imagemagick splitting large pdf into png's - imagemagick

Question

I have a pdf I'd like to split into individual pictures, each page is a picture, I am using the following imagemajick command to do so:
convert -density 400 mypdf.pdf out.png
and it works fine however I have tested it on the first 5 pages of my pdf and it took 10 seconds, at this rate it should take about half an hour to split my pdf, which seems strange to me considering that I'm not really doing anything fancy, I'm not rotating the images or modifying them in anyway, I'd like to know if there is a faster way to do this. Thanks
Also, I'd like to preserve the quality, I was doing it before without the density flag but the quality dropped dramatically.

Answer 1

PDF rendering is a bit of a mess.
The best system is probably GhostScript, and MuPDF, its library form. It's extremely fast and scales well to large documents. Unfortunately the library licensing (AFL) is difficult and you can't really link directly to the binary.
ImageMagick gets around this restriction by shelling out to the ghostscript command-line tool, but of course that means that rendering a page of a PDF is now a many-stage process: the PDF is copied to /tmp, ghostscript is executed with a set of command-line flags to render the document out to an image file in /tmp, this temporary image file is read back in again, a page is extracted and finally the image is written to the output PNG.
On my laptop I see:
$ time convert -density 400 nipguide.pdf[8] x.png
real 0m2.598s
The other popular PDF renderer is poppler. This came out of the xpdf document previewer project, so it's fast, but is only really happy rendering to RGB. It can struggle on large documents too, and it's GPL, so you can't link to it without also becoming GPL.
libvips links directly to poppler-glib for PDF rendering, so you save some copies. I see:
$ time vips copy nipguide.pdf[page=8,dpi=400] x.png
real 0m0.904s
Finally, there's PDFium. This is the PDF render library from Chrome -- it's the old Foxit PDF previewer, rather crudely cut out and made into a library. It's a little slower than poppler, but it has a very generous license, which means you can use it in situations where poppler would just not work.
There's an experimental libvips branch which uses PDFium for PDF rendering. With that, I see:
$ time vips copy nipguide.pdf[page=8,dpi=400] x.png
real 0m1.152s

Answer 2

If you have Python installed, you should try PyMuPDF. It is a Python binding for MuPDF, extremely easy to use and extremely fast (3 times faster than xpdf).
Rendering PDF pages is bread-and-butter business for this package. Use a script like this:
#----------------------------------------------------------------------------------
import fitz
fname = sys.argv[1] # get filename from command line
doc = fitz.open(fname) # open the file
mat = fitz.Matrix(2,2) # controls resolution: scale factor in x and y direction
for page in doc:
pix = page.getPixmap(matrix=mat, alpha=False)
pix.writePNG("p-%i.png" % page.number) # write the page's image
#----------------------------------------------------------------------------------
More to "Matrix":
This form scales each direction by a factor of 2. So the resulting PNG becomes about 4 times larger than the default version in original, 100% size. Both dimensions can be scaled independently. Rotation or rendering only parts of a page is possible also.
More to PyMuPDF:
Available as binary wheel for Windows, OSX and all Linux versions from PyPI. Installation therefore is a matter of a few seconds. The license for the Python part is GNU GPL 3, for the MuPDF part GNU AFFERO GPL 3. So it's open source and freeware. Creating commercial products is excluded, but you can freely distribute under the same licenses.