Edge enhancement

When sharpening the change of image information is referred to when this change distinctness of different image details with each other increases. When sharpening the actual information content (ie, the optical field) does not increase, therefore we also speak of the increase in subjective sharpness impression.

Basics

Color and brightness in photos (digital and film by ) is always generated using a color mixture. Different colors are usually mixed with different intensities - the result is a specific color with a specific brightness.

In order to distinguish more image details from each other ( to raise ) these brightness values ​​must be changed. The change can locally be global ( the whole picture ) Regulations, ( between individual picture elements ) and structural ( changes in dot density and color depth).

Contrast

Contrast defines the difference between light and dark areas. Sharpening is the specific change in this difference.

For this change in contrast, numerous methods exist:

  • The curve for the change of the global density distribution
  • The high-pass filter for the selection of specific brightness values
  • The post-exposure for individual contrast change
  • U.v.a.

Contrast change

With all the changes in contrast can the subjective impression of sharpness of an image change.

Part of the contrast changing process (Example: Brightness Control ) can be used for general image enhancement - yet they also affect the impression of sharpness.

Other contrast changing process (Example: unsharp masking ) are purposefully used to increase the sharpness impression - of course also changes the perceived brightness of the entire image.

The difference between all contrasting changing process is:

  • The effective range of the contrast change (pixels Exactly, global, edges Stressing ... )
  • The application of contrast change ( smooth transitions, hard edges, subtracting as in the Unsharp Mask ... )

History

The technique can also change subjective impression of sharpness of an image, is already more than a hundred years old. Using fine brush and special ink were used for the sharpness important image details (mostly eyes and contours) traced delicate. This method was so successful that it was used until the late 20th century by most professional photographers for such purposes. Even today can be personalized with the appropriate software retouch so that improved their sharpness.

The second, also very old method is that of contrast change. Using different developer chemicals and paper and targeted post-exposure could be globally manipulated the contrast throughout the image. In the second half of the 20th century, the contrast change was a significant jump in quality: the unsharp masking (where the contrast is locally reinforced at the edges within an image). Both techniques were able to successfully compete in the digital photo age.

With the beginning of the 1990s (very unspectacular ) heralded a new era in photography. At this time 95% of the photos were made ​​in commercial labs. The large laboratories then introduced the technique of automatic image optimization. In a split second each (then analog ) photo was analyzed and manipulated. This technique could individually Dodge in tenths of seconds each image, Burn, blurred mask etc. The software, which was based on this method, required years of fine tuning ( they had to be "trained" ). Therefore, in the course of time, a gradual transition to eventually completely different viewing habits. The best way to observe this change on a comparison between photos of the 1980s and 1990s. Although the technique ( use of most photographers ) mm film has not changed significantly in this period, you can see from the photos this time clearly see the difference.

In the years of the millennium, the mass distribution of digital photography began. To cover the then serious deficiencies sharpness of digital technology, the entire image production line was adapted to the user profiles. A typical example of this is the profile of the " clipper ": pleasure and family motifs are 10 cm × 15 cm for the photo album can be printed in size. The printers were adjusted so that they the black level increased significantly more than it would have been necessary. This meant that the motifs very strong and contrasting looked ( sharpness ). Thus, the weakness of the printer was laminated at the same time to be able to delicate colors very poorly printed. The restriction to 10 cm × 15 cm, the physical lack of sharpness of digital images was barely visible.

Use

Today sharpening one of the fundamental applications of graphics filters in image processing and is also automatically used in most imaging techniques. This " sharpening filter " are exactly matched in scope of application and the change in contrast in an image. Typical filters are

  • The edge enhancement to the simple increase in the sharpness impression
  • The unsharp masking to sophisticated image optimization

Other applications are

  • The aperture correction to improve the steepness of low pass filters used in electronic imaging systems.
Hammer Self-information Color mixing High-pass filter#Image Image editing
712172
de