I want to compress bitmaps to PNG with different compression levels as these levels are available in JPEG compression in C#. I have 20 to 30 images of different sizes to process in 1 sec. Is there any library to achieve this compression in PNG with different compression levels?
A lossless compression method does not have to be single quality level. I mean, some lossless compression methods have some parameters to choose speed/compression ratio trade-off. It's up to author.
As to PNG compression, it's actually bunch of filters and deflate algorithm. Considering deflate has a redundant encoding stage (e.g. two different compressors can produce completely different output, yet still they can be decompressed by any valid decompressor), it's no wonder several programs outputs different PNGs. Note that, I'm still not taking filters into account. There are several filters and there is no "best" filter i.e. it's quality varies by image.
Due to it's redundant feature, some people wrote PNG optimizers which take a regular PNG as input to produce smaller PNG without any perceptual loss. Some of them applies some tricks such as filling completely transparent area with some predictable colors to increase compression. But, in general they tweak parameters in a brute-force fashion.
Now, simplest answer for your question could be that you have two options:
- If you're going to run your executable on a desktop environment, you can use one of these tools as an optimizer. But, in shared hosting you can't use them due to possible privileges.
- You can write your own PNG writer in .NET that takes into account some brute-force parameter tuning.
According to this answer C#: Seeking PNG Compression algorithm/library PNG in C# is a lossless compression library, e.g. does not support multiple quality levels.
The wiki page on PNG (http://en.wikipedia.org/wiki/Portable_Network_Graphics) seems to confirm the compression format is lossless (e.g. has only a single compression level)
Some googling suggests there is research on lossy versions of PNG
来源：https://stackoverflow.com/questions/6922533/png-compression-in-net