Edges and Microcontrast

Edges and Microcontrast

The effects of this tool are only visible at a preview scale of 1:1 or more. Use a detail window (click on the Window-add.png icon under the main preview panel) to inspect a part of the image, or zoom the main preview to 100% (also called 1:1) Magnifier-1to1.png.


1 General

Unlike Unsharp Mask, Edges is a true sharpening algorithm. It does not introduce halos, it can be used on noisy images and it works in the Lab color space. It emphasizes only the edges, and can be combined with Microcontrast to also enhance the texture.

Both algorithms were originally implemented by Manuel LLorens.

2 Edges

This tool sharpens any edges that have sufficient contrast for them to be considered an edge. In other words, it sharpens edges that are already sharp, ignoring edges that do not have enough contrast. The algorithm is not affected by image noise and does not generate halos.

This type of sharpening can make the edges look a bit unnatural, as if they had been "cut out". Also, if the settings are too high, the resulting edges may exhibit Aliasing. This is why you should be careful when applying it to images with curved edges. However, when straight lines predominate (especially if they are not diagonal), it is a useful method of sharpening, especially if you reduce the size of the image at the end of processing.

To get the best results, the following settings are recommended:

  1. Iterations: number of iterations carried out by the algorithm. A high number produces an overly sharp effect around the edges. With a value set to 2 this problem can already be observed in some cases. In general, 1 or 2 iterations give the best results.
  2. Quantity: number of adjacent pixels to be analyzed when deciding what constitutes an edge. Higher values produce sharper edges and a greater "saw-tooth" effect.
  3. Luminance only: the tool works in the L*a*b* color space and with this option, only the L* component is enhanced.
On the left, the original image. On the right, the image with the default "Edges" settings. The arrows indicate much sharper edges when the image is at full scale (at 100%), but if you open the image to view it at 200% or 300% or more, you can clearly see the sawtooth effect and some slight posterization around the edges

Additional information can be found here:: https://web.archive.org/web/20110625093654/http://www.rawness.es/sharpening/?lang=en

3 Microcontrast

"Microcontrast" can be defined as contrast on a pixel level[1], as opposed to "local contrast" which pertains to contrast between larger (lower frequency) areas.

The Microcontrast tool increases the contrast of a pixel relative to its neighbors, effectively leading to an apparent increase in texture. The intention is to allow recovering texture lost due to noise reduction. It does not introduce halos.[2]

Example of Microcontrast.

The tool's controls are progressive and allow you to choose a balance between increasing the contrast at the pixel level and the appearance of artifacts:

  • Contrast threshold: sets the minimum contrast at which the tool will act on the pixels.
  • Quantity: the intensity of the effect. The higher the value, the greater the difference between the pixels.
  • Uniformity: to the left, the algorithm tends to respect the initial contrast gradients. To the right, the contrasts are more intense and the initial contrast gradients are ignored, which makes the image harsher.
  • Matrix: defines the area that will be used to calculate the contrast variation. There are two possibilities, a 3x3 pixel matrix around the pixel being analyzed, or a 5x5 pixel matrix. By default, it will be 5x5, giving a more intense effect, the 3x3 matrix will be more appropriate for noisy images.