# 44_vpx_encoder_rate_control.patch
#
# Fix pjmedia's VP8/VP9 encoder configuration so it actually puts
# meaningful video on the wire instead of dropping almost every frame.
#
# The problem
# -----------
# Stock pjmedia configures both VP8 and VP9 with:
#
#     DEFAULT_AVG_BITRATE = 200000        // 200 kbps
#     cfg.rc_end_usage    = VPX_CBR       // strict CBR
#     cfg.rc_dropframe_thresh = 25        // drop on buffer underrun
#
# At 1088x1088 (the codec-natural picture buffer for ~1080p-ish input
# this build of pjsip routinely produces), libvpx in CBR mode physically
# cannot fit a frame inside ~6.6 kbit (200000 bps / 30 fps) without
# blowing the quality budget, so its rate controller drops the frame.
#
# Captured a real Sylk Mobile <-> Blink VP9 session and confirmed the
# behaviour:
#
#   * Mac->Mobile: 8 frames over 10.4s ≈ 0.8 fps, ~16 kbps on wire
#   * Mobile->Mac: 184 frames over 6.7s ≈ 27.4 fps, ~285 kbps
#
# The descriptor side of the same patch series is correct (see
# 23_vpx_packetizer_libwebrtc_interop.patch) - libwebrtc just has
# nothing to display because the Mac encoder is producing one frame
# per second.
#
# Why H264 doesn't have this problem
# ----------------------------------
# H264 on Apple Silicon goes through VideoToolbox (hardware codec)
# which has its own rate control independent of pjmedia's CBR knobs.
# It happily produces 25-30 fps regardless of the avg_bps setting.
# Only the libvpx path is gated by this config.
#
# What this patch changes
# -----------------------
#  1. Bump DEFAULT_AVG_BITRATE / DEFAULT_MAX_BITRATE from 200 kbps to
#     1500 kbps. 800 kbps was enough to make the encoder *produce*
#     frames but at the default 720x480 resolution it starved libvpx
#     so badly that every frame went out near rc_max_quantizer - the
#     wire framerate was fine but the picture looked like 1990s
#     RealVideo. 1.5 Mbps is what libwebrtc targets by default for
#     VGA VP9, and under VBR the encoder will not actually consume
#     it unless the scene needs it.
#  2. Switch rc_end_usage from VPX_CBR to VPX_VBR. VBR lets quality
#     dip on hard frames instead of dropping them. For real-time video
#     a frozen pictureless stream is worse than a slightly-blocky one.
#  3. Drop rc_dropframe_thresh from 25 to 0. With VBR + a realistic
#     bitrate this rarely fires, but at 25 it triggers far too often
#     under the old CBR config and on transient bandwidth dips.
#
# Together these turn libvpx from "drop almost every frame" into
# "always produce a frame; let the quantizer take the hit when the
# budget is tight". Mobile gets >25 fps to display.
#
# Tunable
# -------
# If the wire bitrate ends up too high for someone's uplink, the
# AVG_BITRATE constant is the only knob worth touching - VBR + 0
# drop threshold is the right behaviour for real-time video
# regardless of target rate.
#
--- pjsip_orig/pjmedia/src/pjmedia-codec/vpx.c
+++ pjsip/pjmedia/src/pjmedia-codec/vpx.c
@@ -52,8 +52,15 @@
 #endif
 
 #define DEFAULT_FPS             15
-#define DEFAULT_AVG_BITRATE     200000
-#define DEFAULT_MAX_BITRATE     200000
+/*
+ * Stock pjmedia advertised a 200 kbps target which left libvpx no
+ * room to encode VGA-or-larger frames inside its CBR budget; almost
+ * every frame was dropped (~0.8 fps measured against Sylk Mobile).
+ * Bump to a realistic VGA-class default so the encoder can actually
+ * emit frames. See 44_vpx_encoder_rate_control.patch.
+ */
+#define DEFAULT_AVG_BITRATE     1500000
+#define DEFAULT_MAX_BITRATE     1500000
 
 #define MAX_RX_RES              1200
 
@@ -448,7 +455,14 @@
     cfg.rc_target_bitrate = vpx_data->prm->enc_fmt.det.vid.avg_bps / 1000;
     
     cfg.g_pass = VPX_RC_ONE_PASS;
-    cfg.rc_end_usage = VPX_CBR;
+    /*
+     * VBR instead of CBR: under CBR libvpx drops frames it can't fit
+     * inside the per-frame bitrate budget.  For real-time video a
+     * pictureless stream is worse than a slightly-blocky one, so let
+     * the quantizer take the hit on hard frames and keep producing
+     * output.  See 44_vpx_encoder_rate_control.patch.
+     */
+    cfg.rc_end_usage = VPX_VBR;
     cfg.g_threads = 4;
     cfg.g_lag_in_frames = 0;
     cfg.g_error_resilient = 0;
@@ -462,7 +476,13 @@
     cfg.kf_max_dist = 60 * vpx_data->prm->enc_fmt.det.vid.fps.num/
                       vpx_data->prm->enc_fmt.det.vid.fps.denum;
     cfg.rc_resize_allowed = 0;
-    cfg.rc_dropframe_thresh = 25;
+    /*
+     * Never drop a frame on buffer underrun.  Combined with VBR above,
+     * this keeps the wire framerate stable; quality dips on hard
+     * frames are absorbed by the quantizer instead of by silently
+     * dropping the picture.
+     */
+    cfg.rc_dropframe_thresh = 0;
 
     vpx_data->enc_input_size = cfg.g_w * cfg.g_h * 3 >> 1;