Masking
Encyclopedia
In art, craft, and engineering, masking is the use of materials to protect areas from change, or to focus change on other areas.
Masking can describe either the techniques and materials used to control the development of a work of art by protecting a desired area from change; or a phenomenon that (either intentionally or unintentionally) causes a sensation to be concealed from conscious attention.
The term is derived from the word "mask", in the sense that it hides the face from view.
's dexterity and choice of applicator to control where paint is laid. Examples include the use of a stencil or masking tape to protect areas which are not to be painted.
to hold the mask in place while work is performed. Some, such as masking tape and frisket, come with adhesive pre-applied. Solid masks are readily available in bulk, and are used in large painting jobs.
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" highlight
s, 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.
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; filter
s or other adjustments are then applied, using the mask to selectively block portions of the image.
" 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.
, 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, 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.
version of the image is produced, either by desaturation or by calculating selected ratios of the image's color channel
s, 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.
) 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.
/"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.
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.
Masking can describe either the techniques and materials used to control the development of a work of art by protecting a desired area from change; or a phenomenon that (either intentionally or unintentionally) causes a sensation to be concealed from conscious attention.
The term is derived from the word "mask", in the sense that it hides the face from view.
In painting
Masking materials supplement a painterPainting
Painting is the practice of applying paint, pigment, color or other medium to a surface . The application of the medium is commonly applied to the base with a brush but other objects can be used. In art, the term painting describes both the act and the result of the action. However, painting is...
's dexterity and choice of applicator to control where paint is laid. Examples include the use of a stencil or masking tape to protect areas which are not to be painted.
Solid masks
Most solid masks require an adhesiveAdhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
to hold the mask in place while work is performed. Some, such as masking tape and frisket, come with adhesive pre-applied. Solid masks are readily available in bulk, and are used in large painting jobs.
- PaperPaperPaper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
products- Kraft paperKraft paperKraft paper or kraft is paper or paperboard produced from chemical pulp produced in the kraft process.Pulp produced by the kraft process is stronger than that made by other pulping processes; acidic sulfite processes degrade cellulose more, leading to weaker fibers, and mechanical pulping...
- Butcher paperButcher paperButcher paper is a kraft paper. Originally sold to butchers for the purpose of wrapping meat and fish, butcher paper is now used for a wide variety of purposes, notably in primary education where it is used for arts and crafts, such as hanging artwork. Many high schools use butcher paper for...
- Masking tapeMasking tapeMasking tape is a type of pressure sensitive tape made of a thin and easy-to-tear paper, and an easily released pressure sensitive adhesive. It is available in a variety of widths. It is used mainly in painting, to mask off areas that should not be painted...
- Kraft paper
- PlasticPlasticA plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
film- FrisketFrisketA frisket is any material that protects areas of a work from unintended change.-Letterpress:On a sheet-fed letterpress printing machine, a frisket is a sheet of oiled paper that covers the space between the type or cuts and the edge of the paper that is to be printed...
- PolyesterPolyesterPolyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
tape
- Frisket
- StencilStencilA stencil is a thin sheet of material, such as paper, plastic, or metal, with letters or a design cut from it, used to produce the letters or design on an underlying surface by applying pigment through the cut-out holes in the material. The key advantage of a stencil is that it can be reused to...
s- Silk screen
Liquid masks
Liquid masks are preferred where precision is needed; they prevent paint from seeping underneath, resulting in clean edges. Care must be taken to remove them without damaging the work underneath.- LatexLatexLatex is the stable dispersion of polymer microparticles in an aqueous medium. Latexes may be natural or synthetic.Latex as found in nature is a milky fluid found in 10% of all flowering plants . It is a complex emulsion consisting of proteins, alkaloids, starches, sugars, oils, tannins, resins,...
or other polymerPolymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s - Molten waxWaxthumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
- GessoGessoGesso is a white paint mixture consisting of a binder mixed with chalk, gypsum, pigment, or any combination of these...
, typically a substrate for painting, but can also be applied to achieve masking effects
In photography
Masks used for photographyPhotography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
are used to enhance the quality of an image.
Representations of a scene—whether film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
, video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
display, or printed
Printing
Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....
—do not have the dynamic contrast range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
available to the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
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" highlight
Highlight
Highlight may refer to:* In photography, any of the brightest parts of a subject, in contraposition to "shadow"* An especially significant or interesting detail or phenomenon.Highlight is also used in the following expressions:...
s, and "crushed" or "muddy" shadow
Shadow
A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the...
areas, where clipping
Clipping (photography)
In digital photography and digital video, clipping is a result of capturing or processing an image where the intensity in a certain area falls outside the minimum and maximum intensity which can be represented. It is an instance of signal clipping in the image domain...
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
Recursion
Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from...
.
Masks used to produce other effects are similar to those used in painting.
Film
The basic methods of controlling exposureExposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an 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
Foil (chemistry)
A foil is a very thin sheet of metal, usually made by hammering or rolling a piece of metal. Foils are most easily made with malleable metals, such as aluminium, copper, tin, and gold. Foils usually bend under their own weight and can be torn easily. The more malleable a metal, the thinner foil can...
, foam
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...
, and paper
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
) to the photographer's hands.
To create a photographic mask, a sheet of negative
Negative (photography)
In photography, a negative may refer to three different things, although they are all related.-A negative:Film for 35 mm cameras comes in long narrow strips of chemical-coated plastic or cellulose acetate. As each image is captured by the camera onto the film strip, the film strip advances so that...
film is contact-exposed
Contact print
A contact print is a photographic image produced from film; sometimes from a film negative, and sometimes from a film positive. The defining characteristic of a contact print is that the photographic result is made by exposing through the film negative or positive, onto a light sensitive material...
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
Digital imaging
Digital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...
techniques: the inverse
Inverse
Inverse may refer to:* Inverse , a type of immediate inference from a conditional sentence* Inverse , a program for solving inverse and optimization problems...
of the working image is reduced to an image mask; filter
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
s 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 "marqueeMarquee
Marquee may refer to:* A large tent, open-sided and installed outdoors for temporary functions* "Marquee", a song by Superchunk from their 1997 album Indoor Living* Marquee Cinemas, a movie theater chain in the United States...
" 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 filterNeutral density filter
In photography and optics, a neutral density filter or ND filter can be a colorless or grey filter. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition.The purpose of standard photographic...
, 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-whiteBlack-and-white
Black-and-white, often abbreviated B/W or B&W, is a term referring to a number of monochrome forms in visual arts.Black-and-white as a description is also something of a misnomer, for in addition to black and white, most of these media included varying shades of gray...
film, such as Kodak Technical Pan
Technical Pan
Technical Pan was an almost panchromatic black-and-white film produced by Kodak. While it could reproduce the visible light spectrum, it leaned to the red, and so unfiltered outdoor shots would render blues, most notably the sky, with additional darkening and reds with some lightening. These unique...
, 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
Printing registration
In color printing, registration is the method of correlating overlapping colors on one single image. There are many different styles and types of registration, many of which employ the alignment of specific marks.-Purpose:...
, and collectively exposed with additional time to compensate for the presence of the mask.
Digital
Contrast masking is made simpler with digital editing. A grayscaleGrayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
version of the image is produced, either by desaturation or by calculating selected ratios of the image's color channel
Channel (digital image)
Color digital images are made of pixels, and pixels are made of combinations of primary colors. A channel in this context is the grayscale image of the same size as a color image, made of just one of these primary colors. For instance, an image from a standard digital camera will have a red, green...
s, 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
Gamma correction
Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems...
, 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 (acutanceAcutance
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...
) 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
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
.
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 apertureAperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
/"fast" lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
.
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 radiusRadius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
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
Image noise
Image noise is random variation of brightness or color information in images, and is usually an aspect of electronic noise. It can be produced by the sensor and circuitry of a scanner or digital camera...
.)
Unsharp masking is computationally more complex than other sharpening algorithms, but results in a higher-quality remedy. Deconvolution
Deconvolution
In mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...
allows for truer sharpening, but is much more complex than unsharp masking.
External links
- 3M, producer of various products including pre-taped masking film
- Associated Paint, maker of Masking Liquid H20
- Grafix, maker of Liquid Frisket