/bitmap2ttf

Convert bitmap fonts to ttf fonts using font forge.

Primary LanguagePython

Bitmap2ttf converts (monochrome) bitmap fonts into ttf fonts.

There are three parts:

  • outliner.py:

    Traces each bitmap into a series of polygons.

  • convert.py:

    Makes the polygons into svg files, and writes a script for font forge to convert the svgs into a ttf font.

  • loader of your choice/implementation:

    • pcftotty.py:

      Loader for PCF bitmap fonts. Usage: pcftottf font.pcf font.ttf

    • amigatottf.py

      Loader for amiga diskfonts (eg topaz). Usage: amigatottf topaz/11 topaz.ttf Amiga diskfonts are multiple files in a directory. The output will be named after the directory.

    • Something else

      The program is structured to make it easy to implement a loader for a font format of your choice. Define a click command to load the font, wrap it with @converter and make it return a tuple containing (glyphs, ascent, descent) where glyphs is a dict of {codepoint: bitmap} mappings.

Scaling

When a ttf font is displayed at a certain size, that size is the vertical size from the top of the tallest character to the bottom of the lowest descender. It is not usually possible to set the character width independently.

Assuming a fixed font with character bitmaps 20 tall and 12 wide, then the resulting ttf should be set to 20px height. It will then be 12px wide and the strokes will exactly line up to screen pixels. (Assuming device pixel ratio is 1, but that is a whole other matter.)

However, there is a problem: many bitmap fonts are not designed for square pixels. There are two situations where this is important:

  1. When the font is designed for a half-height progressive screen mode like 720x256. In a screen mode like this the pixels are twice as tall as they are wide. When displayed on a modern computer they will appear vertically squashed. This is easy to fix - just double the vertical size of each character shape before conversions.

  2. When the font is designed for a display system that doesn't use exactly square pixels, such as PAL. In the PAL format, pixels are around 1.2 to 1.333 times wider than they are tall. Fonts like this will appear narrower than they should on a modern PC. This can also be fixed by scaling the character shapes. However, because the scaling factor is not an integer this leads to a problem: suppose the character is 20x12 and you scale the width by 1.2. 1.2 * 12 = 14.4. Now the character won't fit horizontally on the pixel grid when it is displayed at 20 pixels tall. This causes aliasing which makes the font look bad. There isn't really any solution for this - you have to choose whether you want the font to have pixel perfect outlines, or whether a particular body of text should have the right aspect ratio. Depending on the font renderer and intended purpose of the font, you might get better results with one or the other.

The bitmap2ttf converters accept a horizontal and vertical scaling factor in the --xscale and --yscale options. Note however that only the ratio between them is important. A font will always be displayed 20px tall and with the horizontal width scaled to match regardless of its internal dimensions. So scaling 2x in both directions will have no visible effect on the resulting font.