Application of Common Image Formats in Electronic Publishing System IV

(D) PNG image format

PNG is the abbreviation of the first letter of the English word for portable network image, and it is a newly-formed and widely used format in recent years.

1. PNG image structure analysis PNG is transmitted in the form of data streams. Similar to GIF, PNG also decomposes the data stream into several data blocks. The PNG image format file (or data stream) consists of an 8-byte PNG file signature field and more than three chunks organized according to a specific structure.

PNG defines two types of data blocks, one is called a critical data block, which is a standard data block; the other is called an auxiliary data block, which is an optional data block. The key data blocks define 4 standard data blocks, which are present in each PNG file. PNG read-write software must also support these data blocks.

The four standard data blocks in the key data block are: header data block, palette data block, image data block, and image end data block.

In addition to the header data block indicating the start of the data block must be placed first, the end-of-image data block indicating the end of the PNG file is placed outside the last, and the storage order of other data blocks is not limited.

The PNG file format specification establishes 10 auxiliary data blocks for expanding the storage of image information.
Based on the above rules, the PNG file structure is as follows: (1) In the beginning of the PNG data stream is the signature of the PNG image format; (2) The subsequent data stream is the data block representing the image information; (3) PNG file The end of the END - file end data block.

2. PNG image feature analysis As an image format mainly based on web design, PNG can be widely used, mainly because it has many advantages that JPEG and GIF both commonly used in web design do not have. It also draws on the many strengths of both and has improved its deficiencies. The following is a brief analysis of the comparison with JPEG and GIF:

(1) Advantages 1 Both GIF and JPEG color modes can store up to 48-bit powerful color images.
2PNG can compress image files to the limit to facilitate network transmission, but it can retain all information related to image quality.
3 more optimized transmission display;
4 transparent background image display, PNG provides 0 to 255 level transparent information;
5 good compatibility.

(2) Disadvantages 1PNG is a "single picture" file format and does not support animation applications:
2 Because PNG uses a lossless compression method, although PNG image files of the same image quality are smaller than JPEG image files, JPEG can appropriately sacrifice the picture quality and obtain a smaller file size than PNG.
3PNG does not support CMYK mode;
4PNG does not store dpi information.
Since PNG does not support CMYK color mode and does not store dpi information, some people think that PNG is a destructive compression format. This is a misunderstanding. The PNG image format is designed for web browsing and there are no printing and printing requirements. The main factor that restricts the wider application of PNG images is that it cannot produce animation effects. PNG image format developers have realized this and have developed new image formats based on PNG images.

Conclusion Based on the above analysis, we summarized the characteristics and application scope of several common image formats as follows.

(a) BMP is the simplest image format. No loss of any information of the original image is the most realistic reproduction of the original image. BMP is one of the most commonly used image formats under the Windows system and is generally used for the distortion-free preservation of original images and the preservation of button images in web page design and software user interfaces.

(b) TIFF is a complex, flexible and comprehensive image format. TIFF also does not lose the original image information, its flexible and comprehensive features suitable for cross-platform use. TIFF image format is one of the most commonly used image formats in printing. It can preserve the effects of various image special effects processing, so TIFF is also an image format commonly used in the field of multimedia.

(3) JPEG is an excellent compression standard, based on which the image format has the most superior compression performance and is the mainstream image format in the Internet.

(D) PNG is an excellent web design image format. It inherits the main advantages of the GIF and JPEG image formats, saves images in the form of data streams, compresses image data to the limit but retains all information related to image quality, and is suitable for network transmission. Therefore, PNG is the best choice for web page images.

In this paper, several main image formats are assembled. Through a series of analysis and summary, the structural characteristics and application scope of these several image formats are clarified, which is beneficial to the selection of image formats in practical applications.