Learning About PNG
There is no uncompressed variant of PNG. It is possible to store uncompressed
data by using only uncompressed deflate blocks (a feature normally used to
guarantee that deflate does not make incompressible data much larger).
However, PNG software must support full deflate/inflate; any software that
does not is not compliant with the PNG standard. The two most important
features of PNG---portability and compression---are absolute requirements
for online applications, and users demand them. Failure to support full
deflate/inflate compromises both of these objectives.