Re: Why is it that png files are not dependable over the long haul

So there is a compile option for zlib to allow small frames?  That's why
firefox and chrome WORK?

chromium is #define ZLIB_VERSION "1.2.11"
but... there are changes.

#elif defined(Z_CR_PREFIX_SET)
#    undef gzgetc
#    define gzgetc(g) \
          ((g)->have ? ((g)->have--, (g)->pos++, *((g)->next)++) \
                     : (Cr_z_gzgetc)(g))

    state->check = 1L;      /* 1L is the result of adler32() zero length
data */

several misc changes in deflate.c and crc32.c (which actually is expected
to hook into like Cr_z_crc_* )

----
Firefox has no modifications to zlib (names and type compatibility)
/* This header does prefixing as below, but with an updated set of names. */
#include "mozzconf.h"

but has mods to 1.6.34 for at least APNG and seems all the changes are in
#ifdef blocks; no existing code seems to be modified...

which is just some additional block types(?)

#ifdef PNG_READ_APNG_SUPPORTED
      else if (chunk_name == png_acTL)
         png_handle_acTL(png_ptr, info_ptr, length);

      else if (chunk_name == png_fcTL)
         png_handle_fcTL(png_ptr, info_ptr, length);

      else if (chunk_name == png_fdAT)
         png_handle_fdAT(png_ptr, info_ptr, length);
#endif

---------------
So firefox is really vanilla (again unless there's some compile flag/option
that is getting set) Zlib build is pretty simple.

mozilla build does enable optimzations for platforms with intrinsics (neon,
SSE...) when building PNG...


On Wed, Aug 1, 2018 at 9:25 PM Cosmin Truta <[email protected]> wrote:

> Cosmin Truta wrote:
>
> > The PNG specification (and zlib, and deflate, etc.) have strict rules
> > for error detection, but no rules for error recovery.
>
> Correction: the PNG spec does have rules for error recovery, for
> ancillary chunks (e.g. signal, and then drop the bad ancillary chunks,
> and then continue) but not for critical chunks. The error that we're
> dealing with here is inside IDAT, which is a critical chunk containing
> a critically-important, unfortunately broken zlib datastream.
>
> Also: general rules for error recovery do not specifically include
> rules for data recovery. For example: if you have a chunk with
> spurious data, it's intuitive to simply discard the spurious data. But
> other than that, where nothing is spurious, and nothing is redundant
> (as it is in our case), it is necessary to guess (because it is not
> possible to absolutely know) how to recover the data. And guessing is
> what we're doing here to solve this problem.
>
> Sincerely,
> Cosmin
>
>
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