Re: cOMp chunk [was: EXIF support in PNG]

On Sun, Jan 1, 2017 at 10:49 AM, Glenn Randers-Pehrson <[email protected]>
wrote:

> What I'm proposing is a set of chunks which the application never sees; if
>> I ask the encoder will write an sPLT as cOMP/sPLT but I never seer that; I
>> give the encoder an sPLT.  When an app reads this using a decoder which
>> supports cOMP the decoder reports an sPLT, not a cOMP.
>>
>
> You cannot count on applications not seeing the chunk.
> You can, however, count on the fact that some applications won't recognize
> it, and will report cOMP but not the embedded sPLT.
>

I'm still proposing something that doesn't *require* any support in the
application, only the decoder and encoder.


>> I'm talking about a cOMP chunk that contains a chunk that doesn't have
> its own compression, either because the chunk
> spec doesn't allow it or because the author chose not to compress it.  The
> cOMP chunk should contain a compression_method byte in order to
> future-proof the chunk against improved compression methods that might
> come along, just as IHDR, zTXt, etc. do.
>

Well, yes; I was talking about adding keywords.  I'm certainly interested
in the possibility of allowing different compression methods (e.g. RLE)
though it would slow down adoption.

The chunk I'm talking about is just a piece of math that transparently
results in a smaller PNG from the application point of view.  It's a
completely reversible lossless transform that *can* be implemented
invisibly within the current specification.  It it carries extra
information there will be bug reports from someone, somewhere that the
information is lost by some application and then the chunk ceases to be
transparent.

Or, to put it another way, when (20 years time?) all PNG *decoders* support
cOMp/cOMP/COMP/COMp I can find quite a few PNGs and, completely, totally,
losslessly make them smaller, and no app will know unless it asks.

John Bowler

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most 
engaging tech sites, SlashDot.org! http://sdm.link/slashdot

_______________________________________________
png-mng-misc mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/png-mng-misc