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
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.