using \definebodyfont with multiple size declarations -- Why?

[Martin Althoff <martin.althoff@yahoo.com>]

As someone new to Context, I am trying to get to grips with font handling. Reading through a fair amount of documentation it is not always to clear to me what recommended practice in todays Luatex/MKIV enviroment is.

My setup is "Minimals" updated 26.7.2010 (luatex 0.60.2, MTXrun 2010.07.22) setup on Mac OS X 10.6.4. I use it to produce documents I need in a language teaching environment. As I am starting of without legacy documents, there is no need for MKII or any other type of Tex.

Confusing for me is the use of "\definebodyfont" in typescripts. I read in "co-fonts.pdf" (no release date, internal creation date 18/11/2009):

"because Latin Modern comes in design sizes, there was a need to associate a specific font with each bodyfont size. You may find yourself in a similar situation when you attempt to create a typescript for a ‘professional' commercial font set."

Looking at the example:

\definebodyfont [10pt,11pt,12pt] [rm] [tfe=Serif at 48pt, ite=SerifItalic at 48pt] 
\tfe Big {\it Words}.

What I simply don't understand is the need for multiple font size declarations. I can (kind of) see that it might be necessary (looking at typescripts), if the whole typescript resolves to "design sizes" where each point size is a single font file. But for ttf/otf fonts, ie. where the font is scaled?


This minimal example, including a "special" font (TitleFont) works (almost) fine for me. 

% the only two statements to declare the fonts I want to use
\usetypescriptfile[type-otf] % being default, I could even miss this declaration 
\setupbodyfont[pagella,10pt]

\definefont [WonderFont] [BoldItalic sa 4.8]

\starttext
{\WonderFont The Big Title}
\input aesop-de
\stoptext

I do get one consistant error (the "5pt" can vary depending on the size I give in "setupbodyfont"): 

{/Users/martin/contextMini/tex/texmf-context/fonts/map/pdftex/context/mkiv-px.map}
virtual math    > the mapping is incomplete for 'pxmath' at 5pt

Though, the document looks fine. Does the error hint I am missing something? What is "good practice" in Luatex/MKIV to access the desired fonts?


My questions: 

- when do I need to use the multiple point sizes at all in a Luatex/MKIV environment?
- when would I need to provide that information in typescript files I need to create? (simplefonts is very slow to load fonts with phonetic symbols)

Any tips to help my understanding would be appreciated!





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