| OpenCV 4.3.0 Open Source Computer Vision

Load Caffe framework models

## Introduction

In this tutorial you will learn how to use opencv_dnn module for image classification by using GoogLeNet trained network from Caffe model zoo.

We will demonstrate results of this example on the following picture.

Buran space shuttle

## Source Code

We will be using snippets from the example application, that can be downloaded here.

#include <fstream>
#include <sstream>
#include <opencv2/dnn.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>
#include "common.hpp"
std::string keys =
"{ help h | | Print help message. }"
"{ @alias | | An alias name of model to extract preprocessing parameters from models.yml file. }"
"{ zoo | models.yml | An optional path to file with preprocessing parameters }"
"{ input i | | Path to input image or video file. Skip this argument to capture frames from a camera.}"
"{ framework f | | Optional name of an origin framework of the model. Detect it automatically if it does not set. }"
"{ classes | | Optional path to a text file with names of classes. }"
"{ backend | 0 | Choose one of computation backends: "
"0: automatically (by default), "
"1: Halide language (http://halide-lang.org/), "
"2: Intel's Deep Learning Inference Engine (https://software.intel.com/openvino-toolkit), "
"3: OpenCV implementation }"
"{ target | 0 | Choose one of target computation devices: "
"0: CPU target (by default), "
"1: OpenCL, "
"2: OpenCL fp16 (half-float precision), "
"3: VPU }";
using namespace cv;
using namespace dnn;
std::vector<std::string> classes;
int main(int argc, char** argv)
{
CommandLineParser parser(argc, argv, keys);
const std::string modelName = parser.get<String>("@alias");
const std::string zooFile = parser.get<String>("zoo");
keys += genPreprocArguments(modelName, zooFile);
parser = CommandLineParser(argc, argv, keys);
parser.about("Use this script to run classification deep learning networks using OpenCV.");
if (argc == 1 || parser.has("help"))
{
parser.printMessage();
return 0;
}
float scale = parser.get<float>("scale");
Scalar mean = parser.get<Scalar>("mean");
bool swapRB = parser.get<bool>("rgb");
int inpWidth = parser.get<int>("width");
int inpHeight = parser.get<int>("height");
String model = findFile(parser.get<String>("model"));
String config = findFile(parser.get<String>("config"));
String framework = parser.get<String>("framework");
int backendId = parser.get<int>("backend");
int targetId = parser.get<int>("target");
// Open file with classes names.
if (parser.has("classes"))
{
std::string file = parser.get<String>("classes");
std::ifstream ifs(file.c_str());
if (!ifs.is_open())
CV_Error(Error::StsError, "File " + file + " not found");
std::string line;
while (std::getline(ifs, line))
{
classes.push_back(line);
}
}
if (!parser.check())
{
parser.printErrors();
return 1;
}
CV_Assert(!model.empty());
Net net = readNet(model, config, framework);
net.setPreferableBackend(backendId);
net.setPreferableTarget(targetId);
// Create a window
static const std::string kWinName = "Deep learning image classification in OpenCV";
namedWindow(kWinName, WINDOW_NORMAL);
VideoCapture cap;
if (parser.has("input"))
cap.open(parser.get<String>("input"));
else
cap.open(0);
// Process frames.
Mat frame, blob;
while (waitKey(1) < 0)
{
cap >> frame;
if (frame.empty())
{
waitKey();
break;
}
blobFromImage(frame, blob, scale, Size(inpWidth, inpHeight), mean, swapRB, false);
net.setInput(blob);
Mat prob = net.forward();
Point classIdPoint;
double confidence;
minMaxLoc(prob.reshape(1, 1), 0, &confidence, 0, &classIdPoint);
int classId = classIdPoint.x;
// Put efficiency information.
std::vector<double> layersTimes;
double freq = getTickFrequency() / 1000;
double t = net.getPerfProfile(layersTimes) / freq;
std::string label = format("Inference time: %.2f ms", t);
putText(frame, label, Point(0, 15), FONT_HERSHEY_SIMPLEX, 0.5, Scalar(0, 255, 0));
// Print predicted class.
label = format("%s: %.4f", (classes.empty() ? format("Class #%d", classId).c_str() :
classes[classId].c_str()),
confidence);
putText(frame, label, Point(0, 40), FONT_HERSHEY_SIMPLEX, 0.5, Scalar(0, 255, 0));
imshow(kWinName, frame);
}
return 0;
}

## Explanation

1. Firstly, download GoogLeNet model files: bvlc_googlenet.prototxt and bvlc_googlenet.caffemodel

   Also you need file with names of ILSVRC2012 classes: classification_classes_ILSVRC2012.txt.

   Put these files into working dir of this program example.

2. Read and initialize network using path to .prototxt and .caffemodel files
   Net net = readNet(model, config, framework);
   net.setPreferableBackend(backendId);
   net.setPreferableTarget(targetId);
   You can skip an argument `framework` if one of the files `model` or `config` has an extension `.caffemodel` or `.prototxt`. This way function cv::dnn::readNet can automatically detects a model's format.

3. Read input image and convert to the blob, acceptable by GoogleNet

   VideoCapture cap;
   if (parser.has("input"))
   cap.open(parser.get<String>("input"));
   else
   cap.open(0);

   cv::VideoCapture can load both images and videos.

   blobFromImage(frame, blob, scale, Size(inpWidth, inpHeight), mean, swapRB, false);

   We convert the image to a 4-dimensional blob (so-called batch) with `1x3x224x224` shape after applying necessary pre-processing like resizing and mean subtraction `(-104, -117, -123)` for each blue, green and red channels correspondingly using cv::dnn::blobFromImage function.

4. Pass the blob to the network
   net.setInput(blob);
5. Make forward pass
   Mat prob = net.forward();
   During the forward pass output of each network layer is computed, but in this example we need output from the last layer only.
6. Determine the best class
   Point classIdPoint;
   double confidence;
   minMaxLoc(prob.reshape(1, 1), 0, &confidence, 0, &classIdPoint);
   int classId = classIdPoint.x;
   We put the output of network, which contain probabilities for each of 1000 ILSVRC2012 image classes, to the `prob` blob. And find the index of element with maximal value in this one. This index corresponds to the class of the image.
7. Run an example from command line
   ./example_dnn_classification --model=bvlc_googlenet.caffemodel --config=bvlc_googlenet.prototxt --width=224 --height=224 --classes=classification_classes_ILSVRC2012.txt --input=space_shuttle.jpg --mean="104 117 123"
   For our image we get prediction of class `space shuttle` with more than 99% sureness.