical grouping of glyphs identified by a name, a size and a style.
  • Font Name. "The font name describes, in broad terms, the visual style of the font." How do you tell that a character is part of the Arial font? Or Times New Roman? Well, they tend to share certain visual characteristics, such as whether they have serifs, or whether they are rounded or angular or represent alphabetic characters or symbols. 
  • Font Size. "The font size describes the maximum height of the character cell, in pixels. The standard font always has the font size of 19." Rather than describing characters in terms of "points", the UEFI specification describes them in terms of the height of the character cell, in pixels. The UEFI specification does not use points, because (a) it only uses bitmapped fonts and (b) although points technically refers to 1/72nd of an inch, it actually does little to describe how much screen space a font will take up (except in comparison with another point size for the same font).
  • Font Style. "The font style describes standard visual modifies to the base visual style of a font." In other words, you can have Arial, Arial + Bold, Arial + Italic, Arial + Bold and Italic, etc. They are still an Arial font, but they have additional stylistic modifiers. The UEFI specification explicitly supports bold, italic, underline, double-underline, embossed, outline and shadowed.
The glyph is the image representation of the character, with each dot in the image represented by a single bit, on or off. The off pixels are either not drawn (for transparent drawing) or else drawn in the background color. The on pixels are drawn in the foreground color. For example, for the letter A, it might look like this:

But this isn't enough information to draw the glyph. For one thing, we don't know how much space to leave between this glyph and other glyphs that we have drawn. This is one of the key differences between this font and the simple font in the previous articles. Simple fonts have the empty space above, below, before and after the character image built into the glyph. Proportional fonts track this sort of information separately, using three key ideas:
  1. The character cell.
  2. The baseline.
  3. The character advance.
Character Cell

The character cell is a box that the glyph's image is positioned inside of. Each font has a character cell and each character glyph's image is placed relative to the top-left corner of that cell, or box. For example:

The baseline (or Offset Y) allows UEFI firmware to draw text from different fonts on the same line and still have everything line up correctly. The baseline is calculated as the number of pixels above the Origin of the tallest character image.


In this picture, the baseline is the height of the tallest character (A). Notice that the character 'q' starts 3 pixels below the baseline, while both the 'A' and 'x' start on the baseline and the quotation mark (") starts 8 pixels above the baseline. The gap between the Origin and the left-most pixel of the glyph is known as Offset X. For the 'A' and 'q' and quotation mark, this is 1 pixel. For the 'x', this is 0 pixels.

Character Advance
The character advance is the number of pixels to move the Origin to the right. The character advance determines how much whitespace to put to the left and right of the glyph.

All of these details about offsets and baselines, advances and cells may seem intimidating, especially in a firmware environment. Many basic tasks become a lot more difficult when using proportional fonts: drawing text, editing a string or selecting text (with a mouse!). Fortunately, the EFI_HII_FONT_PROTOCOL provides functions which not only draw a string to a buffer or to the screen, but also report back information about the width and height of each character drawn. We will investigate these in more detail later.

Next week we will return to how the fonts are encoded.

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