Custom Bitmap Fonts

soxfox42 · December 8, 2025, 2:32am

Is anyone aware of a way to add custom fonts to a watchapp without converting from TTF? In my current watchface I pretty much reimplemented font rendering, since it didn’t seem like the SDK had a way to add a custom hand-drawn bitmap font, but I would prefer not to need that workaround.

irek · December 8, 2025, 7:21am

I might be wrong on that but I believe that there is no way to convert from bitmap font formats. According to documentation TTF is the only supported format.

In my watchfaces I always go for handmade solution with single bitmap image atlas of all glyphs and frame buffer. It’s straightforward when you support only ASCII characters without line wrap. It also makes it easy for me to apply dithering effects and render custom icons.

It would be nice to have support for bitmap font format like BDF as you not always have vector version of the font, especially when you are going for pixel perfect solution. I wander how Raster Gothic system font was generated.

hellcp · December 8, 2025, 9:46am

If you have a look at the firmware resource generation, especially for language packs, you will find that BDF is used and supported for fonts generation there, so adding SDK support wouldn’t be that hard. TTF fonts apparently can hold bitmap font data too, since one of the fonts in firmware also does that.
Have a look at the firefly fonts here:

jrmobley · December 8, 2025, 12:15pm

Oh, that’s interesting. Supporting BDF directly in the build would be neat. And if you are saying that the build already will use bitmap strikes from a TTF, that is also interesting information…

hellcp · December 8, 2025, 12:30pm

In the background it’s all freetype, and freetype supports these features

freakified · December 8, 2025, 12:43pm

I would love to be able to convert straight from bitmap fonts; I could finally do my iPod revival face concept (though I guess I can already do that using TTF as an intermediary, but that is less exciting)

hellcp · December 8, 2025, 1:21pm

That might be a good first contribution to the firmware actually – I think the only change that has to be made is adding .bdf extension to this list:

github.com/pebble-dev/pebble-firmware

tools/resources/resource_map/resource_generator_font.py

6ad458aab


      
                  d.compress = definition_dict.get('compress')
                  d.extended = bool(definition_dict.get('extended'))
                  d.tracking_adjust = definition_dict.get('trackingAdjust')
          
              return definitions
          
          @classmethod
          def generate_object(cls, task, definition):
              font_path = task.inputs[0].abspath()
              font_ext = os.path.splitext(font_path)[-1]
              if font_ext in (".ttf", ".otf"):
                  font_data = cls.build_font_data(font_path, definition)
              elif font_ext == ".pbf":
                  font_data = open(font_path, "rb").read()
              else:
                  raise Exception(f"Unsupported font format: {font_ext}")
          
              return ResourceObject(definition, font_data)
          
          @classmethod
          def build_font_data(cls, ttf_path, definition):

You should also check if renaming the file to .ttf extension allows the BDF font to be used, that might not be intuitive, but from what I’m seeing it should technically work

soxfox42 · December 9, 2025, 9:20am

Thanks for that, I can confirm that a BDF renamed to *.ttf works, and also that the change you suggested should work – I modified my installed SDK and was able to use a .bdf directly.

I’ll try to open a PR for PebbleOS shortly to make that change

Edit: sdk: allow BDF fonts to be added as resources by soxfox42 · Pull Request #578 · coredevices/PebbleOS · GitHub

irek · December 9, 2025, 9:41am

Amazing! I might try to use that in my current project.

BTW @soxfox42 how do you work with BDF? Did you draw font by hand or BDF was generated from something?

soxfox42 · December 9, 2025, 9:51am

To test the conversion I just used an existing freely available BDF. I’m not too happy with the available tools to be honest, I’ll most likely create an Aseprite extension when I actually start working with custom fonts.

soxfox42 · December 9, 2025, 10:34am

The SDK change is merged, so the next SDK update from Core should support BDFs directly!

stefanopace · December 13, 2025, 5:03pm

Hello! For the one interested, i’ve found this repository that i’ve extended to be able to create ttf fonts from images. I’m waiting the maintainer to accept my pull request. Meanwhile you can try it with my fork: GitHub - stefanopace/bitmap2ttf: Convert bitmap fonts to ttf fonts using font forge. .
This is what i’m using to create custom fonts for my watchface and it seems to work well.

I’m also curious, from a performance point of view (i mean battery charge duration), if it is easier to render the TTF fonts or the BDF ones, or if it will make no difference.

hellcp · December 13, 2025, 7:58pm

It doesn’t matter, the fonts are converted to a proprietary format when building an app or face.

irek · December 23, 2025, 11:25pm

I got some good results using gbdfed. It crashed few times and I had to redraw all glyphs by hand. But it works.

Just be aware that font filename affects font build validation. If you have any number in font file name then build tool will assume this is your font height and will validate that during build. I had no idea and it took me many hours to figure this out. I had a random number in filename just for tests and my build was failing because of that.

stefanopace · December 23, 2025, 11:55pm

It also took me an hour to figure out why my 7segments font was rendering with 7 pixel height