I have the same problem, TTF_OpenFont returns NULL and with an error.
TTF_OpenFont("/absolute/path/to/SourceSansPro-Black.ttf", 25);
std::cout << TTF_GetError() << std::endl;
And I get the "Couldn't load font file"
- The .ttf chmod is 777
- I tried running the program with root
I run out of ideas.