Detailed explanation of picture format

In our daily work, we often come into contact with many kinds of picture formats, the most common ones are JPG, BMP, PNG, GIF and TIF, in addition to PSD, AI, TIF, EPS, RAW and so on. Different picture formats have different characteristics and occupy different data spaces. Some pictures with different formats need special software to open. I believe everyone has encountered them. Let's talk about their differences here!

Image formats can be roughly divided into bitmap, vector image (lossless scaling), HDR high dynamic range image (image with illumination information), plane software source file and so on. According to the storage format, it can be divided into lossy compression and lossless compression.

JPG (also known as JPEG) is a standard lossy compression method widely used in photo images. JPEG compressed image files are also commonly known as JPEG files, and the most commonly used extension format is. Jpg, other commonly used extensions.

It also includes. jpeg，。 jpe，。 Jfif and. Giff. JPEG format data can also be embedded in other file formats, such as TIFF file format.

JPEG image format is the most common and commonly used. It can compress images in a small storage space, but it will lose some image data, especially the higher the compression ratio, the lower the image quality. But JPEG compression technology is also its application advantage. 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. At the same time, JPEG is a very flexible format, which has the function of adjusting image quality, allowing files to be compressed with different compression ratios, supporting multiple compression levels, and having more application environments and more convenience.

PNG is a lossless compression image storage format, and its design purpose is to try to replace GIF and TIFF file formats, while adding some functions that GIF file format does not have. Because it uses lossless compression algorithm to compress, the file size of images with the same pixel width and height saved as PNG is often much larger than JPEG, usually several times that of JPEG. Lossless compression does not lose image data, and supports the alpha channel, and completely saves image data without aliasing, so it is generally applied to PS materials or icons, which is why Android and iOS icons use png format.

BMP (Bitmap) is the standard image file format in Microsoft Windows operating system, which is divided into two categories: device-dependent bitmap (DDB) and device-independent bitmap (DIB), and is widely used. It uses bitmap storage format, usually uncompressed, and does not use any other compression except the optional image depth. So BMP files take 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.

GIF (Graphic Interchange Format), originally meaning "Image Interchange Format", is an image file format developed by CompuServe Company in 1987. GIF is very popular because of its small size and relatively clear imaging, especially suitable for the early slow network speed. 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. GIF format can store images of multiple colors. If multiple image data stored in one file are read out one by one and displayed on the screen, the simplest animation can be formed. This is the format of dynamic pictures and expression packs that people usually see on websites.

PSD(Photoshop Document) is a special format of Adobe's famous image processing software Photoshop. This format can store all the information of layers, channels, reference lines, labels and color modes in Photoshop, and it is an uncompressed original file storage format. PSD file can keep all the original operation information, so it will take up a lot of space, but it is more meaningful for later modification and continued operation. PSD file is actually the source file of Photoshop. At present, other software can gradually support native PSD, but some special effects will be merged into an independent pixel layer. If you make an image that needs to be revised repeatedly, it is very necessary to save it in PSD format.

EPS(Encapsulated Post Script) is a comprehensive format among the general exchange formats commonly used in desktop printing systems at present. It consists of text files in PostScript language and (optional) low-resolution representative images described in PICT or TIFF format, such as multi-tone curve, Alpha channel, color separation, clipping path, screen information and tone curve, so EPS format is often used for printing or printout. EPS is the most important format in our image processing work, which is widely used in graphics and layout design under Mac and PC environments.

PDF (Portable Document Format) is a file format developed by Adobe Systems, which is used to exchange files independently of applications, operating systems and hardware. PDF file is based on PostScript language image model, which can ensure accurate color and accurate printing effect on any printer, that is, PDF will faithfully reproduce every character, color and image of the manuscript. PDF files use industry standard compression algorithms, which are usually smaller than PostScript files and easier to transmit and store. It is also page independent. A PDF file contains one or more "pages", which can handle each page independently, especially suitable for multiprocessor systems. In addition, a PDF file also contains the PDF version used in the file and the positioning information of some important structures in the file. It is precisely because of its various advantages that PDF files have gradually become the new favorite of the publishing industry.

The purpose of designing PDF file format by Adobe Company is to support cross-platform and multimedia integrated information publishing, especially for network information publishing. In order to achieve this goal, PDF has many incomparable advantages over other electronic document formats. PDF file format can encapsulate text, font, format, color, graphics and images in one file independent of device and resolution. Format files can also contain hypertext links, sounds, dynamic images and other electronic information, support super-long files, and are highly integrated, safe and reliable.

TIFF(Tagged Image File Format) is a lossless compression format (the highest compression ratio can only be 2 ~ 3 times), which can keep the color and level of the original image, but it takes up a lot of space. For example, a 2-megapixel image occupies almost 6MB of storage capacity, so TIFF is usually used for more professional purposes, such as book publishing and posters, and is rarely used on the Internet. TIFF is a flexible bitmap format, which is mainly used to store images including photos and artistic drawings. By including "tags" in the file header, it can handle multiple images and data in one file. Tags can indicate the basic geometric dimensions of an image, such as image size, or define the arrangement of image data and whether to use various image compression options.

HDR (High Dynamic Range) can provide more dynamic range and image details than ordinary images. According to the LDR (low dynamic range) images with different exposure times, the final HDR image can be synthesized by using the LDR image with the best details corresponding to each exposure time, which can better reflect the visual effect in the real environment. In reality, the real brightness difference, that is, the ratio of the brightness of the brightest object to the brightness of the darkest object, is the eighth power of 10, while the range that human eyes can see is about the fifth power of 10, but ordinary monitors and cameras can only show 256 different brightness. Generally speaking, computer reality cameras can't reach the color range that we can perceive with naked eyes, so hdr image format will be accompanied by light intensity information, which has more brightness data storage than other images, and its way of recording brightness is also different from traditional images. Instead of compressing the brightness information into the color space of 8bit or 16bit in a nonlinear way, it records the brightness information directly, so we can say that it records the illumination information in the image environment. Many HDRI files are provided in the form of panorama, and we can also use it as the environmental background to produce reflection and refraction. It should be emphasized here that HDRI is fundamentally different from panorama. Panorama refers to an ordinary image containing a 360-degree scene, which can be in JPG format, BMP format, TGA format, etc. It belongs to a low dynamic range radiant brightness image, and has no illumination information.

RAW (original image format) is an "unprocessed" image. It can be understood that the RAW image is the original data that CMOS or CCD image sensor converts the captured light source signal into digital signal, and the details of the RAW format image are more rigorous and easier to adjust. RAW files are mostly directly output by cameras, which record the original information of digital camera sensors and some metadata (such as ISO settings, shutter speed, aperture value, white balance, etc. ) is generated by camera shooting. Therefore, RAW is also an unprocessed and compressed format, which can be conceptualized as "original image encoded data" or more vividly called "digital negative".

Usually with HDR high dynamic range lighting information, you can better control exposure, white balance, color saturation and so on when editing pictures in the later stage. Therefore, in order to consult and post-process photos more conveniently, photographers usually record photos in JPEG and RAW formats at the same time, which not only allows users to organize and edit photos (JPEG) with conventional image processing software; RAW can be used to solve the problem when users need to get a finely processed photo or improve the shortcomings of the photo, such as incorrect white balance and lack of highlight/dark details.

For most users, our most commonly used formats should be RAW, PSD and JPEG. If the RAW+JPEG format is used when taking photos, the space occupied by one photo is about 35MB, and more photos in the same scene will reach 100MB. In addition, many people now have the habit of taking photos with them, and they will take photos of several GB spaces if they are not careful. Then we suggest that you prepare at least 32GB memory card for shooting and buy TB-level mechanical hard disk/mobile hard disk for backup and preservation.

If you need to use EPS, TIF, BMP or even more other image formats in your work, it fully shows that your work is very professional and image data is more important, so data preservation is even more important. At this point, you may need to save a photo in multiple backup image formats, which will take up more space. In addition, the amount of material data is not small, I suggest you buy a variety of hard disks for multi-site backup; You can also choose to build a NAS data center to realize data synchronization, * * * sharing and remote operation.