Terms Related to Web Graphics

aliasing

In sound and image generation, aliasing is the generation of a false (alias) frequency along with the correct one when doing frequency sampling. For images, this produces a jagged edge, or stair-step effect. For sound, it produces a buzz.

animated GIF

An animated GIF is a graphic image on a Web page that moves - for example, a twirling icon or a banner with a hand that waves or letters that magically get larger. In particular, an animated GIF is a file in the Graphics Interchange Format specified as GIF89a that contains within the single file a set of images that are presented in a specified order. An animated GIF can loop endlessly (and it appears as though your document never finishes arriving) or it can present one or a few sequences and then stop the animation.

Java, Shockwave, and other tools can be used to build applets that achieve the same effects as an animated GIF. However, these require browsers and operating systems capable of handling the applets. Animated GIFs can be handled by most browsers and are easier to build than comparable images with Java or Shockwave.

antialiasing

Antialiasing is the smoothing of the image or sound roughness caused by aliasing. With images, approaches include adjusting pixel positions or setting pixel intensities so that there is a more gradual transition between the color of a line and the background color. With sound, aliases are removed by eliminating frequencies above half the sampling frequencies.

bit map (or bitmap)

A bit map defines a display space and the color for each pixel or "bit" in the display space. A GIF and a JPEG are examples of graphic image file types that contain bit maps.

A bit map does not need to contain a bit of color-coded information for each pixel on every row. It only needs to contain information indicating a new color as the display scans along a row. Thus, an image with much solid color will tend to require a small bit map.

Because a bit map uses a fixed or raster method of specifying an image, the image cannot be immediately rescaled by a user without losing definition. A vector graphic image, however, is designed to be quickly rescaled. Typically, an image is created using vector graphics and then, when the artist is satisifed with the image, it is converted to (or saved as) a raster graphic file or bit map.

display modes (CGA, EGA, VGA, XGA, and SVGA)

Displays for personal computers have steadily improved since the days of the monochrome monitors that were used in word processors and text-based computer systems in the 1970s. In 1981, IBM introduced the Color Graphics Adapter (CGA). This display system was capable of rendering four colors, and had a maximum resolution of 320 pixels horizontally by 200 pixels vertically. While CGA was all right for simple computer games such as solitaire and checkers, it did not offer sufficient image resolution for extended sessions of word processing, desktop publishing, or sophisticated graphics applications.

In 1984, IBM introduced the Enhanced Graphics Adapter (EGA) display. It allowed up to 16 different colors and improved the resolution to 640 pixels horizontally by 350 pixels vertically. This improved the appearance of the display and made it possible to read text more easily than with CGA. Nevertheless, EGA did not offer sufficient image resolution for high-level applications such as graphic design and desktop publishing.

In 1987, IBM introduced the Video Graphics Array (VGA) display system. This has become the accepted minimum standard for PC clones. Many VGA monitors are still in use today. The maximum resolution depends on the number of colors displayed. You can choose between 16 colors at 640 x 480 pixels, or 256 colors at 320 x 200 pixels. All IBM-compatible computers support the VGA standard.

In 1990, IBM intoduced the Extended Graphics Array (XGA) display as a successor to its 8514/A display. A later version, XGA-2 offers 800 by 600 pixel resolution in true color (16 million colors) and 1,024 by 768 resolution in 65,536 colors.

Most PC displays sold today are described as Super Video Graphics Array (SVGA) displays. SVGA originally just meant "beyond "VGA" and was not a single standard. More recently, the Video Electronics Standards Assocation (VESA) has established a standard programming interface for SVGA displays, called the VESA BIOS Extension. Typically, an SVGA display can support a palette of up to 16,000,000 colors, although the amount of video memory in a particular computer may limit the actual number of displayed colors to something less than that. Image-resolution specifications vary. In general, the larger the diagonal screen measure of an SVGA monitor, the more pixels it can display horizontally and vertically. Small SVGA monitors (14-inch diagonal) usually display 800 pixels horizontally by 600 pixels vertically. The largest monitors (20 inches or more diagonal measure) can display 1280 x 1024, or even 1600 x 1200, pixels.

GIF (Graphics Interchange Format)

A GIF is one of the two most common file formats for graphic images on the World Wide Web. The other is the JPEG.

On the Web and elsewhere on the Internet (for example, bulletin board services), the GIF has become a de facto standard form of image. The format is actually owned by Compuserve and companies that make products that exploit the format (but not ordinary Web users or businesses that include GIFs in their pages) need to license its use.

Technically, a GIF uses the 2D raster data type, is encoded in binary, and uses LZW compression. There are two versions of the format, 87a and 89a. Version 89a (July, 1989) allows for the possibility of an animated GIF, which is a short sequence of images within a single GIF file. A GIF89a can also be specified for interlaced presentation.

A patent-free replacement for the GIF, the PNG format, has been developed by an Internet committee and major browsers will soon be supporting it.

GIF89a

A GIF89a graphics file is an image formatted according to Graphics Interchange Format (GIF) Version 89a (July, 1989). There was an earlier Version, 87a, from May of 1987, but most images you will see on the Web have probably been created in the newer format. One of the chief advantages of the newer format is the ability to create an animated image that can be played after transmitting to a viewer's browser. An animated GIF is a graphic image that moves - for example, a twirling icon or a banner with a hand that waves or letters that magically get larger. In particular, an animated GIF is a file in the Graphics Interchange Format specified as GIF89a that contains within the single file a set of images that are presented to the viewer in a specified order.

interlaced GIF

An interlaced GIF is a GIF image that seems to arrive on your display like an image coming through a slowly-opening Venetian blind. A fuzzy outline of an image is gradually replaced by seven successive waves of bit streams that fill in the missing lines until the image arrives at its full resolution. Among the advantages for the viewer using 14.4 Kbps and 28.8 Kbps modems are that the wait time for an image seems less and the viewer can sometimes get enough information about the image to decide to click on it or move elsewhere.

You can interlace a GIF file by saving it as an interlaced GIF89a file using a program such as Alchemy Mindworks (http://www.mindworkshop.com/alchemy/alchemy.html) shareware program, Graphic Workshop for Windows.

JPEG (Joint Photographic Experts Group)

A JPEG (pronounced JAY-peg) is a graphic image created by choosing from a range of compression qualities (actually, from one of a suite of compression algorithms). 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. Formally, the JPEG file format is ISO standard 10918. The JPEG scheme includes 29 distinct coding processes although a JPEG implementor may not use them all.

Along with the Graphic Interchange Format (GIF) file, the JPEG is a file type supported by the World Wide Web protocol, usually with the file suffix of ".jpg". You can create a progressive JPEG that is similar to an interlaced GIF.

pixel (a contraction of "picture element")

A pixel (a word invented from "picture element") is the basic unit of programmable color on a computer display or in a computer image. Think of it as a logical - rather than a physical - unit. The physical size of a pixel depends on how you've set the resolution for the display screen. If you've set the display to its maximum resolution, the physical size of a pixel will equal the physical size of the dot pitch (let's just call it the dot size) of the display. If, however, you're set the resolution to something less than the maximum resolution, a pixel will be larger than the physical size of the screen's dot.

The specific color that a pixel describes is a some blend of three components of the color spectrum - red, green, and blue. Up to three bytes of data are allocated for specifying a pixel's color, one byte for each color. A true color or 24-bit color system uses all three bytes. However, most color display systems use only eight-bits (which provides up to 256 different colors).

A bitmap is a file that indicates a color for each pixel along the horizontal axis or row (called the x coordinate) and a color for each pixel along the vertical axis (called the y coordinate). A GIF file, for example, contains a bitmap of an image (along with other data).

Screen image sharpness is sometimes expressed as dots per inch (dpi). (In this usage, the term dot means pixel, not dot as in dot pitch.) Dots per inch is determined by both the physical screen size and the resolution setting. A given image will have less resolution - fewer dots per inch - on a larger screen as the same data is spread out over a larger physical area. Or, on the same size screen, the image will have less resolution if the resolution setting is made larger - resetting from 800 by 600 pixels per horizontal and vertical line to 640 by 480 means fewer dots per inch on the screen and an image that is less sharp. (On the other hand, individual image elements such as text will be larger in size.)

Pixel has generally replaced an earlier contraction of picture element, pel.

PNG (Portable Network Graphics)

PNG (pronounced PEENG) is a file format for compressed graphic images that, in time, is expected to replace the GIF format that is widely used on today's Internet. The GIF format, patented by Compuserve (now owned by America Online), and its usage in image-handling software involves licensing or other legal considerations. (Web users can make, view, and send GIF files freely but they can't develop software that builds them without an arrangement with Compuserve.) The PNG format, on the other hand, was developed by an Internet committee expressly to be patent-free. It provides a number of improvements over the GIF format.

Like a GIF, a PNG file is compressed in lossless fashion (meaning all image information is restored when the file is decompressed during viewing). A PNG file is not intended to replace the JPEG format, which is "lossy" but lets the creator make a trade-off between file size and image quality when the image is compressed. Typically, an image in a PNG file can be 10 to 30% more compressed than in a GIF format.

The PNG format includes these features:

• You can not only make one color transparent, but you can control the degree of transparency (this is also called "opacity").
• Interlacing of the image is supported and is faster in developing than in the GIF format.
• Gamma correction allows you to "tune" the image in terms of color brightness required by specific display manufacturers.
• Images can be saved using true color as well as in the palette and gray-scale formats provided by the GIF.

Unlike the GIF89a, the PNG format doesn't support animation since it can't contain multiple images. The PNG is described as "extensible," however. Software houses will be able to develop variations of PNG that can contain multiple, scriptable images.

progressive JPEG

A progressive JPEG is the JPEG equivalent of the interlaced GIF. It's an image created using the JPEG suite of compression algorithms that will "fade in" in successive waves of lines until the entire image has completely arrived. Like the interlaced GIF, a progressive JPEG is a more appealing way to deliver an image in these early days of relatively low bandwidth.

As of mid-1996, not all browsers supported progressive JPEGs. However, Netscape, Mosaic, and Microsoft's Internet Explorer do support it.

A tool for creating progressive JPEGs can be found at In-Touch Technology (http://www.in-touch.com/pjpeg.html).

raster graphics

Raster graphics are digital images created or captured (for example, by scanning in a photo) as a set of samples of a given space. A raster is a grid of x- (horizontal) and y- (vertical) coordinates on a display space. (And for three-dimensional images, a z-coordinate.) A raster image file identifies which of these coordinates to illuminate in monochrome or color values. The raster file is sometimes referred to as a bitmap because it contains information that is directly mapped to the display grid.

A raster file is usually larger than a vector image file. A raster file is usually difficult to modify without loss of information, although there are software tools that can convert a raster file into a vector file for refinement and changes. Examples of raster image file types are: BMP, TIFF, GIF, and JPEG files.

scanner

A scanner captures images from photographic prints, posters, magazine pages, and similar sources for computer editing and display. Scanners come in hand-held, feed-in, and flatbed types and for scanning black-and-white only or color. Very high resolution scanners are used for scanning for high-resolution printing, but lower resolution scanners are adequate for capturing images for computer display. Scanners usually come with software, such as Adobe's Photoshop product, that lets you resize and otherwise modify a captured image.

Scanners usually attach to your personal computer with a Small Computer System Interface (SCSI). An application such as PhotoShop uses the TWAIN program to read in the image.

Some major manufacturers of scanners include: Epson, Hewlett-Packard, Microtek, and Relisys.

Shockwave

Shockwave is a technology that allows multimedia documents created in Macromedia's Director or Flash to be viewed and interacted with over the web within the web browser program. In order to retrieve Shockwave content, one must outfit one's web browser with the specialized decoding software called a "plug-in," which can be downloaded over the Internet for free.

streaming audio/video

This technology allows a web browser to begin playing an audio or video clip while that clip is still being downloaded from the server. This minimizes the amount of time that the user spends waiting for the data to come over the Internet. Examples of software that facilitates this technology are Shockwave and Real Audio.

true color

True color is the specification of the color of a pixel on a display screen using a 24-bit value, which allows the possibility of up to 16,777,216 possible colors. Many displays today support only an 8-bit color value, allowing up to 256 possible colors. The number of bits used to define a pixel's color shade is its bit-depth. True color is sometimes known as 24-bit color. Some new color display systems offer a 32-bit color mode. The extra byte, called the alpha channel, is used for control and special effects information.

vector graphics

Vector graphics is the creation of digital images through a sequence of commands or mathematical statements that place lines and shapes in a given two-dimensional or three-dimensional space. In physics, a vector is a representation of both a quantity and a direction at the same time. In vector graphics, the file that results from a graphic artist's work is created and saved as a sequence of vector statements. For example, instead of containing a bit in the file for each bit of a line drawing, a vector graphic file describes a series of points to be connected. One result is a much smaller file.

At some point, a vector image is converted into a raster image, which maps bits directly to a display space (and is sometimes called a bitmap). The vector image can be converted to a raster image file prior to its display so that it can be ported between systems.

A vector file is sometimes called a geometric file. Most images created with tools such as Adobe Illustrator and CorelDraw are in the form of vector image files. Vector image files are easier to modify than raster image files (which can, however, sometimes be reconverted to vector files for further refinement).

Animation images are also usually created as vector files. Shockwave's Flash product lets you create 2-D and 3-D animations that are sent to a requestor as a vector file and then rasterized "on the fly" as they arrive.

Graphics Terms NOT Related to the Web

AVI

"Audio Video Interleave" AVI is the file extension and therefore the common nickname for a "Video for Windows" file. This characteristically confusing acronym represents the digital video file format offered by Microsoft for use in its operating systems. If it stands for "Video for Windows" then why didn't they select ".VFW"? Perhaps this extension was already in use by the "Veterans of Foreign Wars" but only Microsoft knows the answer for sure.

Banding

The undesirable effect of waves or bands of the same color on a digital graphic. Banding often occurs in images that include finely graduated transitions from one color to another. The effect can be created by sending an image to an output device that can't support the number of shades necessary to preserve the integrity of the image. Banding also occurs when an image is saved without enough color depth in the file. At its worst, banding ends up making images look like they were drawn with cheap magic markers, which is interesting, but rarely the desired effect.

BMP

The standard bit-mapped raster graphics format used in the Windows environment. By convention, graphics files in the BMP format end with a.BMP extension. BMP files can be created with Windows' Paintbrush. Windows uses a fixed color palette for BMP files which cannot be changed, as doing so would make the screen and border colors change too. This means that transferring an image to the BMP format may result in some color shifts when BMP files are imported into Windows applications. BMP files store graphics in a format called device-independent bitmap (DIB).

Color Depth

The amount of color that a computer display is capable of processing. The color depth of a system will range from Black and White (1-bit) to Millions of Colors (32-bit). Color depth is expressed in the number of colors or color bits available per pixel (e.g., 8-bit color refers to a display that is capable of showing 256 colors, 2-bit color refers to the amount of color that you can buy for a quarter). It may help to think of color depth as the size of the box of crayons that a computer has; the bigger the box, the more shades available of each color. If possible, try for the big box with the built-in sharpener.

Color Model

The color model is the means through which a computer system defines how individual colors within a file will be stored and delivered. The standard color models in use in electronic publishing today are RGB (Red, Green, Blue) and CMYK (Cyan, Magenta, Yellow, Black). Generally speaking, RGB is used most often for the display screen and CMYK is typically used for print applications. These two color models take the opposite approach to drawing the page or screen from one another, and when selecting which one will best suit a particular purpose, this should be taken into account.

DIB

Short for device-independent bitmap, the bit-mapped graphics format used by Windows. Graphics stored in DIB format generally end with a .bmp extension. It's called device-independent because colors are represented in a format independent of the final output device. When a DIB image is output (to a monitor or printer), the device driver translates the DIB colors into actual colors that the output device can display.

DXF

Abbreviation of Data Exchange File, a two-dimensional graphics file format supported by virtually all PC -based CAD products. It was created by AutoDesk for the AutoCAD system.

EPS

"Encapsulated PostScript" EPS is a highly popular format for storing digital graphics for use in preparing documents for print. An EPS file contains the complete definition of a given image in the PostScript language that is used by the output device, in addition to a screen-based preview image built right in. It's even better than a shampoo with a built-in conditioner.

IMG

IMG files were originally designed to work with the GEM Paint Program. IMG files handle monochrome and gray level images. The VENTURA Publisher application, working in the same GEM environment, also supported the IMG file format. In order to maintain compatibility, various other desktop publishing applications have added support for importing and exporting this format (although usually not processing it).

PCX

Originally developed by ZSOFT for its PC Paintbrush program, PCX is a bit mapped graphics file format for graphics programs running on PCs. It is supported by most optical scanners, fax programs, and desktop publishing systems. Files in the PCX format end with a ".pcx" (pronounced dot -p-c-x) extension. Version 3 does not contain palette information. Some applications will override this by using the default VGA colors used by Windows as the palette. This may result in a different looking images when using different viewers. Most PC software supports version 5 of the PCX format.

PIC (Pictor)

PIC format files are generated and used by PICTOR, PC-Paint and GRASP. This PIC file format is not compatible with the Lotus 1-2-3 PIC drawing files! 16 color PIC files have a non-common structure in their color manifestations. For this reason, graphic applications (rather then the 3 mentioned above) will produce a temporary scratch file while packing or unpacking a 16 color PIC file.

PICT

PICT is an image file format that is used primarily for screen-oriented graphics (most commonly in a Macintosh environment). Despite the fact that the file architecture is capable of supporting the full content of very detailed graphic files, it is not used as a standard for laying out print documents.

Resolution

The pixel density of an image expressed in such terms as "dots per inch" or "dots per centimeter." Basically speaking, the higher the resolution, the greater the potential for image detail. It is also true that a higher resolution will make a given file take up a larger amount of disk space. It is far more important to match the resolution to the occasion than to simply strive for high resolutions in all your image files. Carrying a high resolution in a file is simply a waste of disk space if the image is going to be used only on a low resolution output device such as the computer display screen. It should also be noted that while it is possible to lower the resolution of a file, it is not possible to raise the resolution of a file without making the image size smaller.

RGB

"Red Green Blue" RGB is one of the two prevailing color models for use in electronic publishing. It is the means through which an image is displayed on a computer screen. RGB is based on a "subtractive" color scheme, which means that it defines white as a state in which all three of the available colors are present in the maximum amounts, and black as a state in which all three colors are completely absent. In RGB mode, colors in between black and white are achieved by witholding or "subtracting" varying amounts of the three available colors.

RLE (Run Length Encoding)

RLE files are actually "DIB" files that use one of the RLE compression routines. A DIB image that has been saved by means of using one of the RLE compression methods would produce an identical file as when saving the same image directly in an RLE format.

TIFF

"Tagged Image File Format" TIFF is a very common format for saving print graphics. It was originally created by the now defunct Aldus Corporation. Through the use of a file structure called "run-length encoding" the data in a TIFF graphic can be stored very efficiently. The resolution, file size, and color depth available in TIFF are basically limitless. The functional advantages are complemented by nearly universal support by electronic publishing applications, make TIFF a natural choice for a wide range of layout and design tasks. The standard file extension for TIFF images is ".TIF".

Video Capture

The act of recording the contents of a video tape into a digital file. If the material to be captured is a single frame of a videotape, then the resulting data can be stored in a single image file, such as a TIFF or PICT file. This is sometimes called a "frame grab." In order to capture a motion segment of a video clip, one must store it in a digital video format such as QuickTime or AVI. A captured digital video clip can then be edited and played back on a computer without the benefit of a video tape device.

WMF

"Windows Metafile" An image file format that was to facilitate the exchange of graphic information between applications in the Microsoft Windows operating system. The WMF format is capable of storing a decent looking screen image and the complete vector data (precise mathematical definition) of a file. So far WMF has not yet gained the universal support among Windows applications that Microsoft was originally aiming for, but never bet against Microsoft.

WPG

WordPerfect® Graphics (WPG) File Format. It first appeared with the release of WordPerfect 5.0, and with the release of version 5.1, the format was changed accordingly. It is advised to use the same format version as the version of WordPerfect in which the image will be used. These files can contain bitmaps, line art, and vector graphics. When using an application rather then WordPerfect for viewing a WPG file containing both bitmapped and vector elements, the vector elements will be discarded. Note that the WPG specification allows files of up to 256 colors.