[mapserver-dev] Removing Bitmap Font Support

[thomas bonfort]

Hi devs,

This will become an RFC for 7.0 unless there's wild objection right away...

With the dropping of the GD renderer, I would like to streamline our
text rendering some more and remove support for BITMAP fonts. I will
stress right away that backwards compatibility (aside from the actual
rendered output) will be maintained through the use of an embedded
truetype font.

Bitmap font support has the following inconveniences:
- They are not supported by the cairo (svg+pdf) and kml renderers
- They do not support rotation
- They only cover the ASCII characters
- They are difficult if not impossible to place correctly as they
don't contain the information as to where their baseline is placed
- This can be left to the appreciation of the reader, but my opinion
is that they are far too ugly for any serious text rendering (c.f.
https://github.com/mapserver/msautotest/blob/branch-6-2/renderers/expected/labels-bitmap.png
)
- All the previous points imply that maintaining their support in the
codebase is brittle and painfull

They are however usefull from a user's point of view, as they allow
creating maps without having to define a fontset. Their simplicity
also probably means that they are somewhat more performant to render
than truetype fonts (I don't have any metrics to back this up).

I am thereby proposing that we drop support for bitmap fonts. To
maintain backwards compatibility, and to still allow quickly creating
mapfiles without referencing and creating a fontset, labels of type
bitmap, and/or labels with no font defined will fallback to an
embedded truetype font that freetype will load from memory. The TINY,
SMALL, MEDIUM, LARGE and GIANT keywords will map to equivalent pixel
sizes (proposed: 5,7,10,13,16).

The downsides I see to this modification are an increased libmapserver
memory footprint (due to the embedded truetype font), and possibly
slower rendering for those specifically using bitmap fonts. In
exchange, we get nicer output, support for curved/angle labels,
support for all latin, greek and cyrillic glyphs, support for all
renderers, and a more maintainable codebase.

regards,
Thomas