From 9c4b973cfedc75f30d5a3af68b036c64621b04af Mon Sep 17 00:00:00 2001
From: Jonas Smedegaard <dr@jones.dk>
Date: Thu, 11 Feb 2021 11:41:31 +0100
Subject: use Bjornsgaard Metric as cross-codec factor

---
 localvideowebencode | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/localvideowebencode b/localvideowebencode
index 8bccd0b..86a753c 100755
--- a/localvideowebencode
+++ b/localvideowebencode
@@ -209,38 +209,41 @@ compression=normal
 #    currently used: VMAF 93 of x264 for 360p30 (equiv. crf 23 at medium)
 # 2) pick reference codec (incl. normal settings)
 # 3) render reference frame format with --compression=quality
-#    * check that bpp is near "refbpp" of 0.12
-#      (otherwise adapt settings or do research and update refbpp)
+#    * establish "Bits Per Pixel" and define as refbpp
+#      currently used: 0.12 (TODO: seems too high - ancient reference)
 # 4) render misc. frame formats (esp. modulus 16) with --compression=quality
-#    * check that bpp derives from refbpp by "rule of .75"
-#      (otherwise adapt settings and/or do research and update rule)
+#    * establish and define curve formula
+#      currently used:  "rule of .75" (TODO: seems slightly too high)
 # 5) render misc. frame formats with --compression=normal
-#    * check that bpp derives from refbpp by "rule of .75"
+#    * verify that bpp derives from refbpp as expected
 # 6) optionally pick dirty/hq settings
 #    * check that --compression=quality has filesize similar as normal
 
 # establish/verify rendering settings for non-reference codec
 #
 # 1) pick normal settings
-# 2) render reference frame format with --compression=quality
-#    * check that this "codec-bpp" deviates reasonably from refbpp
+# 2) render reference frame format with --compression=quality at misc --quality
+#    * establish quality similar to reference encoding, and define as q
 # 3) render misc. frame formats (esp. modulus 16) with --compression=quality
-#    * check that bpp derives from codec-bpp by "rule of .75"
-#      (otherwise adapt settings and/or extend rule)
+#    * establish the "Bjontegaard" bitrate distance, and define as b_dbr
+#      <https://streaminglearningcenter.com/encoding/compute-bd-rate-functions.html>
+#    * establish and define bpp curve
 # 4) render misc. frame formats with --compression=normal
-#    * check that bpp derives from codec-bpp by "rule of .75"
+#    * verify that content quality is comparable to reference encoding
+#    * verify that bpp derives from refbpp as expected
 # 5) optionally pick dirty/hq settings
 #    * check that --compression=quality has similar filesize as normal
 
 # Theora seems to compress ~100% worse than H.264
 # (reportedly 76% worse in 2010, and libtheora has not improved since)
 # <http://compression.ru/video/codec_comparison/h264_2010/>
-factor_theora=200/100
+b_dbr_theora=100
 # VP8 is rumored to compress ~20% worse than H.264
-factor_vp8=120/100
+b_dbr_vp8=20
 # VP9 compresses ~30% better than H.264 for practical use-cases
 # <https://engineering.fb.com/2018/04/10/video-engineering/av1-beats-x264-and-libvpx-vp9-in-practical-use-case/>
-factor_vp9=70/100
+b_dbr_vp9=-30
+
 # Mimic ReplayGain level when loudness is enabled (EBU R128: -23LUFS)
 # <https://auphonic.com/blog/2013/01/07/loudness-targets-mobile-audio-podcasts-radio-tv/>
 lufs=-18
@@ -541,9 +544,9 @@ fi
 if [ -n "$_pixels" ] && [ -n "$_frames" ]; then
 	bitrate_avc=$(perl -E '$refsize=640*360;' \
 		-E "say int( +(($_pixels/\$refsize)**0.75*\$refsize*$_frames*$refbpp) )")
-	bitrate_theora=$(perl -E "say int( +($bitrate_avc*$factor_theora) )") #"
-	bitrate_vp8=$(perl -E "say int( +($bitrate_avc*$factor_vp8) )") #"
-	bitrate_vp9=$(perl -E "say int( +($bitrate_avc*$factor_vp9) )") #"
+	bitrate_theora=$(perl -E "say int( +($bitrate_avc*(100+($b_dbr_theora))/100) )") #"
+	bitrate_vp8=$(perl -E "say int( +($bitrate_avc*(100+($b_dbr_vp8))/100) )") #"
+	bitrate_vp9=$(perl -E "say int( +($bitrate_avc*(100+($b_dbr_vp9))/100) )") #"
 fi
 
 # default per-codec-channel bitrates
-- 
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