ObjC: Fix quality scaling for injected encoders

We missed to implement quality scaling in the original CL
https://codereview.webrtc.org/2977213002/. This CL implements it.

Note that the ObjC interface for scalingSettings is slightly different from the C++
interface in that we require explicit QP thresholds to turn quality scaling on, i.e.
we don't provide default values. I think this is more modular as we want to move
codec specific knowledge out from the WebRTC core. I would like to update the
C++ webrtc::VideoEncoder interface to do the same in another CL.

BUG=webrtc:7924

Review-Url: https://codereview.webrtc.org/2991123002
Cr-Commit-Position: refs/heads/master@{#19202}

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCVideoCodec.mm

@@ -54,3 +54,18 @@
 }
 
 @end
+
+@implementation RTCVideoEncoderQpThresholds
+
+@synthesize low = _low;
+@synthesize high = _high;
+
+- (instancetype)initWithThresholdsLow:(NSInteger)low high:(NSInteger)high {
+  if (self = [super init]) {
+    _low = low;
+    _high = high;
+  }
+  return self;
+}
+
+@end

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCVideoCodecH264.mm

@@ -27,6 +27,11 @@ const size_t kDefaultPayloadSize = 1440;
 
 const char kHighProfileExperiment[] = "WebRTC-H264HighProfile";
 
+// These thresholds deviate from the default h264 QP thresholds, as they have been found to work
+// better on devices that support VideoToolbox
+const int kLowH264QpThreshold = 28;
+const int kHighH264QpThreshold = 39;
+
 bool IsHighProfileEnabled() {
   return webrtc::field_trial::IsEnabled(kHighProfileExperiment);
 }
@@ -171,6 +176,11 @@ class H264VideoToolboxDecodeCompleteCallback : public webrtc::DecodedImageCallba
   return _videoToolboxEncoder->SetRates(bitrateKbit, framerate) == WEBRTC_VIDEO_CODEC_OK;
 }
 
+- (RTCVideoEncoderQpThresholds *)scalingSettings {
+  return [[RTCVideoEncoderQpThresholds alloc] initWithThresholdsLow:kLowH264QpThreshold
+                                                               high:kHighH264QpThreshold];
+}
+
 @end
 
 // Decoder.

webrtc/sdk/objc/Framework/Classes/VideoToolbox/encoder.mm

@@ -757,6 +757,8 @@ void H264VideoToolboxEncoder::OnEncodedFrame(
   bitrate_adjuster_.Update(frame._length);
 }
 
+// TODO(magjed): This function is not used by RTCVideoEncoderH264, but this whole file will be
+// removed soon and inlined as ObjC.
 VideoEncoder::ScalingSettings H264VideoToolboxEncoder::GetScalingSettings()
     const {
   return VideoEncoder::ScalingSettings(true, internal::kLowH264QpThreshold,

webrtc/sdk/objc/Framework/Classes/VideoToolbox/objc_video_encoder_factory.mm

@@ -111,6 +111,13 @@ class ObjCVideoEncoder : public VideoEncoder {
 
   bool SupportsNativeHandle() const { return true; }
 
+  VideoEncoder::ScalingSettings GetScalingSettings() const {
+    RTCVideoEncoderQpThresholds* qp_thresholds = [encoder_ scalingSettings];
+    return qp_thresholds ?
+        ScalingSettings(true /* enabled */, qp_thresholds.low, qp_thresholds.high) :
+        ScalingSettings(false /* enabled */);
+  }
+
  private:
   id<RTCVideoEncoder> encoder_;
 };

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoCodec.h

@@ -104,6 +104,17 @@ RTC_EXPORT
 
 @end
 
+/** QP thresholds for encoder. Corresponds to webrtc::VideoEncoder::QpThresholds. */
+RTC_EXPORT
+@interface RTCVideoEncoderQpThresholds : NSObject
+
+- (instancetype)initWithThresholdsLow:(NSInteger)low high:(NSInteger)high;
+
+@property(nonatomic, readonly) NSInteger low;
+@property(nonatomic, readonly) NSInteger high;
+
+@end
+
 /** Protocol for encoder implementations. */
 RTC_EXPORT
 @protocol RTCVideoEncoder <NSObject>
@@ -118,6 +129,13 @@ RTC_EXPORT
            frameTypes:(NSArray<NSNumber *> *)frameTypes;
 - (BOOL)setBitrate:(uint32_t)bitrateKbit framerate:(uint32_t)framerate;
 
+/** Returns QP scaling settings for encoder. The quality scaler adjusts the resolution in order to
+ *  keep the QP from the encoded images within the given range. Returning nil from this function
+ *  disables quality scaling. */
+- (RTCVideoEncoderQpThresholds *)scalingSettings;
+
+// TODO(andersc): Add implementationName method.
+
 @end
 
 /** Protocol for decoder implementations. */
@@ -135,6 +153,8 @@ RTC_EXPORT
       codecSpecificInfo:(__nullable id<RTCCodecSpecificInfo>)info
            renderTimeMs:(int64_t)renderTimeMs;
 
+// TODO(andersc): Add implementationName method.
+
 @end
 
 NS_ASSUME_NONNULL_END