Hi David,
I actually thought about FT_Add_Module as a visionary interface for
dynamically plugging alternative renderers:
https://github.com/raphlinus/font-rs
https://github.com/mooman219/fontdue
https://github.com/servo/pathfinder
Admittedly, none of those projects explored this possibility or there
are more straightforward ways to achieve this.
I'm not against the idea of being able to plug external renderers to FreeType if it makes sense. The OT-SVG case is a compelling reason to.
However, I want to deprecate the existing module API. It has grown a little too complex for its own good, and there are opportunities to simplify the codebase (making the code both shorter and more maintainable), by getting rid of the ability to add/replace/remove arbitrary modules.
That would not prevent us from introducing a new way to "plug" renderers, probably with a new interface, that would be much more specific, and would deal better with subtle issues like deallocating bitmap buffers that were allocated by the renderer.
In the case of OT-SVG, I haven't looked at the problem in detail, but it looks like this would require a new glyph image type to support (doable), and the ability to provide FreeType with an interface it could use to render these with a third-party library.
Do you know if the format allows compositing SVG and non-SVG glyphs together? That would be ... difficult to support though.
I don't think we want to statically link any SVG renderer to the library by default. There are so many subtleties related to vector graphics rendering, that there is no "good" default choice to make. In other words, any default we select at link time would probably be inappropriate for a lot of developers anyway.
- David
Then there is this whole discussion on how to integrate SVG, which
again comes down to using an alternative renderer module and whether
it is worth hooking inside a single module or Add/Remove multiple
modules.
https://lists.nongnu.org/archive/html/freetype-devel/2019-07/msg00058.html
So I know that nobody uses FT_Add_Module but it is not a bad interface.
Alexei