Graphic images that have been processed by a computer can usually be divided into two distinct categories. Such images are either bitmap files or vector graphics. If you work in image analysis, you need a good comprehension of the bitmap files. These pages tries to describes bitmap data.
Bitmaps images are exactly what their name says they are: a collection of bits that form an image. The image consists of a matrix of individual dots (or pixels) that all have their own colour (described using bits, the smallest possible units of information for a computer).
Lets take a look at a typical bitmap image to demonstrate the principle:
To the left you see an image and to the right a 250 percent enlargement of the top of one of the mountains. As you can see the image consists of hundreds of rows and columns of small elements that all have their own colour. One such element is called a pixel -short for picture element). The human eye is not capable of seeing each individual pixel so we perceive a picture with smooth gradations.
The number of pixels you need to get a realistic looking image depends on the way the image will be used.
Types of bitmap images
Bitmap images can contain any number of colours but we distinguish between four main categories:
- Line-art. These are images that only contain two colours, usually black and white. Sometimes these images are referred to as bitmaps because a computer has to use only 1 bit (on=black, off=white) to define each pixel.
- Grayscale images, which contain various shades of grey as well as pure black and white. Images can be 8-bits, 16-bits (unsigned) or 32-bits. i.e. the precision shades of grey is described by the number of bits granted for each Pixel.
- Multitones: such images contain shades of two or more colours. The most popular multitone images are duotones, which usually consist of black and a second spot colour (often an Pantone colour). The example below contains black and Pantone Warm Red.
- Full colour images. The colour information can be described using a number of colour spaces: RGB (Red, Green , Blue). An RGB image, sometimes referred to as a truecolor image, it defines red, green, and blue color components for each individual pixel. RGB images do not use a look up table. The color of each pixel is determined by the combination of the red, green, and blue intensities stored in each color plane at the pixel's location. Graphics file formats store RGB images as 24-bit images, where the red, green, and blue components are 8 bits each. This yields a potential of 16 million colors.
File formats that are used for bitmap data
Bitmap data can be saved in a wide variety of file formats. Among these are:
- BMP: limited file format.
- GIF: mainly used for internet graphics
- JPEG: or rather the JFIF file format, which is mainly used for internet graphics
- TIFF: the most popular bitmap file format
- FITS: the bitmap file format used in astronomy
BMP is an acronym for Bimap, it is open format image developed by Microsoft and IBM. It is the most simple image format.
BMP images can be 1-bits (Line-art), 4-bits (16 colors for a grayscale image or a duotone image), 8 bits (256 colors for a grayscale image or a duotone image), 16-bits (65 536 colors for a grayscale image or a duotone image), ... or 24-bits (RGB color with 8-bits for each primary color). The BMP format don't use compression, they are generally a big files.
GIF is an acronym for Graphics Interchange Format. GIF is for pictures that use 256 (or fewer) distinct colours and animations that use 256 (or fewer) distinct colours per frame. GIFs are compressed files, and are employed specifically to reduce the amount of time it takes to transfer images over a network connection.
JPEG formatJPEG is an acronym for Joint Photographic Experts Group. JPEGs are compressed files. It is a format at loss, which thus eliminates from information, but one of the strong points of JPEG is that its compression ratio is adjustable.This format is useful to save results and not to analyze images.
In computing, JPEG is a commonly used standard method of lossy compression for photographic images. The file format which employs this compression is commonly also called JPEG; the most common file extensions for this format are .jpeg, .jfif, .jpg, .JPG, or .JPE although .jpg is the most common on all platforms. One of the strong points of JPEG is that its compression ratio is adjustable.This format is especially useful to save results.
TIFF is a flexible and adaptable file format. It can handle multiple images and data in a single file through the inclusion of "tags" in the file header. Tags can indicate the basic geometry of the image, such as its size, or define how the image data is arranged and whether variousimage compression options are used
FITS is an acronym for Flexible Image Transfer System and is the most commonly used file format in astronomy.
FITS is often used to store also non-image data, such as spectra, photon lists, data cubes, and much more. A FITS file may contain several extensions, and each of these may contain a data object.
A major advantage of FITS for scientific data is that the header information is human readable ASCII), so that an interested user can examine the headers to investigate a file of unknown provenance. Each FITS file consists of one or more headers containing ASCII card images (80 character fixed-length strings) that carry keyword/value pairs, interleaved between data blocks. The keyword/value pairs provide metadata such as size, origin, binary data format, free-form comments, history of the data, and anything else the creator desires: while many keywords are reserved for FITS use, the standard allows arbitrary use of the rest of the name-space.