imagemagickでpdfを画像に変換する

macで実行すると

convert -density 600 -geometry 1000 erd.pdf erd.png
convert: no images defined `erd.png' @ error/convert.c/ConvertImageCommand/3230.

とエラーとなる。
gsが入って無いと出る場合があるのでgsを入れる

brew install ghostscript

再度実行すると変換される

convert -density 600 -geometry 1000 erd.pdf erd.png
   **** Warning: considering '0000000000 XXXXX n' as a free entry.

   **** This file had errors that were repaired or ignored.
   **** The file was produced by:
   **** >>>> Mac OS X 10.10.5 Quartz PDFContext <<<<
   **** Please notify the author of the software that produced this
   **** file that it does not conform to Adobe's published PDF
   **** specification.