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OpenCV 4.4 Graph API

G-API: What is, why, what's for?

OpenCV evolution in one slide

Version 1.x – Library inception

  • Just a set of CV functions + helpers around (visualization, IO);

Version 2.x – Library rewrite

  • OpenCV meets C++, cv::Mat replaces IplImage*;

Version 3.0 – Welcome Transparent API (T-API)

  • cv::UMat is introduced as a transparent addition to cv::Mat;
  • With cv::UMat, an OpenCL kernel can be enqeueud instead of immediately running C code;
  • cv::UMat data is kept on a device until explicitly queried.

OpenCV evolution in one slide (cont'd)

Version 4.0 – Welcome Graph API (G-API)

  • A new separate module (not a full library rewrite);
  • A framework (or even a meta-framework);

  • Usage model:

    • Express an image/vision processing graph and then execute it;
    • Fine-tune execution without changes in the graph;
  • Similar to Halide – separates logic from platform details.

  • More than Halide:

    • Kernels can be written in unconstrained platform-native code;
    • Halide can serve as a backend (one of many).

OpenCV evolution in one slide (cont'd)

Version 4.2 – New horizons

  • Introduced in-graph inference via OpenVINO™ Toolkit;
  • Introduced video-oriented Streaming execution mode;
  • Extended focus from individual image processing to the full application pipeline optimization.

Version 4.4 – More on video

  • Introduced a notion of stateful kernels;

    • The road to object tracking, background subtraction, etc. in the graph;
  • Added more video-oriented operations (feature detection, Optical flow).

Why G-API?

Why introduce a new execution model?

  • Ultimately it is all about optimizations;

    • or at least about a possibility to optimize;
  • A CV algorithm is usually not a single function call, but a composition of functions;
  • Different models operate at different levels of knowledge on the algorithm (problem) we run.

Why G-API? (cont'd)

Why introduce a new execution model?

  • Traditional – every function can be optimized (e.g. vectorized) and parallelized, the rest is up to programmer to care about.
  • Queue-based – kernels are enqueued dynamically with no guarantee where the end is or what is called next;
  • Graph-based – nearly all information is there, some compiler magic can be done!

What is G-API for?

Bring the value of graph model with OpenCV where it makes sense:

  • Memory consumption can be reduced dramatically;
  • Memory access can be optimized to maximize cache reuse;

  • Parallelism can be applied automatically where it is hard to do it manually;

    • It also becomes more efficient when working with graphs;

  • Heterogeneity gets extra benefits like:

    • Avoiding unnecessary data transfers;
    • Shadowing transfer costs with parallel host co-execution;
    • Improving system throughput with frame-level pipelining.

Programming with G-API

G-API Basics

G-API Concepts

  • Graphs are built by applying operations to data objects;

    • API itself has no "graphs", it is expression-based instead;
  • Data objects do not hold actual data, only capture dependencies;
  • Operations consume and produce data objects.

  • A graph is defined by specifying its boundaries with data objects:

    • What data objects are inputs to the graph?
    • What are its outputs?

The code is worth a thousand words 

#include <opencv2/gapi.hpp>                            // G-API framework header
#include <opencv2/gapi/imgproc.hpp>                    // cv::gapi::blur()
#include <opencv2/highgui.hpp>                         // cv::imread/imwrite

int main(int argc, char *argv[]) {
    if (argc < 3) return 1;

    cv::GMat in;                                       // Express the graph:
    cv::GMat out = cv::gapi::blur(in, cv::Size(3,3));  // `out` is a result of `blur` of `in`

    cv::Mat in_mat = cv::imread(argv[1]);              // Get the real data
    cv::Mat out_mat;                                   // Output buffer (may be empty)

    cv::GComputation(cv::GIn(in), cv::GOut(out))       // Declare a graph from `in` to `out`
        .apply(cv::gin(in_mat), cv::gout(out_mat));    // ...and run it immediately

    cv::imwrite(argv[2], out_mat);                     // Save the result
    return 0;
}

The code is worth a thousand words

Traditional OpenCV   B_block BMCOL 

#include <opencv2/core.hpp>
#include <opencv2/imgproc.hpp>

#include <opencv2/highgui.hpp>

int main(int argc, char *argv[]) {
    using namespace cv;
    if (argc != 3) return 1;

    Mat in_mat = imread(argv[1]);
    Mat gx, gy;

    Sobel(in_mat, gx, CV_32F, 1, 0);
    Sobel(in_mat, gy, CV_32F, 0, 1);

    Mat mag, out_mat;
    sqrt(gx.mul(gx) + gy.mul(gy), mag);
    mag.convertTo(out_mat, CV_8U);

    imwrite(argv[2], out_mat);
    return 0;
}

OpenCV G-API   B_block BMCOL 

#include <opencv2/gapi.hpp>
#include <opencv2/gapi/core.hpp>
#include <opencv2/gapi/imgproc.hpp>
#include <opencv2/highgui.hpp>

int main(int argc, char *argv[]) {
    using namespace cv;
    if (argc != 3) return 1;

    GMat in;
    GMat gx  = gapi::Sobel(in, CV_32F, 1, 0);
    GMat gy  = gapi::Sobel(in, CV_32F, 0, 1);
    GMat mag = gapi::sqrt(  gapi::mul(gx, gx)
                          + gapi::mul(gy, gy));
    GMat out = gapi::convertTo(mag, CV_8U);
    GComputation sobel(GIn(in), GOut(out));

    Mat in_mat = imread(argv[1]), out_mat;
    sobel.apply(in_mat, out_mat);
    imwrite(argv[2], out_mat);
    return 0;
}

The code is worth a thousand words (cont'd)

What we have just learned?

  • G-API functions mimic their traditional OpenCV ancestors;
  • No real data is required to construct a graph;
  • Graph construction and graph execution are separate steps.

What else?

  • Graph is first expressed and then captured in an object;

  • Graph constructor defines protocol; user can pass vectors of inputs/outputs like

    cv::GComputation(cv::GIn(...), cv::GOut(...))
  • Calls to .apply() must conform to graph's protocol

On data objects

Graph protocol defines what arguments a computation was defined on (both inputs and outputs), and what are the shapes (or types) of those arguments:

Shape Argument Size
GMat Mat Static; defined during
graph compilation
GScalar Scalar 4 x double
GArray<T> std::vector<T> Dynamic; defined in runtime
GOpaque<T> T Static, sizeof(T)

GScalar may be value-initialized at construction time to allow expressions like GMat a = 2*(b + 1).

On operations and kernels

:B_block:BMCOL:

  • Graphs are built with Operations over virtual Data;
  • Operations define interfaces (literally);
  • Kernels are implementations to Operations (like in OOP);
  • An Operation is platform-agnostic, a kernel is not;
  • Kernels are implemented for Backends, the latter provide APIs to write kernels;
  • Users can add their own operations and kernels, and also redefine "standard" kernels their own way.

:B_block:BMCOL: 

digraph G {
node [shape=box];
rankdir=BT;

Gr [label="Graph"];
Op [label="Operation\nA"];
{rank=same
Impl1 [label="Kernel\nA:2"];
Impl2 [label="Kernel\nA:1"];
}

Op -> Gr [dir=back, label="'consists of'"];
Impl1 -> Op [];
Impl2 -> Op [label="'is implemented by'"];

node [shape=note,style=dashed];
{rank=same
Op;
CommentOp [label="Abstract:\ndeclared via\nG_API_OP()"];
}
{rank=same
Comment1 [label="Platform:\ndefined with\nOpenCL backend"];
Comment2 [label="Platform:\ndefined with\nOpenCV backend"];
}

CommentOp -> Op      [constraint=false, style=dashed, arrowhead=none];
Comment1  -> Impl1   [style=dashed, arrowhead=none];
Comment2  -> Impl2   [style=dashed, arrowhead=none];
}

On operations and kernels (cont'd)

Defining an operation

  • A type name (every operation is a C++ type);
  • Operation signature (similar to std::function<>);
  • Operation identifier (a string);
  • Metadata callback – describe what is the output value format(s), given the input and arguments.
  • Use OpType::on(...) to use a new kernel OpType to construct graphs.
G_API_OP(GSqrt,<GMat(GMat)>,"org.opencv.core.math.sqrt") {
    static GMatDesc outMeta(GMatDesc in) { return in; }
};

On operations and kernels (cont'd)

GSqrt vs. cv::gapi::sqrt()

  • How a type relates to a functions from the example?

  • These functions are just wrappers over ::on:

    G_API_OP(GSqrt,<GMat(GMat)>,"org.opencv.core.math.sqrt") {
    static GMatDesc outMeta(GMatDesc in) { return in; }
};
GMat gapi::sqrt(const GMat& src) { return GSqrt::on(src); }

  • Why – Doxygen, default parameters, 1:n mapping:

    cv::GMat custom::unsharpMask(const cv::GMat &src,
                             const int       sigma,
                             const float     strength) {
    cv::GMat blurred   = cv::gapi::medianBlur(src, sigma);
    cv::GMat laplacian = cv::gapi::Laplacian(blurred, CV_8U);
    return (src - (laplacian * strength));
}

On operations and kernels (cont'd)

Implementing an operation

  • Depends on the backend and its API;
  • Common part for all backends: refer to operation being implemented using its type.

OpenCV backend

  • OpenCV backend is the default one: OpenCV kernel is a wrapped OpenCV function:

    GAPI_OCV_KERNEL(GCPUSqrt, cv::gapi::core::GSqrt) {
    static void run(const cv::Mat& in, cv::Mat &out) {
        cv::sqrt(in, out);
    }
};

Operations and Kernels (cont'd)

Fluid backend

  • Fluid backend operates with row-by-row kernels and schedules its execution to optimize data locality:

    GAPI_FLUID_KERNEL(GFluidSqrt, cv::gapi::core::GSqrt, false) {
    static const int Window = 1;
    static void run(const View &in, Buffer &out) {
        hal::sqrt32f(in .InLine <float>(0)
                     out.OutLine<float>(0),
                     out.length());
    }
};
  • Note run changes signature but still is derived from the operation signature.

Operations and Kernels (cont'd)

Specifying which kernels to use

  • Graph execution model is defined by kernels which are available/used;

  • Kernels can be specified via the graph compilation arguments:

    #include <opencv2/gapi/fluid/core.hpp>
#include <opencv2/gapi/fluid/imgproc.hpp>
...
auto pkg = cv::gapi::combine(cv::gapi::core::fluid::kernels(),
                             cv::gapi::imgproc::fluid::kernels());
sobel.apply(in_mat, out_mat, cv::compile_args(pkg));
  • Users can combine kernels of different backends and G-API will partition the execution among those automatically.

Heterogeneity in G-API

Automatic subgraph partitioning in G-API

:B_block:BMCOL: 

digraph G {
rankdir=TB;
ranksep=0.3;

node [shape=box margin=0 height=0.25];
A; B; C;

node [shape=ellipse];
GMat0;
GMat1;
GMat2;
GMat3;

GMat0 -> A -> GMat1 -> B -> GMat2;
GMat2 -> C;
GMat0 -> C -> GMat3

subgraph cluster {style=invis; A; GMat1; B; GMat2; C};
}

The initial graph: operations are not resolved yet.

:B_block:BMCOL: 

digraph G {
rankdir=TB;
ranksep=0.3;

node [shape=box margin=0 height=0.25];
A; B; C;

node [shape=ellipse];
GMat0;
GMat1;
GMat2;
GMat3;

GMat0 -> A -> GMat1 -> B -> GMat2;
GMat2 -> C;
GMat0 -> C -> GMat3

subgraph cluster {style=filled;color=azure2; A; GMat1; B; GMat2; C};
}

All operations are handled by the same backend.

:B_block:BMCOL: 

digraph G {
rankdir=TB;
ranksep=0.3;

node [shape=box margin=0 height=0.25];
A; B; C;

node [shape=ellipse];
GMat0;
GMat1;
GMat2;
GMat3;

GMat0 -> A -> GMat1 -> B -> GMat2;
GMat2 -> C;
GMat0 -> C -> GMat3

subgraph cluster_1 {style=filled;color=azure2; A; GMat1; B; }
subgraph cluster_2 {style=filled;color=ivory2; C};
}

A & B are of backend 1, C is of backend 2.

:B_block:BMCOL: 

digraph G {
rankdir=TB;
ranksep=0.3;

node [shape=box margin=0 height=0.25];
A; B; C;

node [shape=ellipse];
GMat0;
GMat1;
GMat2;
GMat3;

GMat0 -> A -> GMat1 -> B -> GMat2;
GMat2 -> C;
GMat0 -> C -> GMat3

subgraph cluster_1 {style=filled;color=azure2; A};
subgraph cluster_2 {style=filled;color=ivory2; B};
subgraph cluster_3 {style=filled;color=azure2; C};
}

A & C are of backend 1, B is of backend 2.

Heterogeneity in G-API

Heterogeneity summary

  • G-API automatically partitions its graph in subgraphs (called "islands") based on the available kernels;
  • Adjacent kernels taken from the same backend are "fused" into the same "island";

  • G-API implements a two-level execution model:

    • Islands are executed at the top level by a G-API's Executor;
    • Island internals are run at the bottom level by its Backend;
  • G-API fully delegates the low-level execution and memory management to backends.

Inference and Streaming

Inference with G-API

In-graph inference example

  • Starting with OpencV 4.2 (2019), G-API allows to integrate infer operations into the graph:

    G_API_NET(ObjDetect, <cv::GMat(cv::GMat)>, "pdf.example.od");

cv::GMat in;
cv::GMat blob = cv::gapi::infer<ObjDetect>(bgr);
cv::GOpaque<cv::Size> size = cv::gapi::streaming::size(bgr);
cv::GArray<cv::Rect>  objs = cv::gapi::streaming::parseSSD(blob, size);
cv::GComputation pipelne(cv::GIn(in), cv::GOut(objs));
  • Starting with OpenCV 4.5 (2020), G-API will provide more streaming- and NN-oriented operations out of the box.

Inference with G-API

What is the difference?

  • ObjDetect is not an operation, cv::gapi::infer<T> is;

  • cv::gapi::infer<T> is a generic operation, where T=ObjDetect describes the calling convention:

    • How many inputs the network consumes,
    • How many outputs the network produces.

  • Inference data types are GMat only:

    • Representing an image, then preprocessed automatically;
    • Representing a blob (n-dimensional Mat), then passed as-is.
  • Inference backends only need to implement a single generic operation infer.

Inference with G-API

But how does it run?

  • Since infer is an Operation, backends may provide Kernels implementing it;

  • The only publicly available inference backend now is OpenVINO™:

    • Brings its infer kernel atop of the Inference Engine;
  • NN model data is passed through G-API compile arguments (like kernels);
  • Every NN backend provides its own structure to configure the network (like a kernel API).

Inference with G-API

Passing OpenVINO™ parameters to G-API

  • ObjDetect example:

    auto face_net = cv::gapi::ie::Params<ObjDetect> {
    face_xml_path,        // path to the topology IR
    face_bin_path,        // path to the topology weights
    face_device_string,   // OpenVINO plugin (device) string
};
auto networks = cv::gapi::networks(face_net);
pipeline.compile(.., cv::compile_args(..., networks));

  • AgeGender requires binding Op's outputs to NN layers:

    auto age_net = cv::gapi::ie::Params<AgeGender> {
    ...
}.cfgOutputLayers({"age_conv3", "prob"}); // array<string,2> !

Streaming with G-API 

digraph {
  rankdir=LR;
  node [shape=box];

  cap [label=Capture];
  dec [label=Decode];
  res [label=Resize];
  cnn [label=Infer];
  vis [label=Visualize];

  cap -> dec;
  dec -> res;
  res -> cnn;
  cnn -> vis;
}

Anatomy of a regular video analytics application

Streaming with G-API 

digraph {
  node [shape=box margin=0 width=0.3 height=0.4]
  nodesep=0.2;
  rankdir=LR;

  subgraph cluster0 {
  colorscheme=blues9
  pp [label="..." shape=plaintext];
  v0 [label=V];
  label="Frame N-1";
  color=7;
  }

  subgraph cluster1 {
  colorscheme=blues9
  c1 [label=C];
  d1 [label=D];
  r1 [label=R];
  i1 [label=I];
  v1 [label=V];
  label="Frame N";
  color=6;
  }

  subgraph cluster2 {
  colorscheme=blues9
  c2 [label=C];
  nn [label="..." shape=plaintext];
  label="Frame N+1";
  color=5;
  }

  c1 -> d1 -> r1 -> i1 -> v1;

  pp-> v0;
  v0 -> c1 [style=invis];
  v1 -> c2 [style=invis];
  c2 -> nn;
}

Serial execution of the sample video analytics application

Streaming with G-API 

digraph {
  nodesep=0.2;
  ranksep=0.2;
  node [margin=0 width=0.4 height=0.2];
  node [shape=plaintext]
  Camera [label="Camera:"];
  GPU [label="GPU:"];
  FPGA [label="FPGA:"];
  CPU [label="CPU:"];
  Time [label="Time:"];
  t6  [label="T6"];
  t7  [label="T7"];
  t8  [label="T8"];
  t9  [label="T9"];
  t10 [label="T10"];
  tnn [label="..."];

  node [shape=box margin=0 width=0.4 height=0.4 colorscheme=blues9]
  node [color=9] V3;
  node [color=8] F4; V4;
  node [color=7] DR5; F5; V5;
  node [color=6] C6; DR6; F6; V6;
  node [color=5] C7; DR7; F7; V7;
  node [color=4] C8; DR8; F8;
  node [color=3] C9; DR9;
  node [color=2] C10;

  {rank=same; rankdir=LR; Camera C6 C7 C8 C9 C10}
  Camera -> C6 -> C7 -> C8 -> C9 -> C10 [style=invis];

  {rank=same; rankdir=LR; GPU DR5 DR6 DR7 DR8 DR9}
  GPU -> DR5 -> DR6 -> DR7 -> DR8 -> DR9 [style=invis];

  C6 -> DR5 [style=invis];
  C6 -> DR6 [constraint=false];
  C7 -> DR7 [constraint=false];
  C8 -> DR8 [constraint=false];
  C9 -> DR9 [constraint=false];

  {rank=same; rankdir=LR; FPGA F4 F5 F6 F7 F8}
  FPGA -> F4 -> F5 -> F6 -> F7 -> F8 [style=invis];

  DR5 -> F4 [style=invis];
  DR5 -> F5 [constraint=false];
  DR6 -> F6 [constraint=false];
  DR7 -> F7 [constraint=false];
  DR8 -> F8 [constraint=false];

  {rank=same; rankdir=LR; CPU V3 V4 V5 V6 V7}
  CPU -> V3 -> V4 -> V5 -> V6 -> V7 [style=invis];

  F4 -> V3 [style=invis];
  F4 -> V4 [constraint=false];
  F5 -> V5 [constraint=false];
  F6 -> V6 [constraint=false];
  F7 -> V7 [constraint=false];

  {rank=same; rankdir=LR; Time t6 t7 t8 t9 t10 tnn}
  Time -> t6 -> t7 -> t8 -> t9 -> t10 -> tnn [style=invis];

  CPU -> Time [style=invis];
  V3 -> t6  [style=invis];
  V4 -> t7  [style=invis];
  V5 -> t8  [style=invis];
  V6 -> t9  [style=invis];
  V7 -> t10 [style=invis];
}

Pipelined execution for the video analytics application

Streaming with G-API: Example

Serial mode (4.0)   B_block BMCOL 
pipeline = cv::GComputation(...);

cv::VideoCapture cap(input);
cv::Mat in_frame;
std::vector<cv::Rect> out_faces;

while (cap.read(in_frame)) {
    pipeline.apply(cv::gin(in_frame),
                   cv::gout(out_faces),
                   cv::compile_args(kernels,
                                    networks));
    // Process results
    ...
}
Streaming mode (since 4.2)   B_block BMCOL 
pipeline = cv::GComputation(...);

auto in_src = cv::gapi::wip::make_src
    <cv::gapi::wip::GCaptureSource>(input)
auto cc = pipeline.compileStreaming
    (cv::compile_args(kernels, networks))
cc.setSource(cv::gin(in_src));
cc.start();

std::vector<cv::Rect> out_faces;
while (cc.pull(cv::gout(out_faces))) {
    // Process results
    ...
}

Latest features

Latest features

Python API

  • Initial Python3 binding is available now in master (future 4.5);
  • Only basic CV functionality is supported (core & imgproc namespaces, selecting backends);
  • Adding more programmability, inference, and streaming is next.

Latest features

Python API 

import numpy as np
import cv2 as cv

sz  = (1280, 720)
in1 = np.random.randint(0, 100, sz).astype(np.uint8)
in2 = np.random.randint(0, 100, sz).astype(np.uint8)

g_in1 = cv.GMat()
g_in2 = cv.GMat()
g_out = cv.gapi.add(g_in1, g_in2)
gr    = cv.GComputation(g_in1, g_in2, g_out)

pkg   = cv.gapi.core.fluid.kernels()
out   = gr.apply(in1, in2, args=cv.compile_args(pkg))

Understanding the "G-Effect"

Understanding the "G-Effect"

What is "G-Effect"?

  • G-API is not only an API, but also an implementation;

    • i.e. it does some work already!
  • We call "G-Effect" any measurable improvement which G-API demonstrates against traditional methods;

  • So far the list is:

    • Memory consumption;
    • Performance;
    • Programmer efforts.

Note: in the following slides, all measurements are taken on Intel\textregistered{} Core\texttrademark-i5 6600 CPU.

Understanding the "G-Effect"

Memory consumption: Sobel Edge Detector

  • G-API/Fluid backend is designed to minimize footprint:
Input OpenCV G-API/Fluid Factor
MiB MiB Times
512 x 512 17.33 0.59 28.9x
640 x 480 20.29 0.62 32.8x
1280 x 720 60.73 0.72 83.9x
1920 x 1080 136.53 0.83 164.7x
3840 x 2160 545.88 1.22 447.4x
  • The detector itself can be written manually in two for loops, but G-API covers cases more complex than that;
  • OpenCV code requires changes to shrink footprint.

Understanding the "G-Effect"

Performance: Sobel Edge Detector

  • G-API/Fluid backend also optimizes cache reuse:
Input OpenCV G-API/Fluid Factor
ms ms Times
320 x 240 1.16 0.53 2.17x
640 x 480 5.66 1.89 2.99x
1280 x 720 17.24 5.26 3.28x
1920 x 1080 39.04 12.29 3.18x
3840 x 2160 219.57 51.22 4.29x
  • The more data is processed, the bigger "G-Effect" is.

Understanding the "G-Effect"

Relative speed-up based on cache efficiency

\begin{figure} \begin{tikzpicture} \begin{axis}[ xlabel={Image size}, ylabel={Relative speed-up}, nodes near coords, width=0.8\textwidth, xtick=data, xticklabels={QVGA, VGA, HD, FHD, UHD}, height=4.5cm, ] \addplot plot coordinates {(1, 1.0) (2, 1.38) (3, 1.51) (4, 1.46) (5, 1.97)}; \end{axis} \end{tikzpicture} \end{figure}

The higher resolution is, the higher relative speed-up is (with speed-up on QVGA taken as 1.0).

Thank you!