③ GStreamer Bin Elements

Intel® DL Streamer uses GStreamer bin elements to simplify creation of a media analytics pipeline by providing most known scenarios in the form of built single elements, such as inference, detection, classification, tracking, etc. Internally such elements builds sub-pipeline using low-level elements. The diagram below shows high-level sub-pipeline inside Intel® DL Streamer bin elements.

High level bin elements architecture

The diagram shows two branches which are produced by tee element. The upper branch is used for data-processing. The bottom branch is used for preserving original frame.

Pipelines with branches

Pipeline with branches is a bit tricky to write. So, an auxiliary element was introduced – processbin. Is simplifies writing pipelines shown on High level bin elements architecture graph.

Here’s an example of the same pipeline without and with processbin:

# Without processbin
filesrc location=$FILE ! decodebin ! \
tee name=t t. ! queue !  meta_aggregate name=mux ! fakesink \
t. ! videoscale ! videoconvert ! video/x-raw,format=BGRP ! tensor_convert ! \
  openvino_tensor_inference model=$MODEL device=CPU ! \
  queue ! tensor_postproc_detection threshold=0.5 ! mux.

# Using processbin
filesrc location=$FILE ! decodebin ! \
processbin \
  preprocess="videoscale ! videoconvert ! video/x-raw,format=BGRP ! tensor_convert" \
  process="openvino_tensor_inference model=$MODEL device=CPU" \
  postprocess="queue ! tensor_postproc_detection threshold=0.5" \
  aggregate="meta_aggregate" ! \
fakesink

In some way, processbin flattens the pipeline, so it’s easier to write, read, and modify. Internally, it builds sub-pipeline which is shown on High level bin elements architecture diagram.

Pre-processing

Block Pre-processing on diagram High level bin elements architecture may contain one or multiple low-level elements to convert video/x-raw or audio/x-raw buffers into data format and layout required by processing element, according to caps negotiation with processing element.

Video Pre-processing

Typical video pre-processing operations include scaling, color conversion, normalization.

Video Pre-processing Backends for Inference

Pre-processing operations inserted into pipeline between decode and inference operations. By performance and data locality considerations, pre-processing designed to support different backend libraries and can run on CPU or GPU device depending on CPU or GPU device of inference and decode. Intel® Deep Learning Streamer (Intel® DL Streamer) has following pre-processing backends:

• gst-opencv

• vaapi-opencl

• vaapi-tensors

• vaapi

• vaapi-surface-sharing

Some of pre-processing backends follows schema PRIMARY-SECONDARY, where PRIMARY is used for as many operations as possible, and SECONDARY is used for all remaining operations if any:

• gst: GStreamer standard elements

• opencv: low-level elements based on OpenCV library

• vaapi: GStreamer standard and Intel® DL Streamer low-level elements based on media GPU-acceleration interface VA-API

• opencl: low-level elements based on OpenCL library

The following table summarizes default preprocessing backend depending on decode or inference device. Note that preprocessing elements communicate with decode element only by caps negotiation, and assume CPU decode if caps negotiated to memory:System and GPU decode if caps negotiated to memory:VASurface. You can override default pre-processing backend by setting property pre-process-backend in bin elements, however not all combinations of decode and inference devices and pre-processing backends are compatible, and overriding pre-processing backend may impact performance.

Decode device	Inference device	Default Pre-processing Backend
CPU	CPU	gst-opencv
CPU	GPU	gst-opencv
GPU	CPU	vaapi
GPU	GPU	vaapi-surface-sharing

Video Pre-processing Elements

Pre-processing performs differently in case of full-frame inference and per-ROI (Region Of Interest) inference. You can control this using property inference-region in bin elements. In can be set either to full-frame or roi-list.

In case of full-frame inference, pre-processing is normal GStreamer pipeline of scaling, color conversion, and normalization elements executed on full frame.

In case of per-ROI inference, element roi_split inserted before pre-processing elements. The roi_split iterates over all GstVideoRegionOfInterestMeta attached to GstBuffer, and produces as many GstBuffer’s as metadata found in original buffer. Every produced GstBuffer has single GstVideoCropMeta with rectangle (x,y,w,h) according to GstVideoRegionOfInterestMeta in original buffer.

If object-class property is set in bin element, this property passed to roi_split element. As result roi_split may produce less buffers than number of GstVideoRegionOfInterestMeta in original buffer, skipping all GstVideoRegionOfInterestMeta with object class not matching to specified in object-class property. Effectively, all elements inserted after roi_split receive as many buffers per original buffer as number objects on frame require inference operation.

The graph below high-level representation of per-ROI inference:

Per-ROI inference

Batched Pre-processing

The following elements support batched pre-processing for better parallelization and performance:

1. batch_create

2. vaapi_batch_proc

If batch_size property specified in bin element (and passed to inference element), one of these elements negotiate caps with inference element on other/tensors media type having ‘N’ dimension in tensor shape greater than 1.

Element vaapi_batch_proc accumulate internally N frames, then submit VA-API operation on N frames and output single buffer containing pre-processing result of all N frames. Element batch_create accumulates internally N frames (GstBuffer), then pushes them as single GstBufferList containing all N frames.

Inference is performed in batched mode on buffer containing N frames.

Element batch_split inserted after inference element and before post-processing element. This element splits batched frame with N inference results into N frames, so that post-processing element can work in normal mode.

Batched Pre-processing shared across multiple streams

If multiple (M>1) streams run same or similar pipeline with inference on same NN models, specifying property shared-instance-id with identical string in all M streams may help to reduce latency in batched mode. In this case M instances of element vaapi_batch_proc* use single queue to accumulate N (=batch_size) frames from all M streams, so that batched frame may contain data from different streams.

Information about original source of frames composed into batch (ex, timestamp, stream id, ROI id, object id, etc.) attached to buffer as GstStructure-based metadata SourceIdentifierMeta and passed downstream. So that element batch_split can correctly split batched inference result into N results and push each result into corresponding (one of M) stream.

Processing

Block Processing on diagram High level bin elements architecture usually represented as single element.

For inference this is an element that infer a result from trained neural network using some inference engine as backend. An inference element accepts input data and produces a inference result in form of other/tensors.

Currently only one inference engine is supported - OpenVINO™. And the element, which uses it as inference backend, is named openvino_tensor_inference. More inference engines can be supported in the future.

The inference elements sets proper/allowed tensors shape (dims) for input and output caps once NN is read.

Post-processing

The Post-processing box on diagram High level bin elements architecture usually consist of single element.

In case of inference a post-processing element is responsible for decoding output tensor and converting it into metadata (ex., bounding-boxes, confidences, classes, keypoints, etc.). Because different NN models may require different post-processing, there are multiple post-processing elements. In general, every post-processing element that work with tensors starts with tensor_postproc_ prefix.

Bin elements

Intel® DL Streamer provides variety of bin elements to simplify creation of media analytics pipeline. Most of Intel® DL Streamer bin elements internally use auxiliary element processbin to create a processing sub-pipeline.

Element video_inference

This is generic inference element, it serves as base for object_detect and object_classify bin elements. However, it can also be used as is.

It provides full backward compatibility in terms of element properties with gvainference element.

Below are some of pipelines that the video_inference element builds internally based on various parameters, such as input memory type, pre-processing backend, inference device, inference region, etc. The elements tee and aggregate are omitted for simplicity, but in reality they are present in every pipeline.

The gst-opencv pre-processing and full-frame inference on CPU or GPU

The vaapi pre-processing and full-frame inference on CPU

The vaapi-opencl pre-processing and full-frame inference on GPU

The vaapi-surface-sharing pre-processing and full-frame inference on GPU

The queue element can be inserted after inference element to enable parallel inference execution if number of inference requests (nireq) is greater than one.

The queue after inference

Element object_detect

The object_detect element is based on video_inference and sets post-processing element to tensor_postproc_detection by default. It also disables attaching raw tensor data as metadata by default.

Element object_classify

The object_classify element is based on video_inference and sets inference-region property to roi-list by default.