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The library now issues a warning if both background processing and RGB to
gray are used when gamma correction happens. As with previous versions of
the library the results are numerically very incorrect in this case.

There are some minor arithmetic changes in some transforms such as
png_set_background(), that might be detected by certain regression tests.

Unknown chunk handling has been improved internally, without any API change.
This adds more correct option control of the unknown handling, corrects
a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes
it possible to skip IDAT chunks in the sequential reader.

The machine-generated configure files are no longer included in branches
libpng16 and later of the GIT repository.  They continue to be included
in the tarball releases, however.

Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT
stream to set the size of the sliding window for reading instead of using the
default 32-kbyte sliding window size.  It was discovered that there are
hundreds of PNG files in the wild that have incorrect CMF bytes that caused
zlib to issue the "invalid distance too far back" error and reject the file.
Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions,
provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes
and using a 32-kbyte sliding window), by using

    png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
        PNG_OPTION_ON);

and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while
optimizing the CMF bytes in its IDAT chunk correctly.

Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong
length, which resulted in PNG files that cannot be read beyond the bad iTXt
chunk.  This error was fixed in libpng-1.6.3, and a tool (called
contrib/tools/png-fix-itxt) has been added to the libpng distribution.

XIII.  Detecting libpng

The png_get_io_ptr() function has been present since libpng-0.88, has never
changed, and is unaffected by conditional compilation macros.  It is the
best choice for use in configure scripts for detecting the presence of any
libpng version since 0.88.  In an autoconf "configure.in" you could use

    AC_CHECK_LIB(png, png_get_io_ptr, ...

XV. Source code repository

Since about February 2009, version 1.2.34, libpng has been under "git" source
control.  The git repository was built from old libpng-x.y.z.tar.gz files
going back to version 0.70.  You can access the git repository (read only)
at

    git://git.code.sf.net/p/libpng/code

or you can browse it with a web browser by selecting the "code" button at

    https://sourceforge.net/projects/libpng

Patches can be sent to glennrp at users.sourceforge.net or to
png-mng-implement at lists.sourceforge.net or you can upload them to
the libpng bug tracker at

    http://libpng.sourceforge.net

We also accept patches built from the tar or zip distributions, and
simple verbal discriptions of bug fixes, reported either to the
SourceForge bug tracker, to the png-mng-implement at lists.sf.net
mailing list, or directly to glennrp.

XV. Coding style

Our coding style is similar to the "Allman" style
(See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
braces on separate lines:

    if (condition)
    {
       action;
    }

    else if (another condition)
    {
       another action;
    }

The braces can be omitted from simple one-line actions:

    if (condition)
       return (0);

We use 3-space indentation, except for continued statements which
are usually indented the same as the first line of the statement
plus four more spaces.

For macro definitions we use 2-space indentation, always leaving the "#"
in the first column.

    #ifndef PNG_NO_FEATURE
    #  ifndef PNG_FEATURE_SUPPORTED
    #    define PNG_FEATURE_SUPPORTED
    #  endif
    #endif

Comments appear with the leading "/*" at the same indentation as
the statement that follows the comment:

    /* Single-line comment */
    statement;

    /* This is a multiple-line
     * comment.
     */
    statement;

Very short comments can be placed after the end of the statement
to which they pertain:

    statement;    /* comment */

We don't use C++ style ("//") comments. We have, however,