[Regression] "swscale" ignores "param0"/"param1" after 7.1

[b7j6abhi3q]

Summary of the bug: Passing different values of param0 and param1 to the scale filter produces identical output when it shouldn't.
How to reproduce:

% ffmpeg -i input -vf scale=-2:540:flags=bicubic:param0=1:param1=0 -f hash -
SHA256=7083e8d529bc9639a0db1d9aff4253442fa227ac632e19151e863b91611c200d
% ffmpeg -i input -vf scale=-2:540:flags=bicubic:param0=0:param1=0.5 -f hash -
SHA256=7083e8d529bc9639a0db1d9aff4253442fa227ac632e19151e863b91611c200d

ffmpeg version N-118406-g957eb2323a-20250203

[MasterQuestionable]

͏    "The specified values are specific of some scaling algorithms and ignored by others."
͏    Per: https://ffmpeg.org/ffmpeg-scaler.html

͏    Please further verify?

͏    See also: ​https://trac.ffmpeg.org/ticket/10993#comment:2

[Leo Izen]

Can you provide a sample input file for which this is the case?

[b7j6abhi3q]

I should have mentioned the same command works in 7.1, more detailed example in which case the difference is also visually obvious if output to file:

ffmpeg version n7.1-184-gdc07f98934-20250206

% ffmpeg -i carphone_qcif.y4m -vf scale=1440:1080:flags=bicubic,setsar=1 -f hash -
SHA256=f8d1a22960bd14de8933cdd55995dbfe553ab0d3e7fde78c307e7bd92b08f60c
% ffmpeg -i carphone_qcif.y4m -vf scale=1440:1080:flags=bicubic:param0=1:param1=0,setsar=1 -f hash -
SHA256=6556d0e7f59dde9d7c73ff2f9632bfbc388806f0022146a994ea24852b01506
% ffmpeg -i carphone_qcif.y4m -vf scale=1440:1080:flags=bicubic:param0=0:param1=0,setsar=1 -f hash -
SHA256=90261f5618ceba055a46a5e6d18c422d36802576df554795816819e31ab3affb

ffmpeg version N-118431-g6da82b4485-20250206

% ffmpeg -i carphone_qcif.y4m -vf scale=1440:1080:flags=bicubic,setsar=1 -f hash -
SHA256=f8d1a22960bd14de8933cdd55995dbfe553ab0d3e7fde78c307e7bd92b08f60c
% ffmpeg -i carphone_qcif.y4m -vf scale=1440:1080:flags=bicubic:param0=1:param1=0,setsar=1 -f hash -
SHA256=f8d1a22960bd14de8933cdd55995dbfe553ab0d3e7fde78c307e7bd92b08f60c
% ffmpeg -i carphone_qcif.y4m -vf scale=1440:1080:flags=bicubic:param0=0:param1=0,setsar=1 -f hash -
SHA256=f8d1a22960bd14de8933cdd55995dbfe553ab0d3e7fde78c307e7bd92b08f60c

sample from ​https://media.xiph.org/video/derf/y4m/carphone_qcif.y4m

The problem occurred with every input video that I tested and also with the lanczos filter.

[MasterQuestionable]

͏    Regression window:
͏    ​https://github.com/FFmpeg/FFmpeg/commits?since=2025-01-23&until=2025-02-03
͏    ; prominently 2025-02-02 ~ 2025-02-03
͏    (keyword "scale")

͏    Please investigate the exact commit?
͏    Thanks.

͏    ----

͏    Seemingly not within previous window..?
͏    ​https://github.com/FFmpeg/FFmpeg/commits/master/libswscale/graph.c?until=2025-02-07
͏    ​https://patchwork.ffmpeg.org/project/ffmpeg/patch/20250206215857.17894-1-jamrial@gmail.com/

[James]

A fix was sent: ​https://patchwork.ffmpeg.org/project/ffmpeg/list/?series=13878

[James]

Fixed in 268d0b6527cba1ebac1f44347578617341f85c35.