[vox-tech] Changing the print resolution of JPEG images by script

[Henry House]

På lördag, 09 april 2005, skrev Ryan:
> On Friday 08 April 2005 05:24 pm, Ken Herron kherron+lugod-at-fmailbox.com 
> |lugod| wrote:
> > Henry House wrote:
> > > One of the more common operations that I need to do with JPEG images is
> > > to change their print resolution so that when printed (e.g., as included
> > > figures in a LaTeX document) they print at 300 dpi (for example) instead
> > > of 72 dpi.
> >
> > To start with, DPI (dots per inch) implies that each pixel has a
> > physical size, 1/72 of an inch or whatever. As far as I know, JPEG
> > images have a size in pixels (e.g. 106 by 64 pixels for the watertower
> > image on the LUGOD home page), but the pixels have no inherent physical
> > size. Something outside of the image--probably the software used to
> > embed the image into the document--is deciding how large the image
> > should appear on the page. I would expect that you can adjust that, ie
> > tell the software how large to print the image or what DPI to use.
> 
> JPEG files include DPI as metadata, which is used by some programs. I suspect 
> it would be fairly trivial to write a program that changes this.

Yes, the print resolution is definately withing the JPEG file, thoughsince
the programs like Gimp and PDFTeX that understand print resolution as
distinct from  pixel resolution do not have access to any other metadata
beyond that in the file  itself.

Some investigation with google turned up this:

http://sourceforge.net/projects/libexif

   "Most digital cameras produce EXIF files, which are JPEG files with
   extra tags that contain information about the image. The EXIF library
   allows you to parse an EXIF file and read the data from those tags."

This looks promising.

-- 
Henry House
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