What is the PNG Image Format?

Portable network graphics is a lossless compressed bit-picture format. Its design purpose is to try to replace GIF and TIFF file formats while adding some features that GIF file formats do not have. PNG uses a lossless data compression algorithm derived from LZ77, which is generally used in JAVA programs, web pages or S60 programs because of its high compression ratio and small file size.

In early 1995,

Small size Due to bandwidth constraints in network communication, on the premise of ensuring clear and realistic pictures,

PNG

There are three formats of PNG format: 8-bit, 24-bit, and 32-bit. 8-bit PNG supports two different transparent formats (index transparent and alpha transparent). 24-bit PNG does not support transparency. 32-bit PNG adds 24 bits With 8-bit transparent channels, 256 levels of transparency can be displayed.

The numbers after PNG8 and PNG24 represent the maximum color values that this PNG format can index and store. 8 means that the 8th power of 2 is 256 colors, and 24 means that the 24th power of 2 has more than 16 million colors.

PNGGIF png PNG and GIF

1. Generally, the compression rate of static GIF images will be slightly increased after lossless conversion to PNG (provided that the 8-bit index mode is also used).

2. PNG can provide greater color depth support, including 24-bit (8-bit 3 channels) and 48-bit (16-bit 3 channels) true color. Adding alpha channel can further support 64-bit per pixel representation.

3. When a PNG image with a color depth exceeding 8 bits is converted to a GIF, the image quality will decrease due to color separation (reduce the number of colors).

4. GIF natively supports dynamic images. PNG can only be implemented through non-standards. On the basis of PNG, APNG and MNG formats that support animation have been developed, but the popularity is not high. PNG has poor support on older browsers such as IE6.

PNG JPEG png PNG and JPEG

1. JPEG can generate smaller files for photo (or similar) images. This is because JPEG uses a specific lossy encoding method for photo images. This encoding is suitable for low contrast, smooth image color transitions, and high noise. , And the structure is irregular. If you use PNG instead of JPEG in this case, the file size increases a lot, and the improvement in image quality is limited. Correspondingly, if you save text, lines or similar edges with clear, large-area images of the same color area, the compression effect of the PNG format will be much better than JPEG, and the image of high-contrast areas like JPEG will not be damaged. If the image has both sharp edges and the characteristics of a photo image, then a trade-off is made between the two formats. JPEG does not support transparency.

2. Because JPEG is lossy compression, iterative loss will occur. During repeated compression and decoding, information will be continuously lost and the image quality will be reduced. Since PNG is lossless, it is more suitable to save the image to be edited. Although PNG compression of photo images is also effective, there are lossless compression formats specially designed for photo images, such as lossless JPEG2000, Adobe DNG, etc. In general, these formats are not suitable for all images. Images to be published can be saved as JPEG, encoding once with JPEG will not cause obvious image loss.

PNG vs JPEG-LS png PNG vs JPEG-LS

JPEG-LS is an "almost" lossless compression format, which is less well known than the lossy JPEG compression mentioned above. It can be directly compared to PNG, using a standard set of test images. Under the Waterloo Repertoire ColorSet (a set of standard test images), JPEG-LS usually performs 10% -15% better than PNG, but some of these images perform significantly better, about 50% -75%. Therefore, if both formats are supported and sensitive to the size of the image file, you can try both of these formats, which has a greater relationship with the image data itself.

PNGTIFF png PNG and TIFF

TIFF is a format combining quite a few schemes. It is widely used as an intermediate format for image exchange between professional image editing software, so it continuously supports more functions required by applications, and not much support for image manipulation parts that applications do not care about. This also means that many applications can only identify a subset of TIFF, creating more potential confusion.

The most common lossless compression algorithm used by TIFF is LZW. This algorithm, also known as GIF, has been patented until 2003. One variant of TIFF uses the same compression algorithm as PNG, but is not supported by many patented programs. TIFF also provides a special lossless compression algorithm, similar to CCITTGroup IV, which can better compress binary images (such as fax or black and white text) than PNG. PNG only supports non-self-multiplying , while TIFF also supports joint (self-multiplying) .

The PNG specification does not include standards for embedded EXIF (interchangeable image file format) image data, such as images taken by digital cameras. TIFF, JPEG 2000, and DNG all support EXIF.

Early browsers did not support PNG images; JPEG and GIF were the mainstream image formats. Due to the color depth limitation of GIF, all images with color transitions in web pages use JPEG. Either way, JPEG compression causes slight blurring of the image. PNG can be as accurate as possible at the corresponding color depth, while keeping the image file small. PNG has gradually become a better choice for small gradient images, and many browsers already have good support for PNG ^[4] .