A WebPPicture instance is necessary to call WebPReportProgress() which
sets WebPPicture::error_code so as well use WebPEncodingSetError() to
record errors too, instead of functions returning a WebPEncodingError.
However there must be one WebPPicture instance per thread, with error
codes merged at sync time. A mutex could simplify that but it is not
the objective of this change.
https://groups.google.com/a/webmproject.org/g/webp-discuss/c/yOiP8APubgc/m/vCTvxl6ODgAJ
Change-Id: Ia1a8f9d1199202e1c88484ce719b0180a80447ce
When histograms are empty, it is easy to add them.
They should also not be considered when merging histograms
(it is a waste of CPU).
This does not change the compression performance,
just the speed.
Change-Id: I42c721ca0f9c5ea067e73b792aa3db6d5e71d01f
As backward references use the plane code when checking the cost
of a distance, statistics used to compute the cost should use it too.
This provides a small compression improvement at no speed cost.
Change-Id: Icade150929ee39ef6dc0d8b1fc85973086ecf41d
Between each iteration we keep track of the previously found
potential merge hence less work to do.
Change-Id: I2b6237447e79443516a6111727d96c24f10bd98a
this avoids duplicates between these trees and dsp/, e.g., enc/tree.c,
dec/tree.c, making pulling the whole library source tree into one target
possible
BUG=webp:279
Change-Id: I060a614833c7c24ddd37bf641702ae6a5eef1775
