pdfTeX and microtype: Protrusion influence on line-breaking breakpoints

Answer to the Core Questions

1. Does pdfTeX’s line‑breaking algorithm remain agnostic to protrusion settings during breakpoint selection?

Yes. In pdfTeX (1.40 and later) the paragraph builder calculates breakpoints solely from the original box dimensions. The protrusion data that microtype injects via \pdfprotrudechars and related primitives is applied only after the breaker has finished. Consequently, the breaker never “sees” the extended glyph widths, and overfull warnings are generated based on the unprotruded widths.

2. Can microtype settings be adjusted to influence the breaker in pdfTeX, or is this exclusive to LuaTeX?

Microtype offers no option that changes the internal width calculations used by the pdfTeX breaker. Adjusting \SetProtrusion or toggling \pdfprotrudechars merely changes the amount of protrusion applied after the fact. Therefore, influencing the breaker’s decisions is impossible in pdfTeX; this capability exists only in LuaTeX, where protrusion attributes are part of the node model and are considered during line breaking.

How to Verify the Behavior

1. Create a minimal document that would normally overflow the margin without protrusion:

   
   \documentclass{article}
   \usepackage[protrusion=true]{microtype}
   \begin{document}
   This is a very long word that will normally cause an overfull box if the text is not
   protruded beyond the margin.
   \end{document}
   

2. Compile with pdfTeX in PDF mode:

   pdflatex test.tex
   

3. Check the log for an Overfull \hbox warning. The warning will appear because the breaker used the unprotruded width.
4. Compile the same source with LuaTeX:

   lualatex test.tex
   

5. Observe that the overfull warning disappears, confirming that LuaTeX incorporates protrusion during breaking.
6. Optionally, toggle the protrusion flag in pdfTeX:

   \pdfprotrudechars=0
   \pdfprotrudechars=1
   

   and recompile. The presence or absence of the flag does not affect the breakpoint log, only the visual output.

Practical Recommendation

• If you must use pdfTeX and need margin‑consistent line breaks, consider switching to LuaTeX or accepting the post‑processing nature of protrusion in pdfTeX.
• For pdfTeX users, the only way to avoid overfull boxes is to adjust the paragraph content or layout manually; microtype cannot help the breaker.

Missing Diagnostic Detail

Does your workflow compile pdfTeX in PDF mode or DVI mode? Protrusion is ignored entirely in DVI mode, so the behaviour described above only applies when pdfTeX produces PDF output.