Image File Formats By Dr. Rajeev Srivastava 1

Image File Formats Header and Image data. A typical image file format contains two fields namely Dr. Rajeev Srivastava2

Header field They contains various information related to image such as: Format or version identification, Image type (such as B/W, Grey, 8-bit colour, 24- bit colour etc.), Image width and height, Compression type used, Image data format etc.

Image data format The length of header file is normally fixed. If the header length is not fixed, then this information has to be included in header field. Specifies the order in which pixel values are stored (e.g. from left to right and top to bottom) in image data field and whether interlacing is used or not.

Image File Formats A digital image is often encoded in the form of a binary file for the purpose of storage and transmission. Some standard image encoding file formats include: and many more. All these file formats share structural similarities but differ in their technical details. TypesDescription BMPWindows bitmap GIFGraphics Interchange Format PNGPortable network group format JPEGJoint picture expert photographic group format TIFFTagged image file format 5

A general image file format File HeaderImage Data Format ID Image Type Image Data Format Image width and height in pixels Compression Types Pixel Values Colour Map (If Any) 6

Lossy or Lossless Format ID Compression Types Width & Height of Image Image Data Format Image Type File Header Format or version identification B/W, Gray, 8-bit colour, 24-bit colour etc. Specifies the order in which pixel values are stored (Interlacing etc.). Length of header normally fixed otherwise include information No. of Rows and No. of Columns Dr. Rajeev Srivastava7

Some file formats can store multiple images in the same file and may have multiple image data field. Pixel Values Color Map (If any) Image Data Field The pixel values in image data section may be compressed using some standard compression techniques such as run length encoding (RLE). Color Maps Dr. Rajeev Srivastava8

Image File Formats The image data field contains the pixel intensity values and colour map, if any. Some file formats can store multiple images in the same file and may have multiple image data field.

Contd.. The pixel values in image data section may be compressed using some standard compression techniques such as Run Length Encoding (RLE). Compression is generally referred to as reduction of the size of digital data files by removing redundant and/or non-critical information. There are two types of compression techniques: Lossy compression, and Lossless compression.

Image Compression Types LosslessLossy Dr. Rajeev Srivastava11

12 Lossless compression is a type of compression that removes only redundant information. The same compression techniques which can be applied to general files, such as "zip" compression, can also be applied to graphic images. There are also compression techniques which are particular to graphic images. Some graphic file formats incorporate one or more compression techniques.

Lossy compression is a type of compression that deliberately sacrifices some of the original data in return for much higher compression ratios than those achievable with lossless techniques.

Types of images Binary Images: In this format the following method is used for representation -0 (White) and 1 (Black) Gray scale images: Typically 2 k (2, 16, 256, 4096,... ) grey levels, k=number of bits per pixel Indexed colour images: Typically 256 colours (GIF-format) 14Dr. Rajeev Srivastava

RGB colour images Red, green and blue channels, typically 256 levels each: 2 (3*8) = colours (e.g. TIF and JPEG formats) Multispectral images Multiple channels, including visible light. (remote sensing, e.g. Satellite imaging)

Binary Image 16Dr. Rajeev Srivastava

Intensity (Gray-Level) Image 17Dr. Rajeev Srivastava

Coloured Image 18Dr. Rajeev Srivastava

Indexed Color Images 19Dr. Rajeev Srivastava

RGB Color Images 20Dr. Rajeev Srivastava

Some standard image file formats The Device Independent Bitmap (DIB) graphic file format is particular to the Windows operating system. The common suffix for DIB files is ".bmp". Images are typically stored as 3 bytes per pixel, so a 256x256 pixel image requires 193KB of space. Bitmap Graphic File Format (BMP)

Bitmap Graphic File Format (BMP)… The image is stored in scan line order, from bottom to top. The DIB format supports a single image per file. The DIB file format does support compression, but this is seldom used.

Icon File Format Windows icon files (.ico) use an image format similar to bitmap files, but additionally allow any bit or set of bits of the image to be transparent. Icon files can also contain multiple images. The image size must be 8 x 8, 16 x 16, or 64 x 64.

Cursor File Format Windows cursor files (.cur) use an image format similar to bitmap files, but additionally allow any bit or set of bits of the image to be transparent. Cursor files can also contain multiple images. Each image also has a designated "hot spot", which is the effective position of the cursor. Dr. Rajeev Srivastava24

Graphics Interchange Format (GIF) A bit mapped graphics file format which uses LZW (Lempel Ziv Welsh) compression and a maximum of 256 colour palette. Graphics Interchange Format (GIF) files use a 256 entry colour pallette and lossless compression to reduce the size of the file. Dr. Rajeev Srivastava25

GIF… The 256 colour limitation, however, means that colour information will be lost if the image contains more than 256 colours. GIF files commonly have the suffix ".gif", and are appropriate for line drawings, which typically do not use more than 256 colours.

GIF... Photographs and continuous tone images may suffer noticeable colour degradation when stored as GIF files, but in some cases the degradation may be negligible. The GIF specification also supports multiple images stored in a single file as a "slide show", with selectable delays between image displays. This allows GIF images to be used as a form of animation.

Portable Networks Graphics (PNG) The Portable Networks Graphics (PNG) graphics file format was designed to replace GIF. PNG uses lossless compression, and supports partial transparency on a pixel by pixel basis. Transparency is a property of an area on a graphics screen that allows the graphic element behind it to become visible. Dr. Rajeev Srivastava28

JPEG File Interchange Format JPEG (Joint Photographic experts group) is a lossy compression technique for colour images. The JPEG file interchange format uses lossy compression to reduce the size of the file. Dr. Rajeev Srivastava29

JPEG… The compression technique is designed specifically for graphic images, with the goal of removing the information which is least noticeable. The amount of information removed is adjustable. When the image is stored, a "quality factor" is specified. A high quality factor removes the least information, but produces the least compression.

JPEG.. As the quality factor is reduced, more information is removed, and the compression increases. The ideal quality factor is subjective, depends on the image being compressed, and can only be determined through trial and error. Dr. Rajeev Srivastava31

JPEG… JPEG files typically have the suffix ".jpg" or ".jpeg", and are appropriate for photographs and continuous tone images. The JPEG compression techniques produce aliasing with line drawings, and generally JPEG should not be used for line drawings.

Web Browser Support Most web browsers support GIF, JPEG, PNG, and DIB graphics formats. While a few introduced in recent years have inbuilt support for TIFF etc.

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