what sizes are valid for true type fonts?

I am trying to use true type fonts, but finding that when I load different ones, some do not display at various font sizes while others do. It seems, although I am not certain, that some simply do not show below a certain font size. Is there any way to easily tell what sizes are valid for a ttf by examining the file and not simply trying it out in my app?

UPDATE: Considering smilingthax's answer, it would seem like these must be bitmap glyphs since the smaller sizes are not showing up (but using some system font instead I think). Does someone know how I can determine the vali开发者_JAVA百科d sizes I can use with them using the iOS sdk (iPad)?

TTFs can contain Bitmap glyphs and/or Outline glyphs. Outline glyphs are scalable to any size (e.g. "31.4592 pt") although they are often optimized (hinted) for certain sizes (8,10,12,16,....72). You can get these sizes via your Font API (Win32, Freetype, ...). Bitmap glyphs are only available in certain sizes.

For example Freetype's FTFace object contains a .face_flags member which can have FT_FACE_FLAG_FIXED_SIZES set. Also the sizes are accessible via .num_fixed_sizes and the .available_sizes array.