Last week’s article outlined the first two steps on my post-photographic Linux workflow, storing images and cataloguing, organizing, or managing images. In this article I will continue with the last two steps for image processing and correction and exporting and distributing the images.
3 – Image processing and correction
DarkTable is a RAW processing software, but it can be used for a number of image manipulations. It is available only on the Linux/Mac/Solaris platforms, which may be a limitation, but the entire processing is high precision 4×32 bit floating point, GEGL based, with GPU accelerations possible via OpenCL.
DarkTable isn’t just your average image editing tool; in fact, it’s not an image editing tool at all. You won’t create new images with it, and you won’t cut and paste or add layers or layer masks with it. Once you get the feel for how DarkTable works and its UI, you will be able to enhance the images you’ve already created.
Working on images in DarkTable is done non-destructive. In other words, anything applied to the image isn’t actually applied to it — it’s recorded in an XML (XMP sidecar) file associated with the image. This method ensures that the original image remains untouched.
3.1 – Reviewing images
3.1.1 – Rating photos
DarkTable provides star ratings and color labels to help sort and rank images according to the user’s criteria. An image’s star rating and its color labels are displayed in the thumbnail.
I ONLY use the star rating for all my images after importing keeping in mind future usage. My rating system is:
- One-star – Keep
These images aren’t very good, but aren’t accidental photos of my shoes, so they stick around, but are usually excluded from further use.
- Two-stars – Show
These are for decent photos that I either have no inclination to share, or it would be redundant to share.
- Three-stars – Collection (Album)
These are great photos that I keep as a collection for sharing (see 4.2).
- Four-stars – Project
These are the ones that I like to use for my Project365 work (see 4.1).
- Five-stars – Call National Geographic
It’s good to have goals. So far no photos got this rating!
3.1.2 – Key-wording
The ability to tag photos with keywords is yet another way of adding a layer of organization to my photos. While I add general metadata and keywords to my photos during importing, I may add more keywords to specific photos, such as event, activity and/or location names. Key-word tagging makes my photos searchable and is one of the most powerful organization tools in DarkTable.
3.2 – Lens corrections
There are times when I need to take advantage of sensor/lens correction and unique demosaicing algorithms to ensure optimal image quality.
If you measure all the distortion that a lens produces, then you should in theory be able to mathematically manipulate the image data from a camera to remove the distortion. Sounds easy, but it is not. Only recently have personal computers become powerful enough to be able to do this intensive number crunching.
Lens correction was a weakness of Aperture and required a third-party RAW image processing applications such as DxO Optics Pro which automatically and completely corrects for lens distortion (even from fisheyes), color fringing and vignetting, and even corrects for unsharpness as needed. The coexistence of two software applications that deal with RAW files is always complicated, given the lack of compatibility among the different demosaicing engines.
The good news is that with DarkTable there is no need for a third-party application. DarkTable‘s correction group contains the modules that will correct typical problems in an photo such as hotpixels, spot removal, noise, lens correction among others. This group also includes the basic sharpening tools.
DarkTable doesn’t do any lens detection or correction by itself. It depends on two libraries, ** Exiv2** and lensfun, which are provided by the Linux distribution. The Lens correction module is able to correct certain lens flaws, namely distortions, transversal chromatic aberrations (TCA) and vignetting.
3.3 – Optional, additional image processing
I don’t do much image processing to my photos. Mostly I use lens correction, especially for my wide-angle lenses. Since DarkTable has no local brush, gradients and layers support, these tasks would need to be done with other software such as Gimp (GNU image manipulation program).
4 – Exporting and distributing the images
I use Flickr for sharing my photos with family, friends and the public. DarkTable‘s export is modularized into storage and format. DarkTable ships with several storage modules such as save on disk, various web albums, a LaTeX photo book template and more. Format modules are the actual image formats such as JPEG, PNG, TIFF, OpenEXR and more.
In the case of Flickr DarkTable provides a number of options that in most cases are sufficient for me. However, sometimes I need to add more information in that case I use frogr, an external application that provides a direct export interface to Flickr.com.
4.1 Adding my four daily entries to my Project 365
In section 3.1.1 (Rating photos) I will have given hopefully at least 4 stars to four photos for the day in question. After selecting the Day Project folder in question, I will use the filter function to see what I preselected during my import review. Depending on how many four star photos are found I will do the following:
- Less then 4
I will expand the selection by include the photos that received three stars and pick from that selection to get the four I want to use.
- More then 4
I will reduce the selection down to four.
Now that I have the four photos I want I will review them one more time, and if needed perform some lens and image processing (see 3.2) before exporting them Flickr.
4.2 Creating Collections
DarkTable offers a versatile feature to organize your images according to various user defined selection criteria. A set of images which is defined by a specific combination of selection criteria is called a collection. The most basic kind of collection is a film roll – covering all the images which have been imported from a specific folder on disk.
You can easily construct other kinds of collections based on various image attributes like EXIF data, filename, tags etc. Multiple criteria can be logically combined to narrow or extend your collection. DarkTable keeps a list of the most recently used collections for quick access.
The current view in lighttable is called a collection. The collect images panel lets you narrow down the list of visible images to just the ones you want to work with.
Information about all images imported into DarkTable are kept in a database, with various attributes describing each image. You define a collection by applying certain filtering rules to these attributes, creating a subset of images to display in the lighttable view.
The default collection is based on the film roll attribute – it displays all images of the last imported film roll or any other film roll chosen.
Depending on the type of collection, I may upload it to Flickr, unsung the process outlined above (4.).