From Wiki

The Portable Network Graphics Image format[*] is a common and free format used to store and exchange digital bitmapped images of all kinds. For general information see the PNG Wikipedia page[*]

PNG was originally created because of shortcomings and patent issues with widely used GIF format. It uses lossless compression and supports different bit depth and alpha channel transparency. It often compresses better than the different TIFF compressions. However, it is not very well supported by Photoshop[*] (you need a format plugin to support all flavors: [1])

Only recently PNG support of the Panorama tools core library pano12 was fixed.

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