auto_pdf
whether to use PDF images automatically when the output
format is LaTeX, e.g. foo/bar.png will be replaced by
foo/bar.pdf if the latter exists; this can be useful since normally
PDF images are of higher qualities than raster images like PNG when the
output is LaTeX/PDF
dpi
the DPI (dots per inch) value to be used to calculate the output
width (in inches) of the images from the actual width (in pixels) divided
by dpi
; if not provided, the chunk option dpi
is used; if
NA
, the output width will not be calculated