Multi Object Detection using pre-trained SSD Model via Java Inference APIs¶

This tutorial shows how to use MXNet Java Inference APIs to run inference on a pre-trained Single Shot Detector (SSD) Model.

The SSD model is trained on the Pascal VOC 2012 dataset. The network is a SSD model built on Resnet50 as the base network to extract image features. The model is trained to detect the following entities (classes): [‘aeroplane’, ‘bicycle’, ‘bird’, ‘boat’, ‘bottle’, ‘bus’, ‘car’, ‘cat’, ‘chair’, ‘cow’, ‘diningtable’, ‘dog’, ‘horse’, ‘motorbike’, ‘person’, ‘pottedplant’, ‘sheep’, ‘sofa’, ‘train’, ‘tvmonitor’]. For more details about the model, you can refer to the MXNet SSD example.

Prerequisites¶

To complete this tutorial, you need the following:

MXNet Java Setup on IntelliJ IDEA (Optional)
wget To download model artifacts
SSD Model artifacts
- Use the following script to get the SSD Model files :

data_path=/tmp/resnet50_ssd
mkdir -p "$data_path"
wget https://s3.amazonaws.com/model-server/models/resnet50_ssd/resnet50_ssd_model-symbol.json -P $data_path
wget https://s3.amazonaws.com/model-server/models/resnet50_ssd/resnet50_ssd_model-0000.params -P $data_path
wget https://s3.amazonaws.com/model-server/models/resnet50_ssd/synset.txt -P $data_path

Test images : A few sample images to run inference on.
- Use the following script to download sample images :

image_path=/tmp/resnet50_ssd/images
mkdir -p "$image_path"
cd $image_path
wget https://cloud.githubusercontent.com/assets/3307514/20012567/cbb60336-a27d-11e6-93ff-cbc3f09f5c9e.jpg -O dog.jpg
wget https://cloud.githubusercontent.com/assets/3307514/20012563/cbb41382-a27d-11e6-92a9-18dab4fd1ad3.jpg -O person.jpg

Alternately, you can get the entire SSD Model artifacts + images in one single script from the MXNet Repository by running get_ssd_data.sh script

Time to code!¶

1. Following the MXNet Java Setup on IntelliJ IDEA tutorial, in the same project JavaMXNet, create a new empty class called : ObjectDetectionTutorial.java.

2. In the main function of ObjectDetectionTutorial.java define the downloaded model path and the image data paths. This is the same path where we downloaded the model artifacts and images in a previous step.

String modelPathPrefix = "/tmp/resnet50_ssd/resnet50_ssd_model";
String inputImagePath = "/tmp/resnet50_ssd/images/dog.jpg";

3. We can run the inference code in this example on either CPU or GPU (if you have a GPU backed machine) by choosing the appropriate context.

List<Context> context = getContext();
...

private static List<Context> getContext() {
List<Context> ctx = new ArrayList<>();
ctx.add(Context.cpu()); // Choosing CPU Context here

return ctx;
}

4. To provide an input to the model, define the input shape to the model and the Input Data Descriptor (DataDesc) as shown below :

Shape inputShape = new Shape(new int[] {1, 3, 512, 512});
List<DataDesc> inputDescriptors = new ArrayList<DataDesc>();
inputDescriptors.add(new DataDesc("data", inputShape, DType.Float32(), "NCHW"));

The input shape can be interpreted as follows : The input has a batch size of 1, with 3 RGB channels in the image, and the height and width of the image is 512 each.

5. To run an actual inference on the given image, add the following lines to the ObjectDetectionTutorial.java class :

BufferedImage img = ObjectDetector.loadImageFromFile(inputImagePath);
ObjectDetector objDet = new ObjectDetector(modelPathPrefix, inputDescriptors, context, 0);
List<List<ObjectDetectorOutput>> output = objDet.imageObjectDetect(img, 3); // Top 3 objects detected will be returned

6. Let’s piece all of the above steps together by showing the final contents of the ObjectDetectionTutorial.java.

package mxnet;

import org.apache.mxnet.infer.javaapi.ObjectDetector;
import org.apache.mxnet.infer.javaapi.ObjectDetectorOutput;
import org.apache.mxnet.javaapi.Context;
import org.apache.mxnet.javaapi.DType;
import org.apache.mxnet.javaapi.DataDesc;
import org.apache.mxnet.javaapi.Shape;

import java.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class ObjectDetectionTutorial {

    public static void main(String[] args) {

        String modelPathPrefix = "/tmp/resnet50_ssd/resnet50_ssd_model";

        String inputImagePath = "/tmp/resnet50_ssd/images/dog.jpg";

        List<Context> context = getContext();

        Shape inputShape = new Shape(new int[] {1, 3, 512, 512});

        List<DataDesc> inputDescriptors = new ArrayList<DataDesc>();
        inputDescriptors.add(new DataDesc("data", inputShape, DType.Float32(), "NCHW"));

        BufferedImage img = ObjectDetector.loadImageFromFile(inputImagePath);
        ObjectDetector objDet = new ObjectDetector(modelPathPrefix, inputDescriptors, context, 0);
        List<List<ObjectDetectorOutput>> output = objDet.imageObjectDetect(img, 3);

        printOutput(output, inputShape);
    }


    private static List<Context> getContext() {
        List<Context> ctx = new ArrayList<>();
        ctx.add(Context.cpu());

        return ctx;
    }

    private static void printOutput(List<List<ObjectDetectorOutput>> output, Shape inputShape) {

        StringBuilder outputStr = new StringBuilder();

        int width = inputShape.get(3);
        int height = inputShape.get(2);

        for (List<ObjectDetectorOutput> ele : output) {
            for (ObjectDetectorOutput i : ele) {
                outputStr.append("Class: " + i.getClassName() + "\n");
                outputStr.append("Probabilties: " + i.getProbability() + "\n");

                List<Float> coord = Arrays.asList(i.getXMin() * width,
                        i.getXMax() * height, i.getYMin() * width, i.getYMax() * height);
                StringBuilder sb = new StringBuilder();
                for (float c: coord) {
                    sb.append(", ").append(c);
                }
                outputStr.append("Coord:" + sb.substring(2)+ "\n");
            }
        }
        System.out.println(outputStr);

    }
}

7. To compile and run this code, change directories to this project’s root folder, then run the following:

mvn clean install dependency:copy-dependencies

The build generates a new jar file in the target folder called javaMXNet-1.0-SNAPSHOT.jar.

To run the ObjectDetectionTutorial.java use the following command from the project’s root folder.

java -cp "target/javaMXNet-1.0-SNAPSHOT.jar:target/dependency/*" mxnet.ObjectDetectionTutorial

You should see a similar output being generated for the dog image that we used:

Class: car
Probabilties: 0.99847263
Coord:312.21335, 72.02908, 456.01443, 150.66176
Class: bicycle
Probabilties: 0.9047381
Coord:155.9581, 149.96365, 383.83694, 418.94516
Class: dog
Probabilties: 0.82268167
Coord:83.82356, 179.14001, 206.63783, 476.78754

dog_1

The results returned by the inference call translate into the regions in the image where the model detected objects.

dog_2

Next Steps¶

For more information about MXNet Java resources, see the following: