FreeType Glyph Conventions

It is somewhat useful to use sub-pixel positioning when rendering text. This is crucial, for example, to provide semi-WYSIWYG text layouts. Text rendering is very similar to the algorithm described in sub-section 1, with the following few differences :

• The pen position is expressed in fractional pixels.
• Because translating a hinted outline by a non-integer distance will ruin its grid-fitting, the position of the glyph origin must be rounded before rendering the character image.
• The advance width is expressed in fractional pixels, and isn't necessarily an integer.

Which finally looks like :

1. convert the character string into a series of glyph indexes.
2. place the pen to the cursor position. This can be a non-integer point.
3. get or load the glyph image.
4. translate the glyph so that its 'origin' matches the rounded pen position.
5. render the glyph to the target device
6. increment the pen position by the glyph's advance width in fractional pixels.
7. start over at step 3 for each of the remaining glyphs
8. when all glyphs are done, set the text cursor to the new pen position

Note that with fractional pixel positioning, the space between two given letters isn't fixed, but determined by the accumulation of previous rounding errors in glyph positioning.

Adding kerning to the basic text rendering algorithm is easy : when a kerning pair is found, simply add the scaled kerning distance to the pen position before step 4. Of course, the distance should be rounded in the case of algorithm 1, though it doesn't need to for algorithm 2. This gives us :

Algorithm 1 with kerning:

3) get or load the glyph image.
4) Add the rounded scaled kerning distance, if any, to the pen position
5) translate the glyph so that its 'origin' matches the pen position
6) render the glyph to the target device
7) increment the pen position by the glyph's advance width in pixels
8) start over at step 3 for each of the remaining glyphs

Algorithm 2 with kerning:

3) get or load the glyph image.
4) Add the scaled unrounded kerning distance, if any, to the pen position.
5) translate the glyph so that its 'origin' matches the rounded pen position.
6) render the glyph to the target device
7) increment the pen position by the glyph's advance width in fractional pixels.
8) start over at step 3 for each of the remaining glyphs

Of course, the algorithm described in section IV can also be applied to prevent the sliding dot problem if one wants to..

The process of laying out arabic or hebrew text is extremely similar. The only difference is that the pen position must be decremented before the glyph rendering (remember : the advance width is always positive, even for arabic glyphs). Thus, algorithm 1 becomes :

Right-to-left Algorithm 1:

3) get or load the glyph image.
4) Decrement the pen position by the glyph's advance width in pixels
5) translate the glyph so that its 'origin' matches the pen position
6) render the glyph to the target device
7) start over at step 3 for each of the remaining glyphs

The changes to Algorithm 2, as well as the inclusion of kerning are left as an exercise to the reader.
 
 

Laying out vertical text uses exactly the same processes, with the following significant differences :
 

The baseline is vertical, and the vertical metrics must be used instead of the horizontal one.

The left bearing is usually negative, but this doesn't change the fact that the glyph origin must be located on the baseline.

The advance height is always positive, so the pen position must be decremented if one wants to write top to bottom (assuming the Y axis is oriented upwards).

Through the following algorithm :

1) convert the character string into a series of glyph indexes.
2) place the pen to the cursor position.
3) get or load the glyph image.
4) translate the glyph so that its 'origin' matches the pen position
5) render the glyph to the target device
6) decrement the vertical pen position by the glyph's advance height in pixels
7) start over at step 3 for each of the remaining glyphs
8) when all glyphs are done, set the text cursor to the new pen position

As you probably know, the acronym WYSIWYG stands for 'What You See Is What You Get'. Basically, this means that the output of a document on the screen should match "perfectly" its printed version. A true wysiwyg system requires two things :

device-independent text layout

Which means that the document's formatting is the same on the screen than on any printed output, including line breaks, justification, ligatures, fonts, position of inline images, etc..

matching display and print character sizes

Which means that the displayed size of a given character should match its dimensions when printed. For example, a text string which is exactly 1 inch tall when printed should also appear 1 inch tall on the screen (when using a scale of 100%).

It is clear that matching sizes cannot be possible if the computer has no knowledge of the physical resolutions of the display device(s) it is using. And of course, this is the most common case ! That's not too unfortunate, however  because most users really don't care about this feature. Legibility is much more important.

When the Mac appeared, Apple decided to choose a resolution of 72 dpi to describe the Macintosh screen to the font sub-system (whatever the monitor used). This choice was most probably driven by the fact that, at this resolution, 1 point = 1 pixel. However; it neglected one crucial fact : as most users tend to choose a document character size between 10 and 14 points, the resultant displayed text was rather small and not too legible without scaling. Microsoft engineers took notice of this problem and chose a resolution of 96 dpi on Windows, which resulted in slightly larger, and more legible, displayed characters (for the same printed text size).

These distinct resolutions explain some differences when displaying text at the same character size on a Mac and a Windows machine. Moreover, it is not unusual to find some TrueType fonts with enhanced hinting (tech note: through delta-hinting) for the sizes of 10, 12, 14 and 16 points at 96 dpi.
 

As for device-independent text, it is a notion that is, unfortunately, often abused. For example, many word processors, including MS Word, do not really use device-independent glyph positioning algorithms when laying out text. Rather, they use the target printer's resolution to compute hinted glyph metrics for the layout. Though it guarantees that the printed version is always the "nicest" it can be, especially for very low resolution printers (like dot-matrix), it has a very sad effect : changing the printer can have dramatic effects on the whole document layout, especially if it makes strong use of justification, uses few page breaks, etc..

Because the glyph metrics vary slightly when the resolution changes (due to hinting), line breaks can change enormously, when these differences accumulate over long runs of text. Try for example printing a very long document (with no page breaks) on a 300 dpi ink-jet printer, then the same one on a 3000 dpi laser printer : you'll be extremely lucky if your final page count didn't change between the prints ! Of course, we can still call this WYSIWYG, as long as the printer resolution is fixed !!

Some applications, like Adobe Acrobat, which targeted device-independent placement from the start, do not suffer from this problem. There are two ways to achieve this : either use the scaled and unhinted glyph metrics when laying out text both in the rendering and printing processes, or simply use wathever metrics you want and store them with the text in order to get sure they're printed the same on all devices (the latter being probably the best solution, as it also enables font substitution without breaking text layouts).

Just like matching sizes, device-independent placement isn't necessarily a feature that most users want. However, it is pretty clear that for any kind of professional document processing work, it is a requirement.