Masks used for
photography are used to enhance the quality of an image. Representations of a scene—whether
film,
video display, or
printed—do not have the
dynamic contrast range available to the
human eye looking directly at the same scene. Adjusting the contrast in an image helps restore some of the perceived qualities of the original scene. These adjustments are typically performed on "blown-out" highlights, and "crushed" or "muddy"
shadow areas, where
clipping has occurred; or on
desaturated colors. Photographic masks are peculiar in that they are produced from the image they will alter, an exercise in
recursion. Masks used to produce other effects are similar to those used in painting.
Controlling exposure Film The basic methods of controlling
exposure are
dodging and burning, which respectively lighten (reduce exposure) and darken (increase exposure) areas of an image. The tools a film photographer uses range from shaped pieces of black material (such as studio
foil,
foam, and
paper) to the photographer's hands. To create a photographic mask, a sheet of
negative film is
contact-exposed to the original film negative or
slide positive in a particular way. Both films are then combined to produce a processed positive. The process is similar when applied using
digital techniques: the
inverse of the working image is reduced to an
image mask;
filters or other adjustments are then applied, using the mask to selectively block portions of the image.
Digital Image editors offer at the very least a "Select All" command and a rectangular "marquee" selection tool. (The word "
marquee" describes the "crawling ants" border used to highlight the active region.) Once a selection is created, further changes to the image will be confined to that area. To continue editing the rest of the image, the selection is either "deselected" or the entire image is selected. Advanced suites offer more ways to select portions of an image, as well as ways to combine these selections through.
Selection masks can be switched between an editable
greyscale image and a mask. They allow the user to create a mask using the suite's painting tools.
Contrast masking When the contrast range of an image needs to be adjusted, a
contrast mask is a simple solution. The processed image resembles what would be achieved when exposing through a
neutral density filter, but the effects are focused highly upon the extreme regions of the image. The blocking areas of the mask coincide with the highlights of the image, and the permissive areas with the shadows, resulting in more detail appearing in each.
Film The mask is often made from high-quality
black-and-white film, such as
Kodak Technical Pan, which allows for a degree of softening on the mask. Its processing time is reduced so as to not completely oppose the original negative. Both negatives are combined and
registered, and collectively exposed with additional time to compensate for the presence of the mask.
Digital Contrast masking is made simpler with digital editing. A
grayscale version of the image is produced, either by desaturation or by calculating selected ratios of the image's
color channels, inverted, and blurred. The mask and original image are blended together to produce the final processed image. Some image editors allow for refinement of the effect by changing the strength of the blend. Contrast masking can be considered to be the opposite of
gamma correction, which adjusts the midtones of an image. Effects similar to contrast masking can be achieved by adjusting the
response curves of an image.
Unsharp masking A derivative of contrast masking is
unsharp masking, an unusual term for a process intended to
increase the
apparent sharpness (
acutance) of an image. Unsharp masking uses a blurred form of the image to increase contrast along regions of moderate contrast difference. Around edges, the blur region causes highlights to overexpose and shadows to underexpose. Taken to an extreme, the edges become overly visible and detract from the quality of the image—this is referred to as
halation. Unsharp masking does not increase the
actual sharpness, as it cannot recover details lost to blurring.
Film Unsharp masking allows the photographer to sharpen areas that have become blurred in the original negative, due to long shutter speed/exposure time, or from using a wide
aperture/"fast"
lens. When creating the unsharp mask, extra space or diffusing material is added between the image and the mask to produce the necessary blur.
Digital Unsharp masking has become automated in digital editing, with higher-end suites offering the process as a "tool" or "filter" in their standard sharpening kits—the actual creation of a mask is bypassed in favor of calculations that represent the mask's effect. The process depends on three factors: the
radius of the blur, the
strength of the effect, and the
threshold degree of contrast above which the effect will be applied. (Adjusting the threshold allows the editor to apply the effect selectively upon moderately defined edges and ignore
image noise.) Unsharp masking is computationally more complex than other sharpening algorithms, but results in a higher-quality remedy.
Deconvolution allows for truer sharpening, but is much more complex than unsharp masking. ==References==