HOME
        TheInfoList






Image file formats are standardized means of organizing and storing digital images. An image file format may store data in an uncompressed format, a compressed format (which may be lossless or lossy), or a vector format. Image files are composed of digital data in one of these formats so that the data can be rasterized for use on a computer display or printer. Rasterization converts the image data into a grid of pixels. Each pixel has a number of bits to designate its color (and in some formats, its transparency). Rasterizing an image file for a specific device takes into account the number of bits per pixel (the color depth) that the device is designed to handle.

Image file sizes

The size of raster image files is positively correlated with the number of pixels in the image and the color depth (bits per pixel). Images can be compressed in various ways, however. A compression algorithm stores either an exact representation or an approximation of the original image in a smaller number of bytes that can be expanded back to its uncompressed form with a corresponding decompression algorithm. Images with the same number of pixels and color depth can have very different compressed file size. Considering exactly the same compression, number of pixels, and color depth for two images, different graphical complexity of the original images may also result in very different file sizes after compression due to the nature of compression algorithms. With some compression formats, images that are less complex may result in smaller compressed file sizes. This characteristic sometimes results in a smaller file size for some lossless formats than lossy formats. For example, graphically simple images (i.e. images with large continuous regions like line art or animation sequences) may be losslessly compressed into a GIF or PNG format and result in a smaller file size than a lossy JPEG format.

For example, a 640 * 480 pixel image with 24-bit color would occupy almost a megabyte of space:

640 * 480 * 24 = 7,372,800 bits  = 921,600 bytes = 900 KiB

With vector images the file size increases only with the addition of more vectors.

Image file compression

There are two types of image file compression algorithms: lossless and lossy.

Lossless compression algorithms reduce file size while preserving a

Image file formats are standardized means of organizing and storing digital images. An image file format may store data in an uncompressed format, a compressed format (which may be lossless or lossy), or a vector format. Image files are composed of digital data in one of these formats so that the data can be rasterized for use on a computer display or printer. Rasterization converts the image data into a grid of pixels. Each pixel has a number of bits to designate its color (and in some formats, its transparency). Rasterizing an image file for a specific device takes into account the number of bits per pixel (the color depth) that the device is designed to handle.