Sunday, February 17, 2013

Digitized picture correction with digiKam – Part 4

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 Part 1 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 this part I describe how to achieve good pictures quality without using unreasonable amounts of time.

The workflow 

I have thousands of pictures to check. It takes a lot of time (which I do not have in unlimited amounts), but I also want pictures to look good. I have developed a typical workflow which is a compromise between time and quality. The result is pictures that are good enough (although probably not perfect) and I work fast enough to see the finish line.

In my workflow I first correct picture orientation and sizing. This is fast, it only takes some seconds.

Then I carefully adjust the colors for one picture in the sequence of similar ones. This can be the first picture in the sequence, but sometimes I choose the most typical or the most beautiful one. I start with the Adjust all Levels Automatically button or with the Select white area button. I usually find the result too aggressive therefore I continue with check of each channel (first 3 colors then luminosity) separately. While checking resulting picture and both histograms I try to slightly decrease the low margin and slightly increase the high margin for input intensity. It can take several minutes, but I do not continue until I am completely satisfied with color balance.

Final step is to adjust brightness gamma input. I decrease it if the picture looks too bright or increase it if the picture looks too dark. While changing gamma I concentrate on correct brightness of the most important areas of the picture, which are typically the faces of the people. Gamma can be entered with precision of 0.01 but the difference is so small, that I can’t see any difference on the picture, therefore I increment or decrement gamma in steps of 0.10 by clicking on the middle bar to the left or right side of the marker. It is very fast and can be done is seconds. Then I apply the settings, save the picture and select next one.

For other pictures in the sequence I only correct orientation and sizing and try to increment or decrement gamma, both are done in seconds. Other color correction settings, which were remembered from previous pictures I leave unchanged.

I will repeat the steps again with keyboard commands used:
  • Picture orientation (if required): Shift-Ctrl-left or Shift-Ctrl-right to rotate 90 degrees left or right and Ctrl-H to mirror
  • Correct vertical lines (if required): Ctrl-R for free rotation
  • Crop (if required): select the rectangle with the mouse and Ctrl-X
  • Adjust colors (for 1 picture in the sequence): Ctrl-L to open Levels Adjust and set input levels for 3 colors and luminosity
  • Adjust brightness (try for each picture): Levels Adjust is already opened, set gamma and apply changes.
  • Save and advance to next picture: Ctrl-S and PageDown
The bottom line is that using described workflow I am able to correct one tray (80 pictures) in a little more than half an hour or as I calculate it, approximately half a minute per picture. I take some time every 5 or 10 pictures to correctly adjust colors then I process following several pictures very quickly. I am using keyboard key combinations as much as possible, but I keep mouse close by for tasks (like cropping) that are more effective by clicking and dragging.

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