pdftoppm defaults to rasterizing the MediaBox, while PDF viewers (pdf.js
in the reader, and every other viewer) display the CropBox. For PDFs
whose page 1 is the full print cover wrap (back cover + spine + front
cover in one landscape page) with a CropBox covering only the front
cover - e.g. No Starch's XeTeX-built 'Algorithmic Thinking' - the
fallback stored the entire spread as a squashed landscape cover, while
the reader correctly showed just the front cover.
Pass -cropbox so the rendered cover always matches what the reader
displays. Poppler falls back to the MediaBox when a PDF defines no
CropBox, so PDFs with identical boxes (the common case) render exactly
as before.
John O'Keefe
9c8337d0a8