acutance

In photography, acutance is the edge contrast of an image.

Acutance is related to the amplitude of the derivative of brightness with respect to space.

Some noise also increases acutance (apparent sharpness).

Several image processing techniques, such as unsharp masking, can increase the acutance in real images.

The resulting print appears more acute than one made without the unsharp mask: its acutance is increased.

Artificially increased acutance has drawbacks.

The technique was first used in Germany during the 1930s as a way of increasing the acutance, or apparent resolution, of photographic images.

As with other forms of image sharpening, edge enhancement is only capable of improving the perceived sharpness or acutance of an image.

Overshoot is often undesirable, particularly if it causes clipping, but is sometimes desirable in image sharpening, due to increasing acutance (perceived sharpness).

Coarse grain or noise can, like sharpening filters, increase acutance, hence increasing the perception of sharpness, even though they degrade the signal-to-noise ratio.

This can cause clipping, and is an artifact (see also ringing artifacts), but it increases acutance (apparent sharpness), and can be desirable.

A well-known property of Rodinal is its high acutance, because the Rodinal formula contains no silver solvent.

Edge enhancement is an image processing filter that enhances the edge contrast of an image or video in an attempt to improve its acutance (apparent sharpness).

A derivative of contrast masking is unsharp masking, an unusual term for a process intended to increase the apparent sharpness (acutance) of an image.

The actual sharpness of the image has been decreased because the transition takes place across 4 pixels, but the apparent sharpness is increased because of the greater acutance.

Due to the nature of the human visual system, an image with higher acutance appears sharper even though an increase in acutance does not increase real resolution.

Low-pass filtering and resampling often cause overshoot, which increases acutance, but can also reduce absolute gradient, which reduces acutance.

While ringing artifacts are generally considered undesirable, the initial overshoot (haloing) at transitions increases acutance (apparent sharpness) by increasing the derivative across the transition, and thus can be considered as an enhancement.

Acutance in the left line was artificially increased by adding a 1 pixel wide darker border on the outside of the line and a 1 pixel wide brighter border on the inside of the line.

Perceived sharpness is a combination of both resolution and acutance: it is thus a combination of the captured resolution, which cannot be changed in processing, and of acutance, which can be so changed.

One definition of acutance is determined by imaging a sharp "knife-edge", producing an S-shaped distribution over a width W between maximum density D and minimum density D - steeper transitions yield higher acutance.