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Adobe Certified Associate in Visual Communication Using Adobe Photoshop Cs6 / CC
Topics : Identifying Design Elements When Preparing Images : Understand key terminology when working with digital images.
DIGITAL IMAGES are electronic snapshots taken of a scene or scanned from documents, such as photographs, manuscripts, printed texts, and artwork. The digital image is sampled and mapped as a grid of dots or picture elements (pixels). Each pixel is assigned a tonal value (black, white, shades of gray or color), which is represented in binary code (zeros and ones). The binary digits ("bits") for each pixel are stored in a sequence by a computer and often reduced to a mathematical representation (compressed). The bits are then interpreted and read by the computer to produce an analog version for display or printing.
Pixel Values: As shown in this bitonal image, each pixel is assigned a tonal value, in this example 0 for black and 1 for white.
digital images: Terms are grouped in the following categories: Applications/Software, Camera Capture: Analog and Digital, Color, Color Calibration, Computer Terminology, Digital Image Description, File Formats, File Size, File Storage, Hardware, Lighting Descriptions, Monitors, Printers and Papers, Photoshop, Resolution, Scanners, Techniques, Text.
APPLICATIONS / SOFTWARE :
- ADOBE ACROBAT Adobe’s freeware software application for creation and viewing of Portable Document Format (PDF) files that can display a document as it was originally designed without having the particular software or fonts used to create the file.
- APPLICATION (OR APPLICATION PROGRAM) A software program that performs a specific task. Some common types of applications are word processing, page layout, database, and graphics programs.
- OPEN SOURCE An application or format whose code is non-proprietary. Example: Adobe’s Digital Negative format (DNG).
- OPERATING SYSTEM The system used by the computer to control basic input and output operations and allow the operation of software applications. Examples include Mac OS, Windows, UNIX, and Linux.
- PAINT PROGRAMS Graphics programs that treat images as collections of individual dots or picture elements (pixels).
- PHOTOSHOP File extension: .psd. The proprietary Adobe Photoshop file format that supports all the applications features including: layers, masks, channels. This file format may not be able to be accessed by other applications.
- PROPRIETARY The design of programs, systems or equipment that are owned by an entity. This technology may be licensed for use in other applications or systems. All camera raw formats are proprietary with the exception of the open-source Adobe Digital Negative format.
- QUICKTIME A video and sound playback application originally developed by Apple Computer. Often used for the playback of photographic images in sequence or for screencasts.
- SYSTEM SOFTWARE The component of a computer system that supports application programs by managing system resources such as memory and 1/0 devices.
CAMERA CAPTURE: ANALOG AND DIGITAL
- AF Abbreviation for " Autofocus ". Used in digital cameras.
- ANALOG In the field of electronics, it refers to a signal that varies regularly and continuously over its range. For example, speedometers or thermometers are analog devices. Analog signals can represent many different real-world things, like video, audio (i.e., music) and physiological waveforms (i.e., heartbeats, respiration).
- APERTURE PRIORITY An exposure mode on an automatic or autofocus digital camera that lets you set the aperture while the camera sets the shutter speed for proper exposure. If you change the aperture, or the light level changes, the shutter speed changes automatically. Other than for sports photography, aperture priority is the most common automatic preference in photography. It can also be explained as an automatic exposure system in which the lens aperture is set by the photographer, and the camera then sets the shutter speed.
- A.S.A. American Standards Association. A term indicating a film's sensitivity to light. The term as been replaced in current practice by the term ISO (International Standards Organization).
- ASPECT RATIO The relationship of the horizontal to the vertical measures of an image or display. The horizontal value is placed first (i.e., 3:2). It is found by dividing the vertical number of pixels (height) by the horizontal number of pixels (width) leaving it in fractional format. The most common aspect ratios are 4:3 and 16:9. The 4:3 aspect ratio is the ratio for many computer monitors, Tvs and classic films. The newer 16:9 aspect ratio is used for High Definition Television (HDTV) and widescreen video.
- BAYER PATTERN In digital cameras, CMOS and CCD pixels on the imager sensors are sensitive to light photons but are not, by themselves, sensitive to color. The pixels will create a black and white image. In order to distinguish between colors, filters are put on top of a pixel to allow only certain colors to pass, turning the "rods" of the array into "cones". Since all colors can be broken down into an RGB or CMYk pattern, individual primary or complementary color schemes are deposited on top of the pixel array. After being read from the sensor, software takes the different values of the pattern and recombines the colors to match the original picture. There are a variety of different filters, the most popular being the Bayer Filter Pattern (also known as RGBG). There are twice as many green filters as the other colors because the human eye is more sensitive to green and therefore green color accuracy is more important to us than the accuracy of other colors:
- BLOOMING A visual effect caused by overexposing an image sensor in a digital camera to too much light resulting in a leakage into adjacent photo sites. Blooming is similar to overexposure in film photography, except that in digital imaging this results in vertical and horizontal streaks appearing from the light source in the image.
- BRACKETING To make several exposures, some greater and some less than the exposure that is calculated to be correct. Bracketing allows for error and permits selection of the best exposure after development.
- BURST IMAGES A series of digital images taken in rapid succession, captured at a preset speed. Burst images are usually used to capture a person or object in motion or to compensate for hand-held images shot in low light. The number of images captured will vary, depending on how long the shutter button is held down and how much memory is available. Digital cameras generally let you take 5 to 30 shots consecutively, and have burst modes good for 2.5 to 10 frames per second for as long as the buffer holds out.
- CAPTURE Digitally acquiring image information with a device such as a scanner or digital camera.
- CCD (CHARGE-COUPLED DEVICE) A semiconductor device that produces an electrical output proportional to the amount of light striking each of its elements. A CCD is the principal imaging sensor in electronic cameras, scanners, and video cameras. Currently the most commonly used image sensor. CCDs capture light onto an array of lightsensitive diodes, each diode representing one pixel. For color imagers, each pixel is coated with a film of red, green, or blue (or complementary color scheme) so that each particular pixel captures that one particular color. The pixel, made up of a light sensitive diode, converts the light photon into a charge, and the value of that charge is moved to a single location in a manner similar to a row of people passing buckets of water. At the end of the row, the charge is amplified. Since this "bucket brigade" is accomplished by applying different voltages to the pixels in a succession, the process is called chargecoupling. Because the value in the pixel is
moved by applying different voltages, CCD sensors must be supported by several external voltage generators. The other main type of digital imaging sensor is the CMOS.
- CMOS (COMPLEMENTARY METAL-OXIDE SEMICONDUCTOR) Like CCDs, CMOS imagers include an array of photo-sensitive diodes, one diode within each pixel. Unlike CCDs, however, each pixel in a CMOS imager has its own individual amplifier integrated inside. Since each pixel has its own amplifier, the pixel is referred to as an "active pixel". (note: There are also "passive pixel sensors" (pps) that do not contain this amplifier). In addition, each pixel in a CMOS imager can be read directly on an x-y coordinate system, rather than through the "bucket-brigade" process of a CCD. This means that while a CCD pixel always transfers a charge, a CMOS pixel always detects a photon directly, converts it to a voltage and transfers the information directly to the output. This fundamental difference in how information is read out of the imager, coupled with the manufacturing process, gives CMOS Imagers several advantages over CCDs. Some CMOS imagers have small lenses manufactured directly above the pixel to
focus the light towards the active portion that would otherwise fall on the non-light sensitive portion of the pixel. Microlenses typically can increase the effective fill factor by two to three times. CMOS Pixels are sensitive to light photons but are not, by themselves, sensitive to color. The pixels will capture any kind of light, creating a black and white image. In order to distinguish between colors, filters are put on top of a pixel to allow only certain colors to pass, turning the "rods" of the array into "cones". Since all colors can be broken down into an RGB or CMYk pattern, individual primary or complementary color schemes are deposited on top of the pixel array. After being read from the sensor, software takes the different values of the pattern and recombines the colors to match the original picture. There are a variety of different filters, the most popular being the Bayer Filter Pattern.
- DIGITAL A process that can be represented in a discrete (noncontinuous) form, such as numerical digits or integers.
- DIGITAL CAMERA Any camera system that is capable of capturing image data into a digital file.
- FORMATTING Completely erasing and resetting a camera's memory card. This is usually done as a quick way to erase a full card that you want to reuse or to attempt to fix a card that can't be recognized by the digital camera.
- HUMAN EYE The human eye can discriminate between hundreds of wavelengths as well as the intensity of the light source. The ability to distinguish these characteristics is through two main types of sensory cells in the retina: rods and cones. Cones give us our color vision. The eye can distinguish about 250 shades of each color, and is most sensitive to green. Rods are stimulated by the intensity of light and are responsible for perceiving the size, shape, and brightness of visual images. They do not perceive color and fine detail. The human eye contains about 130 million rods to about 7 million cones. This means that the human eye is much more sensitive to the intensity of the light than its color.
- ISO From the International Organization for Standardization. A measure of film or digital sensor speed. In the ISO arithmetic scale, doubling the speed of a film (halving the amount of light necessary to expose the film) implies doubling the numeric value that designates the film speed. For example, a film rated ISO 200 is twice as sensitive as a film rated ISO 100. The "ISO" speed is essentially the same as the older "ASA" speed. Film speed is the measure of a photographic surface’s sensitivity to light. A greater ISO setting for a given film or digital sensor comes with some loss of image quality. With a digital sensor, this loss is visible as image noise rather than grain.
- LIGHT Visible light is the band of electromagnetic radiation that can be sensed by the human eye. Electromagnetic radiation is the type of energy, traveling in a wave that is produced by an oscillating energy source. Electromagnetic waves also include radio waves, x-rays and gamma rays. While light has properties of waves, the energy carried by light is not distributed in waves, but carried in discrete bundles, giving light the properties of particles. These light "particles" are called photons, and are used to explain how CCD or CMOS imagers in digital cameras transfer light energy to electrical energy. The Visible Light band in the EM spectrum can be broken down into a series of colors, each color corresponding to a different wavelength. The typical spectrum displayed is comprised of seven colors - red, orange, yellow, green, blue, indigo, and violet. The band actually represents a continuum of colors. The bands immediately outside this region - ultraviolet and infrared - are beyond
the range of the human eye.
- METAMERISM This undesired phenomenon describing the visual match of two or more different printed colors under certain viewing conditions but not in all viewing conditions. When present in prints, it can be seen when tilting the print at different angles to the light.
- NOISE An unwanted electrical signal or data that distorts or degrades the signal. The same as static in a phone line or "snow" in a television picture, noise interferes with the image being read and transferred by the imager. In a digital camera, noise is increased by increasing the ISO.
- PUSH To expose film or digital sensor at a higher ISO speed rating than the normal, then to compensate in part for the resulting underexposure by giving greater development than normal. This permits shooting at a dimmer light level, a faster shutter, or a smaller aperture that would otherwise be possible. Pushing is to be avoided at all costs when shooting in Camera Raw.
- SHUTTER LAG In digital cameras, the elapsed time between pressing the shutter release and the actual exposure during which the camera’s focus and exposure mechanisms make last-minute adjustments. Actual elapsed times range from 0.6 to 1.2 seconds under bright light to up to 1 or 2 seconds under low light.
- SHUTTER PRIORITY A mode of automatic exposure by which the photographer manually adjusts the shutter speed and the camera then adjusts the aperture in order to produce a normal exposure.
- SIGNAL TO NOISE S/N ratio is a measure indicating optical signal purity in a digital camera, video camera or audio component. The higher the S/N ratio, the smaller the amount of noise and therefore the signal may be described as one of high definition.
DIGITAL IMAGE DESCRIPTION
- ARTIFACT Unwanted visual defects generated by an input or output device, or a software operation that degrades the image quality.
- BANDING Visible stepping of shades in a printed or displayed image.
- BRIGHTNESS The balance of light and dark shades in an image. Brightness is distinct from contrast, which measures the range between the darkness and lightest shades in a image. Brightness determines the intensity of shades; contrast determines the number of shades you get. Brightness is one of the three dimensions of color; the other two are hue and saturation. The term is used to describe differences in the intensity of light reflected from or transmitted through an image independent of its hue and saturation.
- BRIGHTNESS RANGE The difference in brightness between the darkest and lightest areas of the subject.
- CONTINUOUS TONE An image that has shades of gray or color (as opposed to a halftone or bit-map image). An image that has enough tonal range so that the human eye can't detect obvious "jumps" or "steps" between gray or color levels. A photograph is an example of a contone image. CONTRAST The range between the darkest and lightest components of an Image.
- CONTRASTY Describes a scene, negative, transparency or print with very great differences in light and dark areas. Opposite: flat.
- DITHERING Adjusting the appearance of colors and shades in an image by varying the size and shape of the pixels. Often used to get rid of artifacts and distortion caused by lowering the number of colors in a GIF image file.
- DMAX The darkest area of an image that a device can reproduce and still have detail. A Dmax value of 4.0 would be considered perfect. Dmax (-) Dmin=Dynamic range
- DMIN The lightest area of an image that a device can reproduce and still have detail. A Dmin value of 0.0 would be considered perfect. Dmax (-) Dmin=Dynamic range.
- DYNAMIC RANGE Dynamic range is a measure that indicates the scale of fine gradations being preserved from highlight to shadow. Applied most often to the ability of a scanner or digital camera to register a wide range of tonal values--something from near white to near black. A scanner or digital camera with good dynamic range is able to map input shades correctly to output shades, making images look brighter and giving them more visible detail. Generally the number of bits determines the maximum dynamic range of a scanner. For example a 36-bit scanner has a higher dynamic range than a 24-bit scanner. A perfect dynamic range for a scanner is considered to be 4.0 or Dmin=0 and Dmax=4. The dynamic range of a scanner is most indicative of its overall image quality with 3.0 being a minimum rating for prepress capable scanners.
- GAMMA A measure of the amount of contrast found in an image according to the slope of a gradation curve. High contrast (steep curve) has high gamma and low contrast (shallow curve) low gamma.
- GRADATION A smooth transition between black and white or between one color and another.
- HALO A light line around object edges in an image, usually produced by a sharpening technique.
- HIGHLIGHT POINT The brightest part of an image with detail. Setting a highlight point in an image is perhaps the most important step to achieving a good scan.
- IMAGE ENHANCEMENT The processing of an image to improve elements such as color, tonal range, and defects.
- KEYSTONING The distortion of a projected slide or movie caused by the projector lens axis not being at a 90 degree angle to the screen. The image will appear wider at one edge than on the opposite and the image will not be uniformly sharp.
- LUMINANCE Brightness, often represented by the letter Y.
- MOIRE PATTERN An undesirable pattern in color printing, resulting from conflicting screen angles of overprinting halftones. Moiré patterns can be minimized with the use of proper screen angles or by removal in a graphics application through the use of appropriate filters (i.e., Despeckle Filter in Photoshop).
- POSTERIZATION The conversion of an image to a more elementary form by reducing the number of tonal values, creating an artificial, banded result.
- RASTER The pattern of parallel lines making up the image on a video display screen. The image is produced by controlling the brightness of successive points of the individual lines of the raster.
- SHADOW POINT The darkest tone printable in an image without being black. All tonal values below this threshold will print as black.
- QUARTER TONES Tones between shadow and midtones are known as 3/4 tones and those between highlight and midtones are known as 1/4 tones. Useful in describing the placement of where tones fall when using the Curves adjustment layer in Photoshop.
- VECTOR IMAGE An image represented by mathematically defined shapes, such as lines, polygons, text, and groups of objects as opposed to bitmaps of them. Because a vector is a mathematically defined shape as opposed to an image made up of pixels, vector shapes do not loose quality when enlarged.
- WATERMARK A faint background image on a printed piece or included in digital files as a security feature (such as on printed currency or checks) or to indicate a copyright of an image.
- THUMBNAIL A small version of a larger graphic image used for indexing databases of images or to preview a very large image.
- ARCHIVAL IMAGE A master image meant to last through time. Archival images are of a higher resolution and quality than the digital image on-screen. The file format most often associated with archival images is TIFF, or Tagged Image File Format, as compared to on-screen viewing file format, which are often JPEGs or GIFs. For images captured in the Camera Raw format, the non-proprietary archival format is the Adobe Digital Negative format (DNG).
- CAMERA RAW A file format that allows the user to specify the file type, file creator, and header information and supports up to 48-bit color information with no compression. Due to the amount of information and knowledge required to use this format, beginners seldom use it. RAW is simply the raw data as it comes directly off the CCD or CMOS image sensor in a digital camera, with no in-camera processing performed. Typically this data is 8, 10 or 12 bits per pixel. The advantage is that the image has been neither processed nor white balanced which means that you can process the image to your liking. The disadvantage is that you cannot open these image files with a normal photo application without using a plug-in, typically TWAIN, which can open and then process such images. The Adobe Camera Raw Converter application is an example of such a plug-in.
- Advantages of RAW format
- A true "digital negative", untouched by camera’s processing algorithms
- No sharpening applied
- No gamma or level correction applied
- No white balance applied
- No color correction applied Lossless yet considerably smaller than TIFF Records data over a wider bit range (typically 10 or 12 bits) than JPEG or 8-bit TIFF
- Disadvantages of RAW format
- Requires proprietary acquire module (typically TWAIN) or plug-in to open images
- Images can take longer to process on an average machine
- No universally accepted RAW standard format, each manufacturer (even each camera) differs.
- COMPRESSED A compressed image is one whose file size has been reduced. Some compression schemes maintain the quality of the image (lossless) and some do not (lossy or destructive). See also Compression.
- COMPRESSION A digital process that allows a data file to be condensed usually by removing unnecessary data for adequate representation. This allows the file to be more efficiently stored or transmitted. In some instances a file is then decompressed and restored.
- JPEG is one of the most common formats for 24-bit images. This format selectively reduces chrominance as opposed to luminance information because the human eye is less likely to detect this loss of data.
- DNG (DIGITAL NEGATIVE) An open source format created by Adobe. Use this format to archive any proprietary image created in Camera Raw.
- EXIF Abbreviation for EXchangeable Image File: the file format used by most digital cameras. The EXIF file tags each image with its capture information (date, time, camera model, shutter speed, aperture, flash, etc.). For example, when a typical camera is set to record a JPEG, it’s actually recording an EXIF file that uses JPEG compression to compress the photo data within the file. (See IPTC and XMP).
- GRAPHICS FILE FORMAT A file format used to store any of the file formats used to store images in a digital form; e.g., GIF, JPG, PHOTOSHOP PDF, PSD, CAMERA RAW , TIF, DNG.
- IPTC (International Press Telecommunications Council). This metadata supplies copyright information for a digital capture. (see EXIF and XMP).
- JPEG (Joint Photographic Experts Group) File extension: .jpg. A bitmap, 24-bit color file format that allows the user to specify the amount of lossy compression applied to a continuous tone photographic image file. This file format is supported by most imaging applications. The amount of compression is variable, and more compression leads to increased deterioration of the photographic image. JPEG images can display millions of colors. When you create a JPEG or convert an image from another format to a JPEG, you are asked to specify the quality of image you want. Since the highest quality results in the largest file, you can make a trade-off between image quality and file size. There are several versions of JPEG, some proprietary. JPEG analyses images in blocks of 8 X 8 pixels in size, and selectively reduces detail within each block. At higher compression ratios, the block pattern becomes more visible and there may be noticeable loss of detail. The actual effect depends on the size of
the image when output on a monitor or printer and on the type of subject. This is why you can get as many images into the digital cameras. The results in decompression of the files can cause "blockiness," the "jaggies," or " pixelization " in some digital images. The higher the compression ratio the more the pixelization occurs. The greater the pixel count, the less pixelization.
- LEGACY FILES Files created in an earlier version of an application that may not include support, or may include less support for some features (e.g., CMS) of the newer version of the application.
- LOSSLESS COMPRESSION A form of compression where no data is lost, and, therefore, does not affect image quality.
- LOSSY COMPRESSION A form of compression that attempts to discard unnecessary data. This data loss can affect the quality of the image. JPEG is a lossy compression type.
- LZW (Lempel-Ziv-Welch) A form of lossless block coding that compresses files by assigning codes to repeating patterns of blocks of pixels. Can be chosen when saving a Photoshop file to a TIFF format.
- LINE ART Images containing only one level of contrast or (1) bit depth. A line art image can be either Bitmap or vector.black and white pixels. Line art may also include one-color image, such as mechanical blue prints or drawings.
- METADATA Data about data, or information known about the image in order to provide access to the image. Usually includes information about the intellectual content of the image, digital representation data, and copyright information.
- NATIVE FILE FORMAT The default format in which a specific application saves files.
- PDF (Portable Document Format) File extension: .pdf. This file format was developed by Adobe for use with its software application entitled Acrobat. A bitmap, vector, font, and page layout file format that accurately represents all the elements regardless of the application used to create the file. In the Photoshop PDF file format, only single images can be used and they can include layers.
- TAGGED IMAGE FILE FORMAT (TIFF) A file format for graphics developed by Aldus, Adobe and Apple that’s particularly suited for representing scanned images and other large bit maps. TIFF is the generally accepted interchange standard for digital images. Often used for transporting high quality image files. TIF files may come in compressed or uncompressed versions. TIF compression is lossless and significantly reduces file size with no loss to the quality of the image.
- UNZIP To decompress a file.
- XMP (Extensible Metadata Platform). This is the open W3W standard developed by Adobe for storing and sending metadata with your digital captures. This data will be created in a file that follows your image file. (see EXIF and IPTC).
- GIF Originally created in 1987 by Compuserve, the GIF (Graphics Interchange Format) is another of the most popular image formats found today. GIF images may contain up to 256 colors or shades of grey, and they may also contain transparency and animation. GIF compression is lossless, and file sizes can be more precisely controlled because a user can determine the exact number of colors they would like the file to contain.
- ZIP To compress a file.
- BIT Short for "binary digit". The smallest possible unit of information in any digital system expressed either as 0 or 1.
- BIT DEPTH Number of bits used to specify the brightness or color range of each pixel in an image sensor. This refers to the grey scale range of possibilities for an individual pixel. A pixel with 8 bits per color gives a 24 bit image (8 bits X 3 colors [RGB] equals 24 bits).
- 30/32 bit color is billions of colors
- 24 bit color resolution is 16.7 million colors.
- 16 bit color is 65,536 colors
- 8 bit color is 256 colors
- 8 bit grey scale is 256 shades of grey
- 1 bit is black or white.
- BYTE A unit of measure equal to 8 bits of digital information. The standard measurement unit of a file size. See also Kilobytes, Megabyte and Gigabyte.
- 1000 Bytes of computer information = 1 Kilobyte, Kilobytes represents a million characters of information.
- 1,000,000 Bytes of computer information = 1 megabyte, Megabytes represents a million characters of information.
- Actually one megabyte = 1,048,576 bytes.
- 100,000,000 Bytes of computer information = 1 Gigabyte , Gigabytes represents a 100 million characters of information.
- GIGABYTE (GB) A unit of measurement equal to 1,024 megabytes. Typically, a storage device capable of holding a gigabyte or more of information is a mass storage device using magneto-optical plates.
- KILOBYTE (K) A unit of measurement consisting of 1,024 bytes.
- MEGABYTE (MB) A unit of measurement equal to 1,024 kilobytes, or 1,048,576 bytes.
- TERABYTE (TB) A measure of file size or memory equal to 1,024 gigabytes. Equivalent to exactly 1,099,511,627,776 bytes of information.
- AMBIENT LIGHT Existing light surrounding a subject; the light that is illuminating a scene without any additional light supplied by the photographer. " Available light " and " existing light " are two other terms that mean the same thing.
- CATCHLIGHT The reflection of a light in the subject's eyes in a portrait.
- DIFFUSE HIGHLIGHT A highlight area of a photographic print or transparency that contains detail.
- HIGH KEY An image with a full representation of tones from dark to light but in which the majority of its tones lie above middle gray.
- LOW KEY An image with a full representation of tones from dark to light but in which the majority of its tones lie below middle gray.
- MIDTONE An area of medium brightness, neither a very dark shadow nor a very bright highlight. A medium gray tone in a print.
- NORMAL KEY An image with a full representation of tones from dark to light in which the majority of its tones lie near middle gray.
- SPECULAR HIGHLIGHT The highlight area of an image that contains little to no detail. Example: light reflected off metal objects.
- ACTION A Photoshop command sequence you can record and replay to automate recurrent imaging tasks.
- ANTI-ALIAS An algorithm to smooth the appearance of the jagged lines "jaggies" created by the limited resolution of a graphic display system. Aliasing is caused by insufficient sampling of a digital signal.
- BATCH PROCESSING The performing of a group of computer tasks at the same time. For photographers, this usually means applying some action to two or more image files.
- BEZIER CURVE A type of curved line defined by a mathematical formula and control points. These points serve as handles in graphics programs that can be "gripped" by the mouse pointer to drag the curve into different shapes. Bezier Curves are used in making highly accurate selections using the Path tool and the Direct Selection tool in Photoshop.
- BLACK POINT The darkest shadow in an image histogram used to adjust the tonal range of the image.
- BLACK POINT COMPENSATION A setting in Photoshop that adjusts for differences in black points when converting colors. When selected, the full dynamic range of the source color space is mapped to the full dynamic range of the destination color space. When deselected, the dynamic range of the source color space is simulated in the destination color space.
- BLUR Softening the detail of an image. Blurring can be applied globally or selectively to portions of an image. The most-used blurring process in Photoshop is the Gaussian Blur filter.
- CHANNEL A component of a digital image that carries data for one color layer.
- RGB has three color channels.
- CMYK has four color channels. In each case, when all channels are combined, a full-color image is created.
- CLIPPING A condition where all values lighter than a specific tone are converted to white and all values darker than a specific tone are converted to black. This is usually an unwanted effect because it compresses the dynamic range of the image.
- CLONE Copying pixels of data to new spatial locations in an image.
- CROPPING (OR CROP) The digital or manual process of cutting away undesired parts of an image.
- CURVES Graphic tools in image manipulation software that allow the user to change the contrast and color values of an image.
- DEFRINGE Blends the pixels along the edge of a selection to seamlessly merge it with a new background.
- DESCREEN The process of removing a moiré pattern usually associated with scanning an image that has been through the four-color process (CMYK) and halftone-screened.
- EYEDROPPER This tool takes a sample of a color from an image so that it can be used as the new background or foreground color.
- FEATHERING A technique in many image-editing programs that allows for the softening around the edges of a selection. Feathering actually softens both the inside and the outside of the actual selection outlines.
- FILTER A program or "mask" that alters data in accordance with specific criteria, a formula, or an algorithm.
- FRINGING This occurs when a digital image is artificially sharpened. The term usually refers to a white fringe that is apparent on the edges of objects in the picture. Fringing can also occur as a result of compression.
- GLOBAL COLOR CORRECTION A color correction in a digital image that affects the entire image.
- GAMMA CORRECTION The nonlinear tonal correction editing of an image’s gamma curve. This is typically used to manipulate image shadow detail and to lighten an image without washing out the highlight areas.
- GAUSSIAN BLUR An image softening effect using a bell-shaped Gaussian distribution to soften the image.
- HISTOGRAM A graphic representation of the number of pixels with given color values. A histogram shows the breakdown of how colors and tones are distributed throughout an image.
- LASSO A tool found in many image editing applications allowing the user to select an area of an image by drawing a line around it.
- LAYERING In image editing software, the placement of one image or graphic over another. The opacity of the top image can allow the lower image to show through.
- LOCAL COLOR CORRECTION Color correcting that only affects a selected area of an image.
- MARQUEE A selection tool in image editing programs typically represented by animated dotted lines around the selected area.
- PALETTE The range of color or tone available in the imaging process, or a movable menu of tools or options found in software applications.
- SHARPENING An image enhancement technique in which the contrast between pixels is enhanced. Sharpening is the enhancing of edge detail. It is performed by a mathematical formula that is applied to the entire image or to parts of it. It enhances the visibility of the boundary between light and dark tones in an image.
- THRESHOLD The point at which an action begins or change. For example, the threshold defined in the USM process determines how large a tonal contrast must be before sharpening will be applied to it.
- TOLERANCE In Photoshop tolerance describes a certain distance between adjacent pixels. For example, tolerance is used with the Wand Tool for making selections and the Paint Bucket Tool for painting. The tolerance values can be adjusted. When the Wand Tool is set to a tolerance of one, only a small selection will be created because the distance between the selected pixel value and adjacent pixels is small.
- UNSHARP MASKING (USM) A filtering process used to sharpen images at contrast breaks.
- ADDRESSABLE RESOLUTION The maximum number of pixels an imaging device is capable of manipulating, and not necessarily the same amount the monitor is capable of displaying.
- DPI Dots per Inch. A measurement of resolution or fineness for a printer or scanner. Scanners are given a DPI rating even though they create pixels not dots. DPI is also a scanner’s measure of its enlargement capabilities. DPI is not a good measure of a scanner’s overall image quality. A dot is the smallest unit that can be displayed, scanned, or printed. If a device has a resolution of 360 dpi, it means there are three hundred sixty dots horizontally and three hundred sixty dots vertically. The higher the number of dots per inch, the greater the amount of detail that can be present in the image. However, with greater dpi, comes larger file-size and/or longer display and printing times.
- EIGHT-BIT COLOR Said of images in which each pixel has eight bits of information assigned to it. Eight-bit color can produce 256 colors or shades of gray per channel. (compare to 24-bit color)
- GRAYSCALE A single-channel image consisting of up to 256 levels of gray, with 8 bits of color information per pixel.
- INTERPOLATION A process for increasing image size by using nearby pixels to estimate the color for pixels in the new, larger image. Some digital cameras use interpolation to produce a larger image than the sensor captured or to create digital zoom. Virtually all image editing software support one or more methods of interpolation. How smoothly images are enlarged without introducing jaggies depends on the sophistication of the algorithm. Nearest neighbor interpolation is the simplest method and makes the pixels bigger. The color of a pixel in the new image is the color of the nearest pixel of the original image. If you enlarge 200%, one pixel will be enlarged to a 2 x 2 area of 4 pixels with the same color as the original pixel. Most image viewing and editing software use this type of interpolation to enlarge a digital image for the purpose of closer examination because it does not change the color information of the image and does not introduce any anti-aliasing. Bilinear
Interpolation determines the value of a new pixel based on a weighted average of the 4 pixels in the nearest 2 x -2 neighborhood of the pixel in the original image. The averaging has an anti-aliasing effect and therefore produces relatively smooth edges with hardly any jaggies. Bicubic interpolation is more sophisticated and produces smoother edges than bilinear interpolation. Here, a new pixel is a bicubic function using 16 pixels in the nearest 4 x 4 neighborhood of the pixel in the original image. This is the method most commonly used by image editing software, printer drivers and many digital cameras for resampling images. Photoshop offers two variants of the bicubic interpolation method: bicubic smoother and bicubic sharper.
- JAGGIES A colloquial term for the jagged edges formed on diagonal lines of a raster image, typically large type fonts, when displayed on a device of limited resolving power. (see anti-alias). Jaggies can occur for a variety of reasons, the most common being that the output device (display monitor or printer) does not have enough resolution to portray a smooth line. In addition, jaggies often occur when a bit-mapped image is converted to a different resolution. This is one of the advantages vector graphics has over bitmapped graphics -- the output looks the same regardless of the resolution of the output device. The effect of jaggies can be reduced somewhat by a graphics technique known as antialiasing. Antialiasing smoothes out jagged lines by surrounding the jaggies with shaded pixels. In addition, some printers can reduce jaggies with a technique known as smoothing. The smaller the pixels and the greater their number, the less apparent the "jaggies". Also known as pixelization.
- LPI Lines Per Inch. Also referred to as line screen. The greater the LPI the more continuous looking the image will appear to the human eye. A newspaper is generally printed at 85-100 LPI while magazines are printed from 133-175 or higher.
- PIXEL Short for "Picture Element". The basic building block of computer images. A single square on a computer display or in a digital file. One pixel can contain various amounts of information measured in bits.
- PPI Pixels Per Inch. This measurement is the same as DPI or Dots Per Inch. PPI is more indicative of a monitor where DPI is usually associated with printers.
- PPI (Pixels Per Inch) A measure of the amount of image information density in a digital image.
- PIXELLATION The occurrence of pixels, large enough to become visible individually, when an image is enlarged. Usually it is the result of insufficient data although it is sometimes used as an effect.
- RESAMPLE To change the resolution of an image. Resampling down discards pixel information in an image; resampling up adds pixel information through interpolation.
- RESOLUTION (SPATIAL RESOLUTION OR SPATIAL RELATIONS) The number of pixels per inch in an image or the number of dots per inch used by an output device. Resolution can also refer to the number of bits per pixel.
- SCREEN Refers to computerized and non-computerized methods of converting a continuous tone image into printable patterns. Screen frequencies are usually referred to as LPI (lines per inch).
- IMAGE HARVESTING Using a tripod, the ability to take multiple digital exposures bracketing the exposure values and objects in and out of focus in order to seamlessly integrate some or all into one final image. This technique is especially useful in integrating images of high dynamic range (i.e., an interior room with bright exterior objects viewable through the windows). Replaces the pre-digital practice of bracketing exposures and then being forced to choose one.
- IMAGE PROCESSING Any operation that can be performed on digital data to alter its characteristics and thereby the image that it represents
- PHOTOMONTAGE A photographic composition assembled from pieces of different photographs or of different negatives, closely arranged or superimposed upon each other. Sometimes graphic material is added to the combination.
- REPURPOSE The use of images, text, or information stored in documents for purposes other than its original intent.
- RETOUCHING The manual or digital process of removing imperfections or unwanted portions of an image.
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