From: Niklas Haas <g...@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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