Technology Tales

Batch conversion of DNG files to other file types with the Linux command line

8^th June 2016

At the time of writing, Google Drive is unable to accept DNG files, the Adobe file type for RAW images from digital cameras. While the uploads themselves work fine, the additional processing at the end that, I believe, is needed for Google Photos appears to be failing. Because of this, I thought of other possibilities like uploading them to Dropbox or enclosing them in ZIP archives instead; of these, it is the first that I have been doing and with nothing but success so far. Another idea is to convert the files into an image format that Google Drive can handle, and TIFF came to mind because it keeps all the detail from the original image. In contrast, JPEG files lose some information because of the nature of the compression.

Handily, a one line command does the conversion for all files in a directory once you have all the required software installed:

find -type f | grep -i "DNG" | parallel mogrify -format tiff {}

The find and grep commands are standard, with the first getting you a list of all the files in the current directory and sending (piping) these to the grep command, so the list only retains the names of all DNG files. The last part uses two commands for which I found installation was needed on my Linux Mint machine. The parallel package is the first of these and distributes the heavy workload across all the cores in your processor, and this command will add it to your system:

sudo apt-get install parallel

The mogrify command is part of the ImageMagick suite along with others like convert and this is how you add that to your system:

sudo apt-get install imagemagick

In the command at the top, the parallel command works through all the files in the list provided to it and feeds them to mogrify for conversion. Without the use of parallel, the basic command is like this:

mogrify -format tiff *.DNG

In both cases, the -format switch specifies the output file type, with the tiff portion triggering the creation of TIFF files. The *.DNG portion itself captures all DNG files in a directory, but {} does this in the main command at the top of this post. If you wanted JPEG ones, you would replace tiff with jpg. Should you ever need them, a full list of what file types are supported is produced using the identify command (also part of ImageMagick) as follows:

identify -list format

Harnessing the power of ImageMagick

26^th October 2008

Using the command line to process images might sound senseless, only for 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, while 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, yet a bit of judicious post-processing will turn things around. Here's a command that'll do the trick:

convert -page 792x612+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.