How to identify the format of images in a pdf?

Question

I have received a number of pdf files with images in them. The original images have been lost, so I need to extract them. I have Adobe Acrobat Pro, so I extracted them using Advanced > Document Processing > Export All Images (there are four options: jpeg, png, tiff, jpeg2000). But, I'd like to extract them in the original format, and this is apparently not jpeg: I also tested pdfimages.exe from xpdf as outlined here, and this gave .ppm files, not jpeg.

So I tried ImageMagick's identify, what it gave me was this:

identify images-000.ppm
images-000.ppm PPM 870x1181 870x1181+0+0 8-bit sRGB 3.082MB 0.000u 0:00.000

Does this indicate it was an embedded .bmp? How to tell? I would actually expect a function in Acrobat to identify the format of images, but I couldn't find it.

So, what is the best way to identify the image format of images in a pdf?

(I prefer extraction via Acrobat because of the batch functionality).

Answer 1

AFAIK, the Image XObjects embedded inside PDFs do not store any information about the original image format. At most if it's an embedded JPEG it can be extracted as-is, but for all other cases you end up with a PxM image that you'll need to convert.

Answer 2

The picture is in portable pixmap file format. (See Wikipedia: Netpbm format for details).

The can use the netbmp tools to convert these to a more modern bmp.
The syntax for that is: ppmtobmp images-000.ppm > images-000.bmp.

http://netpbm.sourceforge.net/ is the homepage for netpbm.

Are there multiple images in a document? Or can we just search the PDF for the line with identify images-000.ppm, cut the file from that location and feed it to ppmtobmp? It should not be hard to automate that.