Add support for loading TrueType font files.

[MarronEyes]

In the ImageFormatter class, we can load ttf fonts that already exists in the operating system.
However, the "font_name" argument does not accept a custom font (a filename or a file-like object containing a ttf font).
I hope that this pull request solves this problem.

[birkenfeld]

I think this is a good feature to have, only two questions:

• how to handle different font styles? At the moment this is impossible.
• I think it's more idiomatic to check for hasattr(obj, 'read') to check for file-like objects, no?

[MarronEyes]

• We can maybe ask the user to provide different styles of the font only if he wants to use a custom font.
• You are right. I will change it.

[birkenfeld]

Thanks! Could you please update the documentation of the parameter (in the docstring of ImageFormatter)?

[MarronEyes]

Yes, of course!
I also noticed that the "FreeTypeFont" font object can take multiple styles if the specified font is a variation font. (See https://pillow.readthedocs.io/en/stable/reference/ImageFont.html#PIL.ImageFont.FreeTypeFont.get_variation_names)
So we could associate each style (in the "fonts" dict) with its font object ?

[MarronEyes]

I tested it and cannot instantiate new font objects with different styles because it throws an OSError.
The only solution I think is to use the "set_variation_by_name" method (https://pillow.readthedocs.io/en/stable/reference/ImageFont.html#PIL.ImageFont.FreeTypeFont.set_variation_by_name) to switch the style of the font, but it requires changing the code of the ImageFormatter class.

[MarronEyes]

Sorry for the long inactivity, I wasn't available at the moment.
I added a get_style function to get the specified style of the font in get_font.
The code is tested and it works! The pr can be merged.

[Anteru]

This also looks good to me, but there's no test for it (nor do I see how this can be tested easily.) @birkenfeld , @jean-abou-samra , any concerns with that? I'm not super happy to merge code without tests but I don't see an easy way to test image based output.

[jeanas]

This indicates an invalid font, right? Shouldn't there be a warning or an error?

[jeanas]

What is this case for? I don't see ValueError mentioned at
https://pillow.readthedocs.io/en/stable/reference/ImageFont.html#PIL.ImageFont.FreeTypeFont.set_variation_by_name

[Anteru]

Does it guarantee that no ValueError is raised? I'd guess some of the code could do that ... alternative would be to catch only OSError and pass in all other cases?

[jeanas]

If I understand correctly, a typo in the file name will result in font_name being understood as the name of a font and not a path, and the errors will read 'No usable fonts named ...' or something like that depending on the system. Can we make it explicit rather than implicit and use a separate font_path parameter?

[jeanas]

I'm not super happy to merge code without tests but I don't see an easy way to test image based output.

No, I don't see one either, especially that font stuff is so platform-dependent :( So I think it's OK if this doesn't have tests.

[Anteru]

Merged for 2.16. Let's see if this works in the wild, thanks for the contribution!