How I can load a font file with PIL.ImageFont.truetype without specifying the absolute path?

When I write the code in Windows, this code can load the font file just fine:

ImageFont.truetype(filename='msyhbd.ttf', size=30);

I guess the font location is registered in Windows registry.But when I move the code to Ubuntu, and copy the font file over to /usr/share/fonts/, the code cannot locate the font:

 self.font = core.getfont(font, size, index, encoding) IOError: cannot open resource

How can I get PIL to find the ttf file without specifying the absolute path?