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Perspective correction and snapshots

In this article I'll point your attention to the (most of the time) very accurate module for perspective correction - which is borrowed from Darktable - and a very interesting and unique feature of ART, namely the possibility to create multiple versions of a photo without actually saving one single file to disk. This is done with snapshots.

Okay, so I was walking the streets when I suddenly encountered that famous 'Henri Cartier-Bresson moment', a man perfectly framed by a door! In a reflex I pointed my digital reflex in his direction and took a snapshot of that scene. As you can see, the moment was well chosen (crop).


But as you can see as well, the framing of the whole photo wasn't exactly perfect...


This was due to the speed of the moment, but also because I used my Sigma 10-20 here (on my APS-C camera this translates to a 15-30mm equivalent). You really have to be careful when framing with this wide-angle, because it captures a lot of info! This photo was taken at 15mm, so 22mm eq.

Perspective correction

ART has a very good tool to correct this photo. It's called Perspective Correction and can be found under the Transform tab. It is taken from another open-source raw convertor called Darktable, also a very capable program.


You can use it the manual way by adapting the several sliders by taste, or use the automatic correction. This last method does not always work for all photos, as it tends to transform certain images in a crazy way. But when it works, it works quite well. The three buttons next to Auto correction in the screenshot above are for correcting horizontal lines, vertical lines or both. That gives the following result.



Horizontal + vertical

Well, that last one looks good!


Now we come to a somewhat hidden but really magic part of ART, its system of snapshots. A snapshot in this context means a 'photo' of the current editing state of the file. RawTherapee offers snapshots already for years, but what ART has added is the possibility to save snapshots in the arp sidecar file, for later use.

RawTherapee works this way. You open a photo, make a snapshot and give it a name, 'original' for example. Then you make a black&white version, make a snapshot again and call it 'b&w'. You can switch between the two versions by clicking on the named snapshot. If you hit Ctrl+S, the actual version of the snapshot will be saved, so either the original or the b&w version. Close the photo, re-open it and it wil show the last saved state, that is the last active snapshot. But the snapshot list is empty now.

ART works another way. Open a photo, make a snapshot and call it 'orginal'. Ho, not that fast. To make a snapshot, go to the left panel, under the History pane you'll see the Snapshots pane. Click on the + sign or hit Alt+S, in the list appears an entry 'Snapshot 1'. Click on that and you can rename it, when done hit Enter. Delete a snapshot by clicking on the minus sign.

Then make a b&w version and call it 'b&w'. Make a negative version and name the snapshot 'b&w negative'. Then click on 'original' and over-saturate the photo, snapshot it and call it 'over-saturated'. Click again on 'original' and add some masks, brighten the masks and make a snapshot called 'brightened masks'. Now close the photo, no need to save anything. Quit ART, or not. Re-open the photo and voilĂ , on the left side you see these five snapshots, choose one and continue editing!


Some possible applications are multiple versions of a photo with different exposure and contrast settings, and/or with different sharpening or denoise settings. Or to make multiple black&white versions with several color filters (red, orange, yellow...) and more or less digital film grain, or... whatever you like!

Truc magique!

Why did I call this 'magic'? In the first place, as far as I know, there's no other software in the world that can do this, at least in the open source league [*]. Second, Darktable stores the edit history, so that when you re-open a file, it shows all the steps you applied the last time, from opening the file to the moment you saved it. But when you choose an entry somewhere in the middle of that list and you change the contrast for example, all the edits you did last time after that point are lost.

[*] Edit. A reader pointed me to the fact that this isn't true, Darktable can do that as well by making duplicates with the Duplicate manager in the left pane of the program, essentially the same as snapshots.

And this is the great trick of ART, you can simultaneously have several versions in parallel, take one, disable some masks, change the exposure and save these actions in a new snapshot - without having touched the others. And the original snapshot (the one you took in the last line) is still there!



xx 4/2020