Png This image format includes many subclasses, but in practice it can be roughly divided into 256-color png and full-color png. If you finish it, you can use 256-color png instead of gif and full-color png instead of jpeg. Png fully supports alpha transparent (transparent, translucent, and opaque), although there are two strange phenomena (under ie6).
png is a lossless image format, which also means that you can do anything to png images without causing loss in image quality. This also allows png to be used as a transition format for jpeg editing.
Weird performance of PNG in ie:
Browsers below IE6 support the index transparency of PNG8, but the images must be fully transparent. The translucent png8 displays as fully transparent under the browser. Alpha transparent full color PNG (png32) will appear in ie6 background color (usually gray).From the above, we can summarize:
(a) Fully transparent png8 can be displayed normally in either browser (just like a gif). The translucent png8 displays incorrectly in browsers such as ie6 and below, and other browsers can display translucent normally. This bug does not require special treatment, because it only displays as fully transparent in browsers that do not support translucent, which has little impact on the user experience. Instead, it is an enhanced version of transparent gifs.
(b) There is no good way to solve the second bug, and it can only be handled through the (AlphaImageLoader filter and htc) method.
More about PNG:PNG8
256 color PNG alias, lossless format, smallest size, but only supports up to 256 color
PNG24
The alias of full-color PNG, in addition to the characteristics of png8, can theoretically store 2 colors to the 24th power, but the previous versions of ie6 have problems with this format compatibility.
PNG32
The alias of full color PNG, except for the above characteristics, is slightly larger in size
Compared to GIF
In addition to not supporting animations, PNG8 has all the features of GIF, but the more advantage over GIF is that it supports alpha transparency and better compression. So, most of the time, you should use PNG8 not a GIF (except that very small image GIFs will have better compression).
Compared to JPEG
JPEG has better compression than full-color PNG, so it also makes JPEG suitable for photos. However, the process of editing JPEG is prone to quality loss, so full-color PNG is suitable as a transition format for editing JPEG.
Excerpted from: http://www.cnblogs.com/kei0/archive/2012/08/29/2661947.html