[luatex] What happens to glyphs not in the font?

[Arno Trautmann]

Hi all,

this might be a quite strange question, but it bothers me for some time 
now: What happens when I input a character that is not available in the 
current font?

Consider the following minimal example:

\pdfcompresslevel0
aßa
\bye

In the pdf, I only get the “aa” as expected. However, for a long time 
now I thought, the “ß” would still make its way into the pdf but would 
just not be shown by the viewer. Loading a font, e.g. by using fontspec, 
the ß of course is there. So I inspected the pdf (hence 
\pdfcompresslevel0) and indee I see:

/F15 9.96264 Tf 1 0 0 1 91.925 759.927 Tm [(aa)]TJ

But when I look into some callbacks, I see that still in the 
pre_output_filter the ß node is present, with the width 0 which makes 
sense to me.

So, at what point is the information lost that there is an additional 
glyph node?

cheers
Arno