Pictures are stored in multiple formats, occupying different spaces, indicating which images are compressed and which are not. The more specific the format, the better.

Generally speaking, there are two different types of image formats: lossy compression and lossless compression. 1. Lossy compression Lossy compression can reduce the space occupied by images in memory and disk. When you watch images on the screen, you won't find that it has a great adverse effect on the appearance of images. Because human eyes are sensitive to light, the effect of light on scenery is more important than color, which is the basic basis of lossy compression technology. The characteristic of lossy compression is to keep the gradual change of color in the image and delete the sudden change of color in the image. A large number of experiments in biology have proved that the human brain will use the closest color to fill the lost color. For example, for a white cloud on a blue sky background, the method of lossy compression is to delete some color parts at the edge of the scene in the image. When looking at this picture on the screen, the brain will fill in the missing color part with the color seen on the spot. Using lossy compression technology, some data is intentionally deleted, and the deleted data is not recovered. Undeniably, using lossy compression technology can greatly compress the data of files, but it will affect the image quality. If the lossy compressed image is only displayed on the screen, it may have little effect on the image quality, at least for the recognition of human eyes. However, if the image processed by lossy compression technology is printed by a high-resolution printer, the image quality will be obviously damaged. 2. Lossless compression The basic principle of lossless compression is that the same color information only needs to be saved once. Software that compresses an image will first determine which areas in the image are the same and which are different. Images containing duplicate data (such as blue sky) can be compressed, and only the starting point and ending point of blue sky need to be recorded. However, there may be shades of blue, and the sky may sometimes be blocked by trees, peaks or other objects, which need to be recorded separately. In essence, lossless compression can delete some duplicate data and greatly reduce the image size to be saved on disk. However, the lossless compression method can not reduce the memory occupation of the image, because when reading the image from the disk, the software will fill the missing pixels with appropriate color information. If we want to reduce the memory occupied by images, we must use lossy compression method. The advantage of lossless compression method is that it can better preserve the image quality, but the compression rate of this method is relatively low. However, if the image needs to be printed by a high-resolution printer, it is best to use lossless compression. Almost all image files use their simplified format names as file extensions. From the extension, we can know what format this image is stored in, what software should be used to read/write, and so on.

I. BMP image file format

BMP is an image file format independent of hardware devices, which is widely used. It uses bitmap storage format and does not use any other compression except image depth, so the BblP file takes up a lot of space. The image depth of BMP file can be selected as lbit, 4bit, 8bit and 24bit. When data is stored in a BMP file, images are scanned from left to right and from bottom to top.

Because BMP file format is the standard for exchanging data related to drawings in Windows environment, all graphics and image software running in Windows environment supports BMP image format.

A typical BMP image file consists of three parts: a bitmap file header data structure, which contains information such as the type and display content of the BMP image file; Bitmap information data structure, including BMP image width, height, compression mode, defined color and other information.

Second, the PCX image file format

The formation of the image file PCX has a development process. The first prototype of PCX appeared in the commercial painting software package named PC PAINBRUSH launched by ZSOFT Company. Later, Microsoft transplanted it into the Windows environment and became a sub-function of the Windows system. It was first widely used in Microsoft Windows 3. 1. With the popularization and upgrade of Windows and its powerful image processing capability, PCX, together with GIF, TIFF and BMP image file formats, are supported by more and more graphics and image software tools, and people pay more and more attention to them.

PCX is the earliest file format that supports color images, and now it can support 256 colors at most, as shown in Figure 4-25, which displays 256-color images. PCX designers introduced the color image file format with foresight, making it a very popular image file format.

PCX image file consists of file header and actual image data. The file header consists of 128 bytes, which describes version information, horizontal and vertical resolution of image display equipment, color palette and other information: in actual image data, it indicates image data type and color type. The data in PCX image files are all image data compressed by PCXREL technology.

PCX is the image file format of PC brush. The image depth of PCX can be selected as 1, 4 and 8 bits. Because this file format appeared earlier, it does not support true color. PCX file adopts RLE run coding, and the compressed image data is stored in the file body. Therefore, when the collected image data is written into PCX file format, RLE coding should be carried out; When reading the PCX file, it should be RLE decoded before further display and processing.

Three. TIFF image file format

TIFF(TaglmageFileFormat) image file is a common image file format developed by Aldus and Microsoft for desktop publishing system. TIFF format is flexible and changeable. It defines four different formats: TIFF-B is suitable for binary images, TIFF-G is suitable for black and white gray images; TIFF-P is suitable for color images with palette; TIFF-R is suitable for RGB true color images.

TIFF supports a variety of coding methods, including RGB compression, RLE compression and JPEG compression.

TIFF is one of the most complex image file formats, which is extensible, convenient and modifiable, and can be provided for running and image editing programs in IBMPC and other environments.

TIFF image file consists of three data structures, namely, file header, one or more IFD directories containing marker pointers, and data itself.

The first data structure in a TIFF image file is called an image file header or IFH. This structure is the only part with a fixed position in the TIFF file; The directory of IFD image file is an information block with variable byte length, and the Tag tag is the core part of TIFF file. All the image parameters to be used are defined in the directory of the image file, and each directory entry in the directory contains one image parameter.

Four. GIF file format

GIF (Graphic Interchange Format) originally means "Image Interchange Format", which is an image file format developed by CompuServe Company in 1987. The data of GIF file is a continuous tone lossless compression format based on LZW algorithm. Its compression ratio is generally around 50%, which does not belong to any application. At present, almost all related software supports it, and a large number of software in the public domain are using GIF image files.

The data of GIF image file is compressed by variable length compression algorithm. Therefore, the image depth of GIF varies from lbit to 8bit, which means that GIF can support 256-color images at most. Another feature of GIF format is that images with multiple colors can be stored in a GIF file. If the data of multiple images stored in a file are read out one by one and displayed on the screen, the simplest animation can be formed.

GIF decoding is faster, because interlaced GIF images can be divided into four scans when decoding and displaying. Although the first scan only showed one-eighth of the whole image, and the second scan only showed 1/4, it has already shown an overview of the whole image. When displaying GIF pictures, staggered pictures will make you feel that its display speed seems to be faster than other pictures, which is the advantage of staggered storage.

Verb (abbreviation of verb) JPEG file format

JPEG is the abbreviation of Joint Photographic Expert Group, and its file name is ".. jpg" or ". Jpeg ",which is the most commonly used image file format. It is a lossy compression format organized by the Software Development Association, which can compress images in a small storage space, and duplicate or unimportant data in images will be lost, which will easily cause damage to image data. Especially, if the compression ratio is too high, the quality of the restored image after final decompression will obviously decrease. If you pursue high-quality images, you should not use too high compression ratio. But JPEG compression technology is very advanced. It uses lossy compression to remove redundant image data, and can display very rich and vivid images at the same time, in other words, it can obtain better image quality with the least disk space. Moreover, JPEG is a very flexible format, which has the function of adjusting image quality, allowing files to be compressed at different compression ratios and supporting multiple compression levels. The compression ratio is usually between 10: 1 and 40: 1. The higher the compression ratio, the lower the quality. On the contrary, the smaller the compression ratio, the better the quality. For example, a BMP bitmap file of 1.37Mb can be compressed to 20.3KB Of course, you can also find a balance between image quality and file size. JPEG format mainly compresses high-frequency information and preserves color information well. It is suitable for the Internet, can reduce the image transmission time, can support 24-bit true color, and is also widely used in images that need continuous tone.

JPEG format is the most popular image format on the network at present, which can compress files to a minimum. When stored in JPEG format in Photoshop software, the compression level of 1 1 is provided, which is represented by 0- 10. Level 0 has the highest compression ratio and the worst image quality. The compression ratio can reach 5: 1 even if the quality maintenance with almost no loss of details is adopted. When saving in BMP format, you will get a 4.28MB image file. When saved in JPG format, the file size is only 178KB, and the compression ratio reaches 24: 1. After many comparisons, eight-level compression is the best ratio of storage space and image quality.

JPEG format is widely used, especially on the Internet and CD-ROM. At present, all kinds of browsers support JPEG as the image format, because the file size of JPEG format is small and the download speed is fast.

As an upgraded version of JPEG, JPEG2000 has a compression ratio about 30% higher than that of JPEG, and supports both lossy and lossless compression. An extremely important feature of JPEG2000 format is that it can realize progressive transmission, that is, the outline of the image is transmitted first, and then the data is transmitted step by step, so as to continuously improve the image quality and make the image display from hazy to clear. In addition, JPEG2000 also supports the so-called "region of interest" function, which can arbitrarily specify the compression quality of the region of interest on the image, or select the specified part to decompress first.

Compared with JPEG, JPEG2000 has obvious advantages and backward compatibility, so it can replace the traditional JPEG format. JPEG2000 can be applied to both traditional JPEG market, such as scanners and digital cameras, and emerging fields, such as network transmission and wireless communication.

Sixth, TGA format

TGA format (Tagged Graphics) is an image file format developed by American Truevision Company for its graphics card. The file suffix is ". TGA "has been accepted by the international graphic and image industry. TGA has a simple structure, belongs to the general format of graphics and image data, has great influence in the field of multimedia, and is the preferred format for converting computer-generated images into TV.

The biggest feature of TGA image format is that it can make irregular shapes of graphics and image files. Generally, graphics and image files are square. If there are circular, diamond or even empty image files, TGA can come in handy! TGA format supports compression and uses lossless compression algorithm.

Seven. exchangeable image file

EXIF format is a digital camera image file format advocated by Fuji Company in 1994. In fact, it is the same as JPEG format. The difference is that it can also store exposure data such as shooting date, aperture, shutter and flash data, incidental information and small-size images.

Eight, FPX image file format

The fpx image file format (extended FPX) was jointly developed by Kodak, Microsoft, Hewlett-Packard and Live PictureInc, and officially released in June. 1996. FPX is an image format with multiple resolutions, that is, images are stored in a series of different resolutions. The advantage of this format is that the image quality can still be maintained when the image is enlarged. Further, when decorating an FPX image, only the processed image is processed.

Nine, SVG format

SVG is a scalable vector graphics format. It is an open standard vector graphics language, which can enlarge the graphics display at will, and the edges are extremely clear. The characters in SVG images remain editable and searchable, and there is no font restriction. The generated files are small and fast to download, which is very suitable for designing high-resolution Web graphics pages.

X.PSD file format

This is a special file format of Photoshop image processing software. The file extension is. . Psd can support various image features of layers, channels, masks and different color modes. It is an uncompressed original file storage format. Scanners cannot directly generate files in this format. PSD file sometimes has a large capacity, but because it can retain all the original information, it is the best choice to save the unfinished image in PSD format in image processing.

XI。 CDR file format

CDR format is a special graphic file format of CorelDRAW, a famous drawing software. Because CorelDRAW is a vector graphics drawing software, CDR can record the attributes, location and paging of files. However, it has poor compatibility and can be used in all CorelDraw applications, but other image editing software cannot open such files. Twelve. PCD file format

PCD is the abbreviation of Kodak photo CD, and the file extension is. . Pod, which is a PhotoCD file format developed by Kodak, can only be read by other software systems. This format uses YCC color mode to define colors in images. YCC and CIE color spaces contain much more colors than RGB and CMYK colors of displays and printing devices. Most of the PhotoCD images are of very high quality.

Thirteen. DXF file format

DXF is the abbreviation of graphic interchange format, and the extension is. DXF。 It is a graphic file format in AutoCAD. It stores the graphics in ASCII code, and the size of the graphics is very accurate, so large software such as CorelDraw and 3DS can be called and edited.

Fourteen UFO file format

It is a special image format of the famous image editing software Ulead Photolmapct, which can completely record all the image attributes processed by PhotoMapCT. It is worth mentioning that UFO files use objects instead of layers to record image information.

Fifteen. EPS file format

EPS is the abbreviation of Encapsulated PostScript, which is a cross-platform standard format. The extension is. EPS and PC platform. . Epsf on Macintosh platform is mainly used to store vector images and raster images. EPS format is described in PostScript language, which can save other types of information, such as multi-tone curve, Alpha channel, color separation, clipping path, screen hanging information and tone curve. Therefore, EPS format is usually used for printing or printout. Many EPS format options in Photoshop can achieve full control of printing, and even better than TIFF format in some cases.

Sixteen. PNG image file format

The original name of PNG(Portable Networf Graphics) is "Portable Network Image", which is the latest accepted image file format on the Internet. PNG can provide lossless compressed image files that are 30% shorter than GIF. It also provides 24-bit and 48-bit true color image support and many other technical support. Because PNG is very new, not all programs can use it to store image files at present, but Photoshop can handle PNG image files and can also store them in PNG image file format.