Topic: Digital image processing – step by step order of execution

Recommended order of execution for processing steps to improve images

This article is essentially a sequel to the article Digital image processing – the basics for beginners: standard procedures. As it is intended for more advanced users, it does not go into as much detail as the first article.

1. RAW conversion to a standard image format – if there are difficulties involving tone values use 16 bits
2. Use the automated routines to remove hot pixels and stuck pixels.
3. Lossless rotation in 90° steps
4. Correct errors due to the lens: distortion, chromatic aberration
5. Rotate to correct alignment.
6. Correct perspective.
7. Clipping
8. Scaling (- if the final application for the image is already known – this usually involves reducing.)
9. Retouching and red eye effect correction.
10. Automatic contrast – tone value correction so as to make use of the best possible dynamic range. Keep an eye on the histogram as far as the degree of intensity is concerned (also for the next steps to sharpening, step 15).
11. White balance (applied before step 13 to avoid the possibility that step 12 limits color values due to strong color channel overexposure).
12. Tone value correction (i.e. brightness and contrast)
13. Color correction
14. Color saturation
15. Sharpening
16. De-noising (can also be appropriate before step 4, because at this stage the original noise pattern is still present and many de-noising programs have ready-made parameter sets for various digital camera models).
17. Labeling, framing.

Important: Never work on original files. Make intermediate backups in lossless formats such as TIFF, PNG or BMP.

Metadata - EXIF, IPTC, JPEG comments can be entered either right at the beginning of the processing or at the end.

Special operations such as removal of silk-screen interference lines from silk screen photos or the removal of canvas structures in photographs of paintings should be performed before step 5. For stereo images – whether anaglyphs or cross/parallel view – use images which have already been optimized. Panoramas, on the other hand, should be created using the original images and then the complete panorama optimized instead.

Photomontages are also best created after optimization. The image sections to be superimposed should also already be optimized. Also, creative changes, e.g. for artistic effect, can only be sensibly performed on an image which has already been optimized.

If you want to or have to optimize a large number of images, e.g. holiday pictures, it will hardly be possible to perform all the steps described above on each individual image. This is where the use of a high-quality automatic image optimizing facility is to be recommended. Examples of these are program extensions such as Xe847, I2e or Perfectly Clear. Of course, automatic routines cannot extract every last scrap of quality, but they are a lot better than no treatment at all.



Recommendations from another user:
- Interchange items 5 and 6 in the list, as it is much more difficult to remove distortion from a skewed image than from one which is at least vertically or horizontally aligned.
- Interchange tone value correction and color correction as well as in his opinion the color balance can be more easily assessed in a softer image.

Another user agrees, who always sharpens after de-noising (especially for portraits), although only with ‘sharpen edges’ between 50-100%. This compensates for the ‘washed out’ effect of the de-noiser.


My opinion:

You can certainly swap rotation and perspective. That makes it easier to deal with the perspective, though the other way round is closer to the original. Theoretically the end result should be the same.

Color correction before tone value correction is not the usual order of operations, because of the danger of encountering problems with a single color channel. Don’t forget: The normal white balance is still to come. The color assessment is clearly a matter of practice, both for softer and crisper images.

There is no universally right moment to perform de-noising – sometimes it’s just a matter of trial and error. It can also be a good idea to de-noise several times in small doses.

Cheers
Ralf Eberle