Types of computer image formats

Computer image processing is divided into two categories, namely vector graphics (Vector graphic), also known as vector graphics; bit images (Raster image or bitmap image) also known as raster images. In terms of pre-press terminology, it is divided into line-work, abbreviated as LW, usually lines, geometric figures and text; the other is continuous-tone images, English is Continuous-tone, abbreviated as CT, these are A picture composed of pixels. The image obtained by a scanner or digital camera is a bit image, and the works drawn by most drawing software (such as lllustraor or Freehand) are vector images; the fonts used in computers are also divided into bitmaps and vector images, such as PoStipt Type 1 Fonts include dot matrix characters (for screen display) and vector characters (for printer output).

Vector graphics

Vector means that the quantity can only be expressed in size and direction, so vector graphics use coordinate values ​​to represent some control points, and the points are connected with lines to form graphics, so they are also called outer frame graphics. Because the graphics are recorded using coordinates and frames, the graphics can be enlarged and reduced at will, the effect is the same, and the required memory is much smaller than the bit image.

Bitmap image

Dot matrix means that the image uses many small squares, the term is called pixels (Pixel) to compose the picture of the picture, each pixel is recorded with different colors or shades, so the relative memory or storage required Very demanding. For example, with an 8-bit grayscale image in a square inch, recording with a resolution of 300 dots per inch, composed of 90 × 000 dots with a total of 300 × 300 dots, and recording one point with one byte, then There are 90, O00byte, which is 88KB capacity.

The above are the two commonly used graphics and image recording methods. Different software, fonts and output devices have different requirements, so you should be aware of their similarities and differences, and then use them in a suitable way.

Image storage format

Based on the above image classification reasons, file format storage is also divided into two categories: Pixel-based formats describe images, and these images are composed of pixels. For example, Photoshop files are pixel-based.

Object-based format is mainly to describe layout files, for example, files made with Illustrator, Freehand, PageMaker and QuarkXPress are Object-based.

To know the type of file format, you can actually go to "Saveas" and "Save a copyas" in PhotoShop at a glance. [next]

PS (FostScript)

PostScript was launched by EiAdobe in 1985. It is a page description language, mainly used to describe text and graphics. Through it, it can effectively print wonderful graphic works on paper, film or zinc plate.

Designers can also choose to save as a PostScript file and then deliver it to the output center. Of course, only devices that support PostScript language and version compatibility can output PostScript files.

The biggest advantage of saving as a PostScript file is that the output center can directly output PS files without desktop publishing software. The disadvantage is that it cannot be previewed and modified. If there is a problem with the file output, the output center cannot check or correct the file.

The method of generating a PostScript file is to select the relevant PostScript printer (PPD) in the Print dialog box and print it as a file. It is not very common to use PostScript file output in Hong Kong, because the output center in Hong Kong routinely checks the documents and placed images clearly before output. Since it cannot be changed arbitrarily, not many people will use this file format, but it is more foreign Universally used. PDF is very popular recently, because it can be pre-checked and corrected, but when using Adobe Acrobat Distiller or RIP of the prepress system process to generate PDF, it is also necessary to print and store PostScript first before generating or setting PostScript, so be aware .

EPS (Encapsulated PostScript)

EPS is based on PostScript. EPS can contain a variety of data such as dot matrix, vector, glyph, preview and frame path, etc. It is a versatile file format that is applied to various software to a high degree. It occupies an important position in the file format. Most software can store or place EPS files, or generate EPS files through the printing process. There are two obvious differences between EPS and PostScript files. The first is that we can modify the EPS from other software without returning to the original file. The second EPS does not have page size information, and the EPS file cannot be generated by printing, but most desktops Publishing software can be saved as an EPS file.

Preview (Preview) -EPS preview part can choose lbit black and white or 8bit-256 color, and the resolution is generally 72dpi, easy to put in other software to work faster, if you want to use PC with Tiff.

Encoding-Encoding-ASCII is a coding method introduced earlier, most of the earlier PostScript Levell and 11 RIP can only support ASCII. Later, the Binary encoding method was introduced. The biggest advantage is that the file is half as thin as ASCll and the output is faster, but the old version of the software or RIP may not support it. The JPEG encoding method has a compression function, but it has distortion, and it is not supported by all software and RIP. Please choose carefully.

Clipping path-If you need to do a fallback, you need to save it as EPS and store it together with the object-based clipping path before you can put it into other software for design and layout. [next]

DCS (Desktop Color Separation)

DCS is a type of EPS format, which can be stored in PhotoShop. After the files are stored in DCS, a total of five files will appear, including four color versions of CMYK and preview files, the so-called "Masterfile".

The biggest advantage of DCS is that the output is relatively fast. Because the file has been divided into four-color files, the image transmission time can be shortened by up to 75%, so it is suitable for color separation output of large files.

Another advantage is that the production speed is also relatively fast. In fact, the DCS format is an important part of the OPl (Open Prepress Interface) workflow concept. OPI refers to the placement of low-resolution images during production and the connection of high-resolution images until output. In this way, the production speed can be accelerated. This workflow concept is especially suitable for the production of some multi-image books or large boxes, so the DCS format is only similar to the OPI concept. The low-resolution images are placed in the document. The high-resolution images will be connected.

Since the five files are combined into one image, it should be noted that the names of the five files must be the same, but there is more C, M, Y, K after the original name, you can not change the name of any one.

DCS2.0 is a newer color separation file format, which can include color versions other than CMYK, such as spot colors and Hi-Fi Color, etc., and can also be stored as a single file, which is convenient for delivery and storage, and reduces the occurrence of missed copies .

When storing the DCS format, the Preview in the options is a 72dpi RGB image, which is mainly used for on-screen display, while Composite is a 72dpi CMYK image, which is used to print color manuscripts.

PDF (Portable Document Format)

PDF translated as a portable file, developed by Adobe, is a cross-platform file format, regardless of any platform such as Mac, Windows, Linux and Unix, as long as there is Acrobat or PDF compatible reader to open the PDF file.

Normally, if a file created by a certain software is used with certain words, if others want to open the file, they must have the words used in the software and the file; but if the file is used Saved in PDF format, no matter what software the file is made of, or what font is used, others only need to have Acrobat Reader to open this file. When the file is stored as a PDF, the elements in the file can be compressed for easy storage and transfer. This file format is gradually welcomed by all walks of life, such as e-books, CD Rom production, and print media. However, it should be noted that the RIP of PostScript Levell and ll cannot directly support this format, but the newer PostScript Level III output device can already directly output PDF files. For PDF files more suitable for printing, please refer to PDF / X format. [next]

PDF / X

In order to meet the needs of pre-press and printing, The Committee for Graphic Arts Technologies Standards (CGATS), Digital Distribution of Advertising for Publications (DDAP) and Newspaper Association of America (NAA) in the United States based on PDF, define suitable output for printing The content was standardized and PDF / X was formulated. The first one defined is ANSI PDF / X-1: 1999, which is based on the specifications of PDF1.2; different versions of PDF / X are used to match different workflows.

PDF / X-1a: 2003 (ISO 15930-4: 2003)

PDF / Xla is suitable for four-color printing. This specification stipulates that the font must be embedded, the color mode is CMYK, spot color or grayscale tone, and RGB or Lab cannot be used. The file size, cutting line and bleeding should be included in the file; the "Trapping" function should be "True" or "False", and cannot be set to "Leave Undefinded". The file cannot be encrypted, and there can be no OPI images. The transparent image function cannot be used and must be flattened. The JPEG2000 format is not supported.

PDF / X-3: 2003 (ISO 15930-6: 2003)

PDF / X-3 is an extension of PDF / X-La: 2001, the main difference is that PDF / X3 contains RGB or Lab color mode, suitable for color tube process.

PDF / X-2.2003 (ISO 15930-5: 2003)

PDF / X-2 is based on the specification of PDF / X-3, but it only uses more OPI functions.

TIFF (Tagged Image File Format)

Developed by Aldus, it is a pixet-base-based image format, widely used in Mac and PC computer software, and can describe different color information, including 1-bit black and white, 8 bit grayscale and RGB or CMYK color images . Most software supports TIFF format. PhotoShop can store the clipping path or alpha channel together with images in TIFF format for retreat, but not all software supports it.

Byte Order-Since the Tiff format of PC and Mac is slightly different, this option actually allows you to choose whether you want to use this image on Mac or PC. Do n’t be confused by the nouns, just choose according to your needs That's it.

File compression-LZW Compression is a compression option for the TIFF file format. The LZW (Lempel-Ziv-Welch) compression structure uses shorter information to record repeated and longer data. The quality of the compressed file will not deteriorate, but Not all software and output devices can support this compressed file, so care must be taken when selecting. [next]

PICT

PICT is the native graphics format of the Macintosh system. It is based on the OuickDraw display language and can handle vector graphics and dot matrix images. PICT is commonly used in the exchange between Mac software, such as images stored in Scrapbook.

The PICT format supports Bitmap, Grayscale, Index Color, and single alpha channel (16-bit) RGB images. There are 2-bit, 4-bit or 8-bit / pixel options for grayscale images. There are 16-bit or 32-bit / pixel options for RGB images. PICT is only suitable for non-PostScript output devices, such as general low-end inkjet printers.

JPEG (Joint Photographic Expels Group)

It is a compressed image format with distortion. When you choose to save JPEG, there will be multiple levels of compression settings, the highest is the best color quality, but the file will be larger, if you choose a lower color quality, the compression is the largest, the file is the smallest, such as a The 2MB TIFF compressed to the lowest quality is only 88Kbytes, if compressed to the highest quality it will be 1.1MB.

The flgJPEG format was first introduced mainly for compressing images used in Quick Time, and later it was also used in layout and design because the compressed file transfer speed is faster. Although the file transfer speed is fast, the quality of the compressed image is low, so the general designer may not use this format.

Prepress exchange file format

When customers exchange data files with printers, it is recommended to use some standard and highly portable file formats. These formats are irrelevant to the software and can be accepted by all those involved in the printing process. Some system and application software versions will be resolved. Different problems.

During various processes in the prepress production process, there is an opportunity to work through different prepress systems. Different operating systems have their own unique output file format. In order to solve the compatibility problem between different file formats, if you use some common The format will facilitate exchange.

The following introduces some common prepress production file formats. If you know more about its advantages and use it, you will get twice the result with half the effort. [next]

TIFF / IT

This file format is to meet the needs of DDAP (Digital Distribution of Advertising for Publication) magazine publishing, advertising companies, pre-press platemaking companies and printers. TIFF / IT is an exchange standard for digital advertisements and complete pages. It only contains four primary colors and does not support spot colors. The design of this file format is very similar to the format of Scitex CT / LW, each page contains three files. There is also a page file whose function is to establish a connection between CT and LW.

1-bit TIFF

It is a completely dot matrix file. The current RIP workflow can generate two files after rasterizing a file, one is the RIP internal file, and the other is the 1-bit TIFF on the Internet. Expressed with one bit of data, RIP's job is to translate PostScript or PDF files into mechanical 1-bit data, including outlets, like traditional film functions, so the industry calls 1-bitTIFF digital film.

Delta List

Generated by Heidelberg's Delta workflow, originally created by Linotype-Hell for Delta workflow; it is a file format that has been interpreted by PostScript (commonly known as RIP post-file format), and then it can be screened Output. Some additional digital drafting processes will accept this file format.

Scitex CT / LW

This file format is generated by Creo's Brisque workflow originally belonging to Sai Angel. The CT format is an uncompressed dot matrix file, and the LW file contains high-precision image information after dot matrix. Because extremely high precision is used, the image will be very large, so the RLE compression technology is used to reduce the size of the file. Since the original LW format can only contain 248 colors, and later developed into NLW, this new format can contain 16,000 colors, making the image contained in LW smoother.

The printing industry in Hong Kong uses a variety of standard file formats, but in fact it still relies heavily on workflow to process documents. Therefore, it is best for the color separation plate-making center and the printer to communicate with each other, which standards can be used to produce the best printed parts. [next]

Image file compression

Compress image files to increase transmission speed and reduce file storage requirements. There are two types of file compression: "lossless" and "lossy". Lossy compression merges similar data, resulting in the loss of original data in the final decompressed file. The most commonly used type of image compression in the printing industry is JPEG (Joint Photographic Experts Group). The greater the "loss" value of the compression setting, the more similar information will be summarized in the same value. The large compression in the lossy type will cause the image quality to degrade and have a great influence on the image details.

The ratio of file compression should not exceed 10: 1. This ratio is usually a low compression / high quality setting. But even at a compression ratio of 10: 1 or less, repeated compression and decompression will result in reduced image quality. For example: an image compressed into JPEG format, after being decompressed and edited, is compressed again into JPEG format; a compressed image is decompressed for design, and finally saved as PDF format, will also cause re-compression. This repeated compression / decompression cycle may cause more quality loss.

Lossless compression only merges equal values, so there is no quality loss when the image is decompressed. Examples of lossless compression include: ZIP and LZW formats, and their compression ratio is about 3: 1.

The transmission of digital files does not require data compression, but data compression can speed up the transmission process of large files. Only when the sender and the receiver agree on the compression method, can the data compression be used to transfer the file.

Here are some detailed guidelines for using compression software:

· The degree of compression will affect the image quality. Do not use compression with a ratio greater than 10: 1. It is best to pre-test to determine the compression method.
· Generally, the receiver and the sender are required to have the same software tools in order to complete the decompression process correctly.
· Do not divide files during compression.
There are some commonly used compression application software, and negotiate with the printer which application software to use.
Both the compression and decompression processes require considerable time and hardware resources on the computer, but this is worth it, because compression can get smaller files that the network can handle, thus reducing the transfer time.
· If the application software can control the degree of JPEG compression, set the black and white photos to "best quality" and the four-color image to "high quality". This setting can not only get the advantages of compression, but also ensure high-quality copy effects.

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