Digital image formats

Digital image formats

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  By: Sarah AliHasan 1st April 2013
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  A digital imageis a numeric representation of a two dimensional image. Most software developers created a new custom file format for each application they develop. Using Standardized file formats increases the ease with which images can be exchanged and be readable by other software .
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  Criteria to beconsidered when choosing file format: Type of image: black & white, grayscale, scans from documents, color images, color graphics… etc. Storage size & compression: storage requirements & compression method. Capability: how important is the exchange of image data & how important is the long-term machine readability of the data. Application domain.
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  Some types of digital images are shown next;
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  Raster Vs. Vectordata: Raster images have a finite set of digital values, called picture elements or pixels. Vector graphics are based on vectors (also called paths, or strokes) which lead through locations called control points. Each of these points has a definite position on the x and y axes of the work plan. Raster images contains a fixed number of rows and columns of pixels, while vector images results from mathematical geometry (vector) that consist of infinite number of points having both direction and length.
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  1-Tagged Image FormatFile (TIFF): ►A TIFF file can contain a number of images with different properties . ►A TIFF specification provide a range of different compression methods;( LZW, ZIP, CCITT & JPEG) and color spaces which make it easy to store a number of variations of an image in different sizes & representations together in a single TIFF file. ►The flexibility of a TIFF has made it universal exchange format used in archiving documents , scientific applications, digital photography and video production.
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  Merits of TIFFformat: ►The great advantage of saving files in the same TIFF format is that no information is lost. ►Useful for transporting image files from one application to another or from one computer to another as they are designed to be independent of any particular hardware or software. ► The strength of TIFF format lies in its architecture , which enables new image types & information blocks to be created by defining new “tags” .
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  Demerits of TIFFformat: ►TIFF files encounter errors when loading. In flexibility also lies the weakness of this format where proprietary tags are not always supported. ►The drawback of TIFF files is that the file size is large. They take up a lot of space when stored and take a long time to open in an application. ► Difficult to store. They are not useful as embedded image files in a webpage as they take much too long to download.
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  2- Graphics InterchangeFormat (GIF) : ►Widely popular format; due to its early support for indexed color at multiple bit depths, LZW compression, interlaced image loading. ► Designed for color & grayscale images with maximum 8bit depth (i.e. doesn’t support true color images). GIF files are designed to efficiently encode flat images consisting of large areas of the same color. ►Supports color palletes in the range of (2-256); enabling pixels to be encoded using fewer bits.
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  Merits of GIFformat: ► The limited color range has the advantage that GIF files will be smaller in size. ► Easy to store and display: When saving an image as GIF, you can specify how the image appears as it is downloaded. Select “Interlaced” to display the image gradually in increasing detail as it is downloaded. ► GIF format has the ability to encode simple animations by storing a number of images in a single file for later sequential display. Also can be made into a transparent image.
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  Demerits of GIFformat: ► The limitation of an 8-bit palette, or 256 colors makes the GIF format suitable for storing graphics with relatively few colors such as simple diagrams, shapes, logos & cartoon style images and ineffective for detailed images or dithered images. ► Loss of color variation. GIF is not suitable for subtle color variations retained in your image. Gif formats are very useful for storing grey scale images which are limited to 256 tones of grey, so that no information is lost .
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  3-Portable Network Graphics(PNG): ► A universal image format designed especially for the use of the internet. PNG format was designed to be an alternative to the GIF format . ►PNG files support 3 types of images: 1) true color (with up to 3*16 bits/pixel) 2) grayscale (with up to 16bit/pixel) 3) indexed (with up to 256 colors). ►PNG includes an alpha channel for transparency with a maximum width of 16bit.
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  Merits of PNGformat: ► Unlike GIF, the PNG format is able to retain full 16,000,000 colors. Still the format support one image per file but it allows images of up to 2^30 * 2^30 pixels. ► The PNG format uses a lossless compression method . Demerits of PNG format: ► Relative large file size ensures PNG format may be less suitable for Web or other online service.
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  4-Joint Photographic ExpertsGroup (JPEG): ►It’s a standard defines a compression method used to display continuous tone images (such as photographs) with a goal of achieving an average data reduction of 1:16. ► The level of JPEG compression will determine how much information is lost (this can be set in most programs that offer jpeg compression). ► Jpeg files can be saved with the following options: "standard", "baseline optimized" and "progressive".
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  Algorithm Core ofRGB image: 1) color conversion & down sampling: color transformation from RGB into the Y Cb Cr space is used to separate the actual color components from the brightness Y component.
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  2) Cosine transform& quantization in frequency space: The image is divided up to a regular grid of 8 blocks. The frequency spectrum is computed for each independent block using the discrete cosine transformation. Next the spectral coefficients of each block is quantized using quantization table; size of this table determines the compression ratio. 3) Lossless compression: The quantized spectral components data stream is again compressed using lossless method (arithmetic or Huffman) in order to remove the last redundancy in the data stream.
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  Merits of JPEGformats: ► Retains up to 16,000,000 colors. ► The ability to compress more or less to find a compromise between file size and image quality. Demerits of JPEG format: ► Loses information on pixel frequency values because they may be averaged to the values of those pixels in its immediately surroundings. ► File size larger than gif because of color information.
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  Some JPEG fileformats: *JFIF: What is normally referred to as JPEG is an instance of “JPEG File Interchange Format”(JFIF) .It specifies a file format based on the JPEG standard by defining the remaining necessary elements of the file format. *EXIF: The exchangeable image file format, Is a variant of (JFIF) designed for storing image data originating on digital cameras. It is the practically used file format by all manufacturers as the standard format for storing digital images on memory cards. *JPEG-2000: was designed to overcome some of the weaknesses of traditional JPEG codec. Higher compression ratios achieved here (up to 0.25bit/pixel), the use of larger 64*64 blocks & the replacement of the cosine transform by Wavelet transform.