FreeType Glyph Conventions / V

V. Text Processing

This section demonstrate algorithms which use the concepts previously defined to render text, whatever layout you use. It assumes simple text handling suitable for scripts like Latin or Cyrillic, using a one-to-one relationship between input character codes and output glyphs indices. Scripts like Arabic or Khmer, which need a ‘shaping engine’ to do the character code to glyph index conversion, are beyond the scope (and should be done by proper layout engines like Pango anyway).

1. Writing simple text strings

In this first example, we will generate a simple string of text in the Latin script, i.e., with a horizontal left-to-right layout. Using exclusively pixel metrics, the process looks like:

1. Convert the character string into a series of glyph indices.
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. Increment the pen position by the glyph's advance width (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.

Note that kerning isn't part of this algorithm.

2. Subpixel positioning

This algorithm can be used for hinting modes that don't apply horizontal hinting. It essentially provides WYSIWYG text layout. Text rendering is very similar to the algorithm described in subsection 1, with the following few differences:

• The pen position is expressed in fractional pixels.

• The hinted outline must be shifted horizontally to the proper sub-pixel position.

• The advance width is expressed in fractional pixels, and isn't necessarily an integer.

Here an improved version of the algorithm:

1. Convert the character string into a series of glyph indices.
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 by ‘pen_pos - floor(pen_pos)’.
5. Render the glyph to the target device at ‘floor(pen_pos)’.
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.

3. Simple kerning

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:

1. Convert the character string into a series of glyph indices.
2. Place the pen to the cursor position.
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:

1. Convert the character string into a series of glyph indices.
2. Place the pen to the cursor position.
3. Get or load the glyph image.
4. Add the scaled unrounded kerning distance, if any, to the pen position.
5. Translate the glyph by ‘pen_pos - int(pen_pos)’.
6. Render the glyph to the target device at ‘int(pen_pos)’.
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.

4. Right-to-left layout

The process of laying out right-to-left scripts like (modern) Hebrew text is very 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 Hebrew glyphs).

Right-to-left algorithm 1:

1. Convert the character string into a series of glyph indices.
2. Place the pen to the cursor position.
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.

5. Vertical layouts

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).

Here the algorithm:

1. Convert the character string into a series of glyph indices.
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.