From 7f196c9320ab010fcf659dbf819d83dded5eeb7f Mon Sep 17 00:00:00 2001
From: Felix Ableitner <me@nutomic.com>
Date: Tue, 9 Oct 2018 17:46:26 -0500
Subject: [PATCH] Scale bitrate linearly with FPS

---
 shared/models/videos/video-resolution.enum.ts | 37 +++++++++++--------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/shared/models/videos/video-resolution.enum.ts b/shared/models/videos/video-resolution.enum.ts
index e40e5b58b..13c0fe9a7 100644
--- a/shared/models/videos/video-resolution.enum.ts
+++ b/shared/models/videos/video-resolution.enum.ts
@@ -9,13 +9,13 @@ export enum VideoResolution {
 }
 
 /**
- * Bitrate targets for different resolutions and frame rates, in bytes per second.
+ * Bitrate targets for different resolutions, at VideoTranscodingFPS.AVERAGE.
+ *
  * Sources for individual quality levels:
  * Google Live Encoder: https://support.google.com/youtube/answer/2853702?hl=en
  * YouTube Video Info (tested with random music video): https://www.h3xed.com/blogmedia/youtube-info.php
  */
-export function getTargetBitrate (resolution: VideoResolution, fps: number,
-    fpsTranscodingConstants: VideoTranscodingFPS) {
+export function getBaseBitrate (resolution: VideoResolution) {
   switch (resolution) {
   case VideoResolution.H_240P:
     // quality according to Google Live Encoder: 300 - 700 Kbps
@@ -30,29 +30,36 @@ export function getTargetBitrate (resolution: VideoResolution, fps: number,
     // Quality according to YouTube Video Info: 879 Kbps
     return 900 * 1000
   case VideoResolution.H_720P:
-    if (fps === fpsTranscodingConstants.MAX) {
-      // quality according to Google Live Encoder: 2,250 - 6,000 Kbps
-      // Quality according to YouTube Video Info: 2634 Kbps
-      return 2600 * 1000
-    }
-
     // quality according to Google Live Encoder: 1,500 - 4,000 Kbps
     // Quality according to YouTube Video Info: 1752 Kbps
     return 1750 * 1000
   case VideoResolution.H_1080P: // fallthrough
   default:
-    if (fps === fpsTranscodingConstants.MAX) {
-      // quality according to Google Live Encoder: 3000 - 6000 Kbps
-      // Quality according to YouTube Video Info: 4387 Kbps
-      return 4400 * 1000
-    }
-
     // quality according to Google Live Encoder: 3000 - 6000 Kbps
     // Quality according to YouTube Video Info: 3277 Kbps
     return 3300 * 1000
   }
 }
 
+/**
+ * Calculate the target bitrate based on video resolution and FPS.
+ */
+export function getTargetBitrate (resolution: VideoResolution, fps: number,
+    fpsTranscodingConstants: VideoTranscodingFPS) {
+  const baseBitrate = getBaseBitrate(resolution)
+  // The maximum bitrate, used when fps === VideoTranscodingFPS.MAX
+  // Based on numbers from Youtube, 60 fps bitrate divided by 30 fps bitrate:
+  // 2600 / 1750 = 1.48571428571
+  // 4400 / 3300 = 1.33333333333
+  const maxBitrate = baseBitrate * 1.4
+  const maxBitrateDifference = maxBitrate - baseBitrate
+  const maxFpsDifference = fpsTranscodingConstants.MAX - fpsTranscodingConstants.AVERAGE
+  // For 1080p video with default settings, this results in the following formula:
+  // 3300 + (x - 30) * (1320/30)
+  // Example outputs: 1080p30: 3300 kbps, 1080p60: 4620 kbps, 720p30: 1750, 720p60: 2450
+  return baseBitrate + (fps - fpsTranscodingConstants.AVERAGE) * (maxBitrateDifference / maxFpsDifference)
+}
+
 /**
  * The maximum bitrate we expect to see on a transcoded video in bytes per second.
  */