Font Manager and the notorious FMSwapFont

As mentioned earlier, the next target is to get QuickDraw text rendering to work. For this purpose, we’ve been working on important part required by it, The Font Manager. The most important interface between QuickDraw and Font Manager is the FMSwapFont trap, which takes the FMInput record, and outputs an FMOutput record. On a real Mac, the font substitution has a bunch of special cases which we luckily don’t need to worry about; for example, we don’t need to worry about choosing right font to display the disk-switch alert. However, there are a number of substitutions we need to support:

• Substitute non-present font to default font
• Choosing nearest-size font strike to substitute a non-present (scaled) size of a font
• FOND tables
• Fallback to old FONT resources in case requested FOND does not exist (using the familiar family * 128 + size formula)
• Etc.

Getting the strike loaded and on the screen

Majority of time spent for this milestone so far was investigating and implementing the quirks of the previously mentioned FMSwapFont trap. After finding the correct (or near-correct) strike, we finally got a FONT resource which we could work on. Of course, this meant that as first step dump the raw font bitmap on the screen buffer byte-by-byte 🙂

Of course getting the actual text to output, each of those characters would have to be individually masked and blitted to the correct location. For the first test, the original QuickDraw luckily appears to have “optimized” way to blitting non-clipped, non-scaled, non-styled srcOr-mode text directly to the screen, without having to go through offscreen buffer for styling and clipping. Implementing this was also pretty straightforward, resulting in the following tidy output using different fonts: