Graphics Formats for Linux
`Gerald Graef
`Issue #23, March 1996
`
`Gerald Graefs provides a comprehensive guide through the jumble of graphics formats
`currently available.
`
`If there is a single unifying feature of the computer world, it is non-conformity. In no place is
`this better illustrated than computer graphics. Literally dozens of different formats exist to do
`the same thing: store graphic images. Proprietary formats, independent free standards, and
`international standards all vie for attention. Fortunately, the computer community has settled
`on just a handful of formats. Some, like TIFF, are the jacks-of-all-trades and are widely used.
`Others, like GIF, have a niche that they dominate. But whatever the context, there is always
`more than one way to store images.
`
`This article first provides a brief introduction to graphics on Linux, including format and type,
`compression, file structure, and various features. It then continues with descriptions of the
`common graphics formats found in the Linux world and programs with which to manipulate
`them.
`Format
`Graphics formats come in two basic varieties: raster and vector. Raster formats store the
`entire image pixel by pixel and are the most commonly-used formats. Vector formats, also
`called descriptive formats, do not contain pixel image data; instead, they contain a series of
`commands telling a display program how to draw the image.
`
`The most common use of vector formats is in computer aided drafting (CAD). Describing a
`line, for example, as a starting point, an ending point, and a color, as opposed to many
`colored points in a field of points, allows a program to manipulate the line as an object,
`making it easy to change color, length, and position. Another common use is in raytracing
`programs. These programs take geometric objects and trace the path of light rays to find
`shadows and highlights. Since the rendered objects are geometric shapes, it is much easier to
`store an image as “dodecahedron at (3,4,3) with size 2 and color 27” than as an array of pixel
`values, especially since the scene to be rendered is three-dimensional. Of course, once the
`scene is raytraced, the resulting image is stored in a raster format.
`Type
`Images come in four basic types: monochrome (black and white), grayscale, color palette, and
`true color.
`
`Grayscale and color palette images typically have 2, 4, or 8 bits per pixel. They use lookup
`tables that must be included in the image file; each entry in the color lookup table gives the
`red, green, and blue values for a color. Grayscale images have a similar lookup table for gray
`values. Each color or gray value is in a range of 0-3 for 2-bit images, 0-15 for 4-bit images,
`and 0-255 for 8-bit images. The actual image pixel data is a series of color or gray values
`corresponding to table entries which give the actual color of each pixel.
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`True color images are 24-bit images and are usually stored as 8 bits of red, 8 bits of green,
`and 8 bits of blue per pixel. Some formats may use different color models than RGB. The JPEG
`format, for example, uses a colorspace based on the luminance and two chrominance values
`(YCbCr). True color images do not need a color lookup table or palette.
`Compression
`Compression is integral to most graphics formats. An uncompressed 640x400x256 color image
`requires just over 256,000 bytes, while a true color image of the same size requires three-
`quarters of a megabyte. Although many compression schemes exist, only a few are common
`in graphics applications. LZW, LZ77, Huffman, and run-length encoding are lossless, meaning
`that the image is reproduced exactly when uncompressed. The cosine transform is the basis
`for the “lossy” JPEG format. We aren't concerned with the details of the compression schemes
`here.
`
`JPEG gives by far the best compression, but at the expense of exact image reproduction and
`speed. Acceptable results are common with compression ratios of 25 to 1, that is, the
`compressed file is 25 times smaller than the original. Ratios of 100 to 1 may be possible if
`image quality is not a primary concern.
`
`[FOOTNOTE:A better algorithm does exist, however: fractal image compression. This method
`gives good results at 100 to 1 and is finding many applications, though it has not yet been
`widely implemented as a general purpose graphics format.]
`
`LZW and LZ77 are the best lossless schemes and produce similar results: usually around 2 to
`1 compression. The modified LZ77 used in the PNG graphics format is slightly slower at
`compressing images than LZW, but it creates smaller files and is faster at decompressing.
`LZ77 is also used by the general purpose compression programs ZIP, GZIP, and PKZIP. Until
`recently, the use of LZW was much more common than LZ77. Its use is falling, however,
`because it is a patented algorithm, and UNISYS requires royalties for its use. Nonetheless,
`both GIF and TIFF, two of the three most common formats, use it.
`
`Run length encoding (RLE) is the most common scheme and the least effective. It is, however,
`very fast compared to other compression algorithms. Its basic mechanism is very simple:
`when a string of pixels with the same value is encountered, RLE outputs the pixel value and
`the number of consecutive pixels with that value. Complex images do not compress well with
`this scheme, although monochrome images often compress very well.
`Structure
`Simple formats consist of nothing more than the image data and a short header giving the
`minimum amount of information necessary to display the image: size and number of colors.
`More robust formats often employ a tag or chunk structure. After an initial header, the image
`is stored in a series of chunks, each prefixed by a code telling the decoding software what the
`chunk contains. The biggest chunk is the actual image data. Other chunks may include
`comment blocks, palette information, or other special image information. Software may only
`support some chunks of a given format: if an unknown chunk is encountered, it is simply
`skipped.
`
`Finally, three special features bear closer examination: magic numbers for format
`identification, gamma correction, and alpha transparency masks. In some cases it is possible
`to identify the format of a file by looking at the file itself. Many formats use magic numbers—
`special (unique, it is hoped!) codes—to identify the format. GIF files, for example, have
`”GIF87a” or “GIF89a” at the start of the file. Because these magic numbers are ASCII coded,
`using less, strings or even cat (though that can accidentally change your character set) to
`display them on the screen is good enough for identifying them. For example, to look for GIF
`files, you can use strings filename | grep GIF.
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`Gamma correction is an important part of the proper display of an image. Whenever an image
`is displayed on a monitor, the monitor's characteristics affect the image. How the monitor
`handles varying degrees of brightness is especially important. In general, the brightness on
`the screen is related to the brightness levels of the original image by a simple formula: screen
`brightness equals image brightness raised to the gamma power. The value of gamma for most
`monitors is around 2.5. To compensate for this, image capture hardware and software may
`make an “opposite” operation on the image. The result is an image with a gamma of 0.45.
`When this image is displayed, the gamma of 0.45 and 2.5 cancel each other out, producing an
`apparent gamma of about 1 (called linear brightness scale).
`
`But what does gamma mean visually? An image displayed with a gamma less than one uses
`more of its pixel codes in the darker areas, that is, darker areas have better color resolution. A
`gamma value greater than one uses more of its pixel codes in the lighter regions. A murky
`image, or an image with too much contrast, may need to have its gamma corrected.
`Correcting the gamma of an image is inherently lossy because of round-off error during the
`taking of powers; hence, gamma correction should be performed when the image is displayed,
`keeping the original image file intact without loss.
`
`Alpha transparency masks are a way of hiding, or masking, parts of an image. In addition to
`image data, a value is included in the image file for every pixel. A value of 2bitdepth-1 is
`opaque and the pixel is displayed normally. A value of 0 is transparent, allowing the
`background color of the screen to show through—the image is not visible at all. The simplest
`application is to make a non-rectangular image. Define an array the size of the image and
`then draw a line between two opposite corners. Set the value of each point above the line to
`one, and the values below to zero. The result will be a triangular image.
`The Formats
`BMP
`
`BMP is the native bitmapped format used by Microsoft Windows. It is a minimal format that
`has few features and uses simple run length encoding for data compression. With the
`widespread use of Windows, BMP is the most common format. In many ways BMP is very
`similar to the PCX format, and has assumed the role that PCX once held.
`
`BMP encodes 1, 2, 4, 8, and 24 bit images. OS/2 uses a similar BMP format, the only
`difference being a slightly simpler header. The first two characters of the BMP header are
`always “BM”.
`
`WMF
`
`Another format native to Microsoft Windows is the Windows MetaFile (WMF). WMF uses
`Windows' Graphical Device Interface (GDI) function calls to store images that appear
`repeatedly in applications. The GDI calls provide support for setting up the screen, defining
`regions, colors, text, pixels, lines, polygons and bitmaps. WMF supports uncompressed
`monochrome, color palette, and true color images.
`
`GIF
`
`The Graphics Interchange Format (pronounced “jiff”) was originally developed by the
`Compuserve Information Service as a color replacement for the earlier RLE format. Since the
`1987 release, GIF has become the standard for graphics interchange, especially on networks.
`A second release in 1989 added several new features to the format. GIF is a lossless format
`based on the LZW compression scheme. It uses a tag system to identify extension blocks in
`the file, although only a few tags have been defined. The biggest assets of the format are its
`relative simplicity, excellent compression, and widespread availability.
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`However, the format has two major drawbacks. First, it is based on a copyrighted compression
`scheme and commercial software using it must pay royalties to Unisys, the patent holder.
`Second, GIF can be used only for images with 256 or fewer colors. (Strictly speaking, that is.
`Since GIF supports local color maps, the palette can be changed in the middle of an image
`allowing for more than 256 colors. Unfortunately, much of the available software does not
`support, or supports poorly, this feature, and it is, at best, clumsy to use.) GIF files can be
`identified by the string GIF87a or GIF89a at the start of the file.
`
`JPEG/JFIF
`
`JPEG (pronounced “jay-peg”) is a standard for lossy compression of images. JPEG achieves
`compression by breaking the image into 8x8 boxes of pixels, performing a mathematical
`operation called a cosine transform on each, and throwing out the high frequency/small detail
`components; the more components thrown out, the greater the compression and the poorer
`the image quality. The remaining frequency components are then run length encoded and
`compressed with the Huffman algorithm. To view an image, a JPEG file must be
`uncompressed, decoded, and an inverse cosine transform performed. This three-step process
`makes displaying JPEGs very slow. Fortunately, JPEG produces excellent compression with
`little or no visible image loss.
`
`JPEG itself is not actually an image format but a set of standards for compressing image data.
`JFIF, the JPEG File Interchange Format, is the format commonly referred to as JPEG. JFIF does
`not support all of the features of JPEG, but is intended as a minimal implementation for image
`transfer. A full featured implementation of JPEG is included in version 6.0 of the TIFF format.
`Designers hope the two implementations of JPEG, one minimal and one full featured, will deter
`software vendors from defining proprietary formats based on JPEG (early Macintosh versions
`were especially notorious for incompatibilities).
`
`JPEG works best on real world images; line drawings and cartoons will not compress nearly as
`well as scanned images. Unlike most other formats, JPEG does not store a pixel as red, green,
`and blue values. Instead it uses a format called YCbCr. Since most display hardware uses RGB
`values, the YCbCr values must be converted—yet another step slowing decompression. The
`JFIF header includes the ASCII characters “JFIF” for format identification.
`
`PBM/PGM/PPM
`
`These three formats are intermediate formats used by the PBMPLUS utilities. The acronyms
`stand for Portable BitMap/GrayMap/PixMap. PBM is for monochrome images, PGM for
`grayscale images with up to 256 shades of gray, and PPM for color images using up to 24-bits
`of true color. A fourth “format” is the Portable AnyMap, PNM. PNM is not actually a format
`itself. A program that uses PNM can read and write PBM, PGM, and PPM files. PNM is used for
`utility programs that support multiple image types. For instance, since the image type of a
`TIFF file may not be known, PNM reads the TIFF file and writes the appropriate file type.
`
`Each of the four formats can read the other ones that carry less information. That is, a PGM
`utility reads PGM and PBM, a PPM utility reads PPM, PGM, and PBM. PBM, PGM, and PPM
`utilities always write in their own format, while PNM utilities generally write whatever format
`they have read. The formats store data either as ASCII or binary data and are otherwise basic
`formats consisting of a header and image data. The header consists of a magic number to
`identify the format, image size, and (except for PBM) the number of colors/gray shades. The
`magic strings are PBM P1(P4), PGM P2(P5), and PPM P3(P6), where the first code is the
`code for ASCII data, and the code in parentheses is for binary data. True color images store
`pixel data as a triplet of numbers for RGB data. For more information on the PBMPLUS
`package, see the section on software below.
`
`PCX
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`The PCX format is the native format of Z-Soft's PC Paintbrush program and uses run length
`encoding. As one of the first general purpose formats, its use has been very widespread: few
`programs exist that do not recognize it. PCX is a basic format consisting of a 128 byte header
`followed by image data. Monochrome as well as 4, 8, and 24-bit color images are supported.
`The palette for 4-bit images is included in the header while the 8-bit palette is appended after
`the image data. This non-uniformity is the result of an older format being updated for newer
`hardware. For this reason, the use of PCX has dwindled in favor of the more coherent and
`unified BMP.
`
`PNG
`
`Amazingly, with all the other formats available, there has been an important niche left
`unfilled: a portable, relatively simple, free standard for the lossless exchange of true color
`images—in short, a 24-bit version of GIF with a non-patented compression algorithm. There
`are existing formats that can be used, but only TIFF has the necessary features, and it suffers
`from over-completeness: very few implementations make use of the entire TIFF specification.
`What is needed is a format that is simple enough that any image saved under it can be read
`by any viewer supporting it.
`
`Compuserve called their development specification GIF24, but when a group of Internet
`graphics experts developed PNG (Portable Network Graphics, pronounced “ping”), Compuserve
`adopted it as the successor to GIF. PNG is a natural extension to GIF, although it is not
`backwards compatible because of a change in compression scheme. In addition to the original
`GIF features, PNG supports true color images up to 48 bits and grayscale images to 16 bits, as
`well as full alpha-channel, gamma correction, and detection of file corruption. On 8 bit and
`larger data, PNG can use a preprocessor on the image data prior to compressing. In many
`cases this processing improves the compression efficiency and results in smaller file sizes.
`Expect to see PNG files appearing at an archive near you soon.
`
`PS/EPS
`
`PostScript (PS) is a page description language developed by Adobe Systems that is both a
`descriptive and raster format and has become one of the most common printer languages.
`PostScript files are created by many application programs as a device-independent output
`format. Encapsulated PostScript (EPS) is a limited version of postscript for single pictures and
`is used for images to be included, or “encapsulated”, in programs or postscript files. The first
`line of a postscript file is a line of the form:
`
`%!PS-Adobe-3.0
`
`where the number refers to the PostScript version the file was created under. EPS files append
`EPSF-3.0 to this line. PostScript is a large and versatile language. Display PostScript is an
`implementation of PostScript for controlling video hardware and is used by NeXT computers
`and software.
`
`TGA
`
`The Targa Truevision graphics format was developed for use in Truevision's product line. TGA
`uses run length encoding to store grayscale, color table, and true color images, and can
`include comments, gamma, alpha, color corrections, and a “postage stamp” version of the
`image. TGA was one of the first true color formats and is still used by some applications like
`the Persistence of Vision Ray Tracer. TGA files may contain a block at the end of the file that
`includes the text TRUEVISION-XFILE.
`
`TIFF
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`The Tag Image File Format is the most robust format. As its name suggests, TIFF makes
`liberal use of the tag concept. TIFF files are typically read using random access because the
`tag fields can come in any order; an image file directory provides offsets to the location of
`data in the file. (Even the directory can be anywhere in the file! A short header gives the
`directory location in the file.) The TIFF format defines several classes of image data: bilevel
`(monochrome), 4 to 8-bit grayscale or color palette, 24-bit RGB, and 24-bit YCbCr. It supports
`run length, Huffman, LZW, and JPEG compression. Most implementations do not support all
`TIFF features, making TIFF a potentially aggravating format. However, TIFF has a huge
`number of features (implemented in tags) making it unique. Originally intended for desktop
`publishing, TIFF has spread to video, fax, and document storage, as well as medical and
`scientific imaging. Because of its complexity, it is not commonly used for home applications.
`
`XBM/XPM
`
`X-Windows defines several formats for internal use. X Bitmap (XBM) is an ASCII format for
`including 1 bit images in the C source of a program: XBM images are integral to the code and
`included at compile time, not run time. XBM data is usually included in header files and
`includes two define statements for the width and height of the image followed by a static
`unsigned character array. XBM images are often used for icons and cursor bitmaps in X. X
`Pixmap (XPM) is the equivalent format for color palette images. Its use is identical to XBM
`except for the addition of the color table and three extra define statements for version
`number, color table length, and the number of bytes per pixel. X does define a general format
`for images, the X Window Dump (XWD or WD) format. XWD supports uncompressed raster
`data of all types.
`Programs and Libraries
`Having looked at some of the common graphics formats, we finish up with a quick look at
`some available programs and libraries for converting images.
`
`xv and ImageMagick
`
`These two X programs display, convert, and manipulate images. xv can read and write JPEG,
`GIF, TIFF, PBM/PGM/PPM, and X formats, although it cannot display 24bit images as 24bit. In
`addition to format conversions, xv can perform simple image manipulations, e.g., rotate, flip,
`crop, magnify, and gamma correction. ImageMagick supports the formats xv does, plus TGA.
`It can send output directly to a postscript printer and has more image manipulation
`capabilities than xv: cut, copy, paste, resize, flip, flop, rotate, invert, emboss, and more, on
`the whole or on part of the image.
`
`ghostscript
`
`The canonical way to view and convert from PostScript (including EPS) is ghostscript.
`Ghostscript operates both from the command line options and interactively. Input is always a
`PostScript file and output may be any of several different file formats, printer languages, or
`screen types. By default, ghostscript sends its output to an X terminal, but it can also save
`images to file. Ghostview is a popular front end for ghostscript that improves the screen
`handling.
`
`PBMPLUS
`
`The PBMPLUS utilities are a set of 120 programs for image conversion and manipulation.
`PBMPLUS defines three intermediate formats: PBM, PGM, and PPM (see listing above). The
`basic philosophy is that if there are twenty formats, you need twenty squared, or 400,
`programs/subroutines to convert between them. The use of intermediate formats reduces that
`number to two times twenty, or forty. In addition to the conversion programs, there are a
`number of simple image processing programs: scaling, rotating, smoothing, convolution,
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`gamma, cropping and more. NETPLUS is a newer version of PBMPLUS, but some versions
`suffer from serious bugs.
`
`C Library support
`
`Substantial support exists for C programmers working with graphics. Libraries for JPEG/JFIF,
`TIFF, and PNG are now available. In addition, code fragments for many of the other formats
`are readily available from Internet archive sites.
`
`IJG JPEG/JFIF library
`
`The easiest way to do JPEG programming is to use the Independent JPEG Group's (IJG)
`library. This library is based on the JFIF specification and includes two common programs
`found on many Unix systems for storage and retrieval of JFIF images: cjpeg and djpeg. The
`library is available as source and compiled code.
`
`SGI TIFF library
`
`Like the JPEG library, a full set of routines to implement TIFF is available. Written by Sam
`Leffler and SGI, this library is also available as source and compiled code. The library includes
`programs for converting, dithering, splitting, and displaying information on TIFF files.
`
`PNG toolbox
`
`With the recent announcement of support for the PNG format, Compuserve also announced a
`PNG toolbox. The toolbox uses the zlib library for the LZ77 code and is intended to speed the
`acceptance of the new format. Its use will be free of royalties. A beta version is available on
`Compuserve.
`Graphics Resources
`There are several archive sites on the Internet where programs and further information can be
`found:
`
`anonymous ftp sites:ftp://sgi.com/graphics/tiff TIFF version 6.0 specification and the source
`for the SGI TIFF libraryftp://sunsite.unc.edu/pub/Linux/apps/graphics/convert
`PBMPLUSftp://sunsite.unc.edu/pub/Linux/X11/xapps/graphics
`ImageMagickftp://sunsite.unc.edu/pub/Linux/libs/graphics Compiled version of JPEG and TIFF
`libraries for Linuxftp://sunsite.unc.edu/pub/Linux/apps/graphics/viewers Compiled version of
`ghostscript for Linuxftp://ftp.wuarchive.edu/ Numerous code fragments for image formats are
`scattered throughout this large archivecompuserve.com GRAPHSUPPORT forum, library 20,
`LP071.zip, the beta version of the PNG toolboxwww.uwm.edu/~ggraef. List of links and other
`direct links to format information
`Gerald Graef (ggraef@csd.uwm.edu) is a doctoral candidate in theoretical physics who lives
`on Alpha, Sparc, and Linux computers. His home page is at: www.uwm.edu/~ggraef.
`
`Archive Index Issue Table of Contents
`
`Copyright © 1994 - 2017 Linux Journal. All rights reserved.
`
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