sws_getCachedContext is not a full replacement for the function.

Based on a long debug session with Kostya.

The rationale is that you have a packed format in form
<greyscale sample> <alpha sample> <greyscale sample> <alpha sample>
and shortening greyscale to 'G' might make one thing about Greenscale instead.
An alias pixel format and color space name are provided for compatibility.

Fixes part of Ticket3785

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

This can help "extreme" resizes, e.g with some 4k stuff.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Prevent a division by zero down the codepath.

Sample-Id: 00001721-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

This way all context fields get initialized and can be used without needing to reinit the context

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Fixes ticket #3495.

Both of these dithering methods are from http://pippin.gimp.org/a_dither/ for
GIF they can be considered better than bayer (provides more gray-levels), and
spatial stability - often more than twice as good compression and less visual
flicker than error diffusion methods (the methods also avoids error-shadow
artifacts of diffusion dithers).

These methods are similar to blue/green noise type dither masks; but are
simple enough to generate their mask on the fly. They are still research work
in progress; though more expensive to generate masks (which can be used in a
LUT) like 'void and cluster' and similar methods will yield superior results

Improves rgb -> gray16 conversion

Fixes Ticket3422

The pam and png output files look visually similar, in both cases the
dynamics increase to 0x0 -> 0xfffb.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Fixes Ticket3242
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Fixes artifacts where fast bilinear was used for downscaling chroma

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

makes the first call to sws_getContext() 1ms faster

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Allow some dumb factorizations, slightly decrease spaghetti factor.

Remove duplicated getSubSampleFactors() function. Simplify.

Found-by: wm4
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

See Ticket3028

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Lanczos for general case, sinc for upscaling, Gaussian for
downscaling. According to current literature these scalers
should be the best quality-wise algorithms for each case.

Inspired from a patch by wm4 <nfxjfg@googlemail.com>

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Previously this would have lead to a memcpy(a,a) which violates the
requirement of non overlapping src and dst.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Also give consistent names to init functions.

Bssed on patch by Øyvind Kolås <pippin@gimp.org>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

This allows specifying more dither algorithms without using up flags and
without ambiguities.

Also initialize the new field based on the flags and use it.
Note, improving the logic of the checks is left to subsequent
commits, this here only switches from flags to enum.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Also add missing ff_ prefixes where necessary.

Fixes CID1061054

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>