The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.

Other Chunks

Several other chunks were proposed but never approved as official extensions, mainly due to the perceived lack of need for them. The alignment chunk (aLIG, had it been approved) would have provided centering and baseline information about an image so that it could be aligned more cleanly with surrounding text; this would have been most useful for images with transparent edges. The fingerprint chunk (fING) would have provided a 16-byte MD5 fingerprint of the raw image data, a type of cryptographic signature that could be used to test whether two images were identical. Neither aLIG nor fING was ever put up for a vote, and both proposals have long since expired.

There were also three proposed scientific-visualization chunks, all of which were rejected in formal voting. The false-color chunk (fALS) would have provided false-color information for grayscale images, such as might be used to highlight a tumor in a medical scan or a shock front in a hydrodynamic simulation. The calibration chunks (xSCL and ySCL, but also known as xCAL and yCAL in later proposals) were similar to sCAL in providing information about the physical characteristics of an image subject but would have allowed offsets and different units along the two axes; they thus would have provided full calibration data, not just scaling information.

Note that any of these chunks may be resurrected in the future, as PNG becomes more widely used and as the needs of various PNG-using communities evolve.

Last Update: 2010-Nov-26