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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

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  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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