default_video_quality_analyzer.md

DefaultVideoQualityAnalyzer

Audience

This document is for users of webrtc::webrtc_pc_e2e::DefaultVideoQualityAnalyzer.

Overview

DefaultVideoQualityAnalyzer implements webrtc::VideoQualityAnalyzerInterface and is a main implementation of video quality analyzer for WebRTC. To operate correctly it requires to receive video frame on each step:

1. On frame captured - analyzer will generate a unique ID for the frame, that caller should attach to the it.
2. Immediately before frame enter the encoder.
3. Immediately after the frame was encoded.
4. After the frame was received and immediately before it entered the decoder.
5. Immediately after the frame was decoded.
6. When the frame was rendered.

The analyzer updates its internal metrics per frame when it was rendered and reports all of them after it was stopped through WebRTC perf results reporting system.

To properly inject DefaultVideoQualityAnalyzer into pipeline the following helpers can be used:

VideoQualityAnalyzerInjectionHelper

webrtc::webrtc_pc_e2e::VideoQualityAnalyzerInjectionHelper provides factory methods for components, that will be used to inject VideoQualityAnalyzerInterface into the PeerConnection pipeline:

• Wrappers for webrtc::VideoEncoderFactory and webrtc::VideoDecodeFactory which will properly pass webrtc::VideoFrame and webrtc::EncodedImage into analyzer before and after real video encode and decoder.
• webrtc::test::TestVideoCapturer::FramePreprocessor which is used to pass generated frames into analyzer on capturing and then set the returned frame ID. It also configures dumping of captured frames if requried.
• rtc::VideoSinkInterface<VideoFrame> which is used to pass frames to the analyzer before they will be rendered to compute per frame metrics. It also configures dumping of rendered video if requried.

Besides factories VideoQualityAnalyzerInjectionHelper has method to orchestrate VideoQualityAnalyzerInterface workflow:

• Start - to start video analyzer, so it will be able to receive and analyze video frames.
• RegisterParticipantInCall - to add new participants after analyzer was started.
• Stop - to stop analyzer, compute all metrics for frames that were recevied before and report them.

Also VideoQualityAnalyzerInjectionHelper implements webrtc::webrtc_pc_e2e::StatsObserverInterface to propagate WebRTC stats to VideoQualityAnalyzerInterface.

EncodedImageDataInjector and EncodedImageDataExtractor

webrtc::webrtc_pc_e2e::EncodedImageDataInjector and webrtc::webrtc_pc_e2e::EncodedImageDataInjector are used to inject and extract data into webrtc::EncodedImage to propagate frame ID and other required information through the network.

By default webrtc::webrtc_pc_e2e::SingleProcessEncodedImageDataInjector is used. It assumes webrtc::EncodedImage payload as black box which is remaining unchanged from encoder to decoder and stores the information required for its work in the last 3 bytes of the payload, replacing the original data during injection and restoring it back during extraction. Also SingleProcessEncodedImageDataInjector requires that sender and receiver were inside single process.

Exported metrics

Exported metrics are reported to WebRTC perf results reporting system.

General

• cpu_usage - CPU usage excluding video analyzer

Video

• psnr - peak signal-to-noise ratio: wikipedia
• ssim - structural similarity: wikipedia.
• min_psnr - minimum value of psnr across all frames of video stream.
• encode_time - time to encode a single frame.
• decode_time - time to decode a single frame.
• transport_time - time from frame encoded to frame received for decoding.
• receive_to_render_time - time from frame received for decoding to frame rendered.
• total_delay_incl_transport - time from frame was captured on device to time when frame was displayed on device.
• encode_frame_rate - frame rate after encoder.
• harmonic_framerate - video duration divided on squared sum of interframe delays. Reflects render frame rate penalized by freezes.
• time_between_rendered_frames - time between frames out to renderer.
• dropped_frames - amount of frames that were sent, but weren't rendered and are known not to be “on the way” from sender to receiver.

Freeze is a pause when no new frames from decoder arrived for 150ms + avg time between frames or 3 * avg time between frames.

• time_between_freezes - mean time from previous freeze end to new freeze start.
• freeze_time_ms - total freeze time in ms.
• max_skipped - frames skipped between two nearest rendered.
• pixels_per_frame - amount of pixels on frame (width * height).
• target_encode_bitrate - target encode bitrate provided by BWE to encoder.
• actual_encode_bitrate - - actual encode bitrate produced by encoder.
• available_send_bandwidth - - available send bandwidth estimated by BWE.
• transmission_bitrate - bitrate of media in the emulated network, not counting retransmissions FEC, and RTCP messages
• retransmission_bitrate - bitrate of retransmission streams only.

Framework stability

• frames_in_flight - amount of frames that were captured but wasn't seen on receiver.

Debug metrics

Debug metrics are not reported to WebRTC perf results reporting system, but are available through DefaultVideoQualityAnalyzer API.

FrameCounters

Frame counters consist of next counters:

• captured - count of frames, that were passed into WebRTC pipeline by video stream source
• pre_encoded - count of frames that reached video encoder.
• encoded - count of encoded images that were produced by encoder for all requested spatial layers and simulcast streams.
• received - count of encoded images received in decoder for all requested spatial layers and simulcast streams.
• decoded - count of frames that were produced by decoder.
• rendered - count of frames that went out from WebRTC pipeline to video sink.
• dropped - count of frames that were dropped in any point between capturing and rendering.

DefaultVideoQualityAnalyzer exports these frame counters:

• GlobalCounters - frame counters for frames met on each stage of analysis for all media streams.
• PerStreamCounters - frame counters for frames met on each stage of analysis separated per individual video track (single media section in the SDP offer).

AnalyzerStats

Contains metrics about internal state of video analyzer during its work

• comparisons_queue_size - size of analyzer internal queue used to perform captured and rendered frames comparisons measured when new element is added to the queue.
• comparisons_done - number of performed comparisons of 2 video frames from captured and rendered streams.
• cpu_overloaded_comparisons_done - number of cpu overloaded comparisons. Comparison is cpu overloaded if it is queued when there are too many not processed comparisons in the queue. Overloaded comparison doesn't include metrics like SSIM and PSNR that require heavy computations.
• memory_overloaded_comparisons_done - number of memory overloaded comparisons. Comparison is memory overloaded if it is queued when its captured frame was already removed due to high memory usage for that video stream.
• frames_in_flight_left_count - count of frames in flight in analyzer measured when new comparison is added and after analyzer was stopped.