Using the command line to process images might sound senseless but the tools offered by ImageMagick certainly prove that it has its place. I have always been wary of using bulk processing for my digital photo files (some digitised from film prints with a scanner) but I do agree that some of it is needed to free up some time for other more necessary things. With this in mind, it is encouraging to see the results from ImageMagick and I can see it making a major difference to how I maintain my online photo gallery.

For instance, making thumbnail images for the gallery certainly seems to be one of those operations where command line bulk processing comes into its own and ImageMagick’s own convert command is heaven sent for this one. For resizing images, all that’s needed is the following:

convert -resize 40% input.jpg output.jpg

Add a spot of further shell scripting and even a dash of Perl and the possibilities for this sort of thing become clearer and this is but the pinnacle of the proverbial iceberg. The -rotate switch will do what the name suggests and there are a whole plethora of other options on tap. So long as you have Ghostscript on your system, conversion of graphics to Postscript (and Encapsulated Postscript too) and PDF files is possible with the -page option controlling the margin around the image itself in the resulting outputs. Unfortunately, portrait is the sole orientation on offer but a bit of judicious post processing will turn things around. Here’s a command that’ll do the trick:

convert -page 792×612+72+72 input.png ps2:output.ps

For retrieving image metadata like its resolution and size, the identify command comes into play. The -verbose option invokes the output of all manner of image metadata so using grep or egrep is perhaps advisable, especially for bulking processing with the likes of Perl. Having the ability to stream image metadata makes loading databases like MySQL less of a chore than the manual data entry that has been my way of doing things until now.

The above error message has been greeting me when creating PDF’s with Ghostscript on a Solaris box and does need some translation. If you are directing output to a real printer, I suppose that it is sensible enough: nothing will happen unless you can connect to it. It gets a little less obvious for PDF creation and seems to mean that the pdfwrite virtual device is unable to create the specified output file. A first port of call would be check that you can write to the directory where you are putting the new PDF file. In my case, there seems to be another cause so I’ll have to keep looking for a solution.

Update: I have since discovered the cause of this: a now defunct TEMP assignment in the .profile file for my user account. Removing that piece of code resolved the problem.

My latest adventure in the world computing has led me into the world of automated PDF generation. When my first approach didn’t prove to be completely trouble-free, I decided to look at the idea of going part of the way with it and finishing off the job with the open source utility Ghostscript. It is that which got me thinking about combining bookmarked PDF files and I can say that Ghostscript is capable of producing what I need as long it doesn’t generate any errors along the way. Here’s the command that does the trick:

gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=final.pdf source_file1.pdf source_file2.pdf

The various switches of the gs command have very useful roles with dBATCH ensuring that Ghostscript shuts down when all is done, dNOPAUSE removing any prompts that would otherwise be given, q for quiet mode, sDEVICE using Ghostscript’s own PDF creation functionality and sOutputFile creates the output file, stopping Ghostscript from sending it to its default stream. All of this applies to Windows Ghostscript too, though the name of the executable is gswin32c for 32-bit Windows instead of gs.

When it comes to any debugging, it is useful to consider that Ghostscript is case sensitive with its command line switches, something that I seen to trip up others. I am getting initial device initialisation so it strikes me that dropping some of the ones that reduce the number of messages might help me work out what’s going on. It’s a useful idea that I have yet to try.

There is also online documentation if you fancy learning more and Linux.com have an article that considers other possible PDF combination tools as well. All in all, it’s nice to have command line tools to do these sorts of things rather than having to use GUI applications all of the time.

Here’s another one of those things that I discovered while being clumsy: in Firefox, click on your middle mouse button/wheel while hovering over a tab and it will close it; you don’t even need to click on the close icon. Evince, the PDF viewer favoured by Ubuntu, also makes use of the middle mouse button: for panning your way through documents using the hand tool. In a moment of lateral thinking, I tried the same trick with Adobe Reader and, in version 7.x, it works in the same way. On Windows at least, Adobe Reader 8.x is a different animal and features automatic scrolling, a very useful proposition for the reading of eBooks if the text doesn’t pass by you too quickly, and even a moderately reliable read aloud feature.