Question: My font has an empty "interior". As show below:

Font with white background:

[![White Background](https://i.stack.imgur.com/q83HW.png)](https://i.stack.imgur.com/q83HW.png)

Font with blue background:

[![Blue Background](https://i.stack.imgur.com/DGm9J.png)](https://i.stack.imgur.com/DGm9J.png)

How can we make the "interior" of the text always be white, without using a background? I'll be using the Unity game engine.

And of course, I can't edit the .ttf file. As @itchee pointed out: A TTF font is always in one color only. It can only specify opaque and transparent parts (technically, only the opaque parts are specified and everything else is transparent). The application that displays that font can chose one single color for the opaque part. But it cannot make "some parts black and some parts white". My question is how to achieve what I want **using the Unity game engine**. Any C# scripts are welcome.

As @fafase said, a filling algorithm, for each character, looking for surrounded zone and filling them would be ideal.

Answer: 
You could use bitmap fonts.
They aren't supported natively by unity, so you can use an asset from the asset store called [CJFinc: Bitmap Font Tools](https://www.assetstore.unity3d.com/en/#!/content/27216). If you aren't willing to pay that much I found a nice tutorial [here](http://www.benoitfreslon.com/unity-generate-and-import-a-bitmap-font-with-a-free-tool).

But so far, my most flexible idea, and probably what I would do is to create a second font that you can place behind your first font. You would keep your original font at a black color. The second font would be simple shapes that fit the shape of your fill, and that you set to whatever color you want. Here's a quick sketch of what I mean:

[![explain](https://i.stack.imgur.com/oe9dE.png)](https://i.stack.imgur.com/oe9dE.png)

You can use [fontforge](https://fontforge.github.io/en-US/) to create your custom fonts. It might take you quite a bit of time, but It will then work consistently, simply, and without many changes to your scripts.
While it is possible to edit TTF fonts, you cannot make the "background" white.

TTF fonts can only specify opaque and transparent parts (technically, only the opaque parts are specified and everything else is transparent). The application that displays that font can chose one single color for the opaque part. But it cannot make "some parts black and some parts white".