Monday, February 04, 2013

Digitized picture correction with digiKam – Part 1

In About picture correction and work optimization post we have decided to use digiKam and in DigiKam installation and configuration (in 4 parts) we have installed and configured digiKam, so we are ready to do some real work. Please keep in mind this is not a manual (you can find that here). It is simply a selection of commands that work great for me with detailed explanation and some theoretical background.

Again I managed to write too much for a single post so I had to split it into several parts:
  • In this part I talk about digiKam Image Editor and picture orientation and sizing commands.
  • In Part 2 I present some color and histograms theory.
  • In Part 3 I go into details of color and brightness correction with "Levels Adjust" command.
  • In Part 4 I describe how to achieve good pictures quality without using unreasonable amounts of time.
My digitized pictures are saved in two main folders, each with subfolders with the same names as I used for original slide trays. The first is named "dia-scan-original", where I save unchanged results of scanning. The second one is named "dia-scan-corrected", where I save (what a surprise) corrected and tagged pictures. I copy one tray subfolder at a time from "dia-scan-original" to the "/home/mirc/Pictures" folder on the Linux virtual machine. After I have made the corrections and tagging I copy the subfolder to "dia-scan-corrected". This way I always have a recovery copy in case I totally screw-up something in digiKam and it does not take too much time or disk space anyway.

When you start digiKam you see your picture collection (probably folder /home/userid/Pictures) on the left and thumbnails of pictures in selected folder on the right. You can also choose to have only one line of thumbnails and preview of selected picture under it. Now click the button Image Editor and you get a column (with resizable width) of thumbnails on the left and selected picture on the rest of the screen. It can be zoomed in or out and panned as desired, to have a good look at some important detail, but that is expected of any picture editor. What I like is the natural way to select next or previous picture with Page Down or Page Up keyboard buttons.

(as in my previous posts all buttoncommand or keyboard key names in the following text are marked like this)

Picture orientation and sizing

Theoretically there are 8 possibilities how to orientate a slide (it can be rotated 90 degrees 4 times multiplied by 2 front side possibilities) and exactly one is correct. Normally, all the slides in a tray should be oriented correctly, but if they were ever taken out and returned I bet another 6 pack of Laško beer that at least one was not returned correctly. I also remember how I once managed to drop the tray including 50 slides to the table. And when I am scanning negatives (not slides) I first cut and frame them so I can use our Reflecta DigitDia 5000 scanner, but I do not care for orientation when I insert them into the tray, because it is faster to rotate them later on computer.

There are three digiKam commands I use to correct above mistakes: Rotate Left (shortcut Shift-Ctrl-left) and Rotate Right (shortcut Shift-Ctrl-right) which both rotate 90 degrees and Flip Horizontally (shortcut Ctrl-H) which I think of as mirror.

As you probably know from the theory of perspective the natural verticals should remain vertical in the picture. But I sometimes forget this and hold the camera slightly left or right. Now if the main motive is a ball it does not really matter, but if picture contains objects we subconsciously know should be vertical (like traffic signs or building edges) it is very disturbing. This can be corrected with command Free Rotation (shortcut Ctrl-R) at the cost of some cropped picture edges. Here is an example:

As a member of the dia-scan-consortium I am using the Reflecta DigitDia 5000 scanner. The included CyberView software manages to correctly crop the slides most of the time. But sometimes (mostly when pictures are extremely dark) it fails and the resulting JPG has a black border. Even more frequent is the case, when I decide that a smaller frame of the picture (a sort of digital zoom) presents a better motive.

The command to correct above mistake is Crop to Selection (shortcut Ctrl-X). Of course you should first use the mouse to select the remaining part of the picture. When selecting I am not overly sensitive to get the correct (4:3) aspect ratio, I find it more important to get a good motive, even if the results is a perfect square. Another example:

Now we have a picture with people correctly standing on their feet, with left to right inscriptions (or right to left if the picture was taken in Egypt), with traffic signs being vertical and without black borders. After your fingers remember the required commands, it literally takes a second or two to correct picture orientation and sizing.

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