Determines which objects in an document belong in the foreground and which belong in the background. A value of 0 indicates that most objects should be placed in the background; a value of 100 specifies that most objects should be placed in the foreground. The default is 80, which is suitable for most documents.
Examples
pdftodjvu --threshold-level=50 document.pdf
[document.djvu]
pdftodjvu
--threshold-level=60 document.ps [document.djvu]