From: Niklas Haas <ffmpeg@haasn.xyz> To: ffmpeg-devel@ffmpeg.org Cc: Niklas Haas <git@haasn.dev> Subject: [FFmpeg-devel] [PATCH 10/11] tests/swscale: constrain reference SSIM for low bit depth formats Date: Mon, 17 Mar 2025 11:43:56 +0100 Message-ID: <20250317104357.307832-10-ffmpeg@haasn.xyz> (raw) In-Reply-To: <20250317104357.307832-1-ffmpeg@haasn.xyz> From: Niklas Haas <git@haasn.dev> Sometimes, the reference SSIM is significantly higher than the SSIM level expected for the test. This is the case when the source format has a much lower bit depth than the destination format. In this case, the fact that legacy swscale does not accurately preserve the source dither pattern gives it an unfair advantage in a direct comparison, leading to false positives. For example, conversion like rgb4 -> rgb565 should be lossless, but swscale low passes / downscales the input chroma, throwing away massive amounts of detail. This gives it a higher SSIM score since the lowpassed result removes some of the dither noise that was present in the source. --- libswscale/tests/swscale.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/libswscale/tests/swscale.c b/libswscale/tests/swscale.c index bce495db90..117ed2144e 100644 --- a/libswscale/tests/swscale.c +++ b/libswscale/tests/swscale.c @@ -321,6 +321,18 @@ static int run_test(enum AVPixelFormat src_fmt, enum AVPixelFormat dst_fmt, goto error; get_ssim(ssim_sws, out, ref, comps); + + /* Legacy swscale does not perform bit accurate upconversions of low + * bit depth RGB. This artificially improves the SSIM score because the + * resulting error deletes some of the input dither noise. This gives + * it an unfair advantage when compared against a bit exact reference. + * Work around this by ensuring that the reference SSIM score is not + * higher than it theoretically "should" be. */ + if (src_var > dst_var) { + const float src_loss = (2 * ref_var + c1) / (2 * ref_var + src_var + c1); + ssim_sws[0] = FFMIN(ssim_sws[0], src_loss); + } + ssim_ref = ssim_sws; } -- 2.48.1 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
next prev parent reply other threads:[~2025-03-17 10:46 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2025-03-17 10:43 [FFmpeg-devel] [PATCH 01/11] tests/swscale: allow choosing specific flags and dither mode Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 02/11] tests/swscale: allow testing only unscaled convertors Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 03/11] tests/swscale: print speedup numbers in color Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 04/11] tests/swscale: use yuva444p as reference Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 05/11] tests/swscale: switch from MSE to SSIM Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 06/11] tests/swscale: print performance stats on exit Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 07/11] tests/swscale: check supported inputs for legacy swscale separately Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 08/11] tests/swscale: remove stray whitespace in scanf format Niklas Haas 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 09/11] tests/swscale: calculate theoretical expected SSIM Niklas Haas 2025-03-17 10:53 ` Niklas Haas 2025-03-17 10:43 ` Niklas Haas [this message] 2025-03-17 10:43 ` [FFmpeg-devel] [PATCH 11/11] tests/swscale: allow setting log verbosity Niklas Haas
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