Image Classification - MXNet - Amazon SageMaker

Image Classification - MXNet

The Amazon SageMaker image classification algorithm is a supervised learning algorithm that supports multi-label classification. It takes an image as input and outputs one or more labels assigned to that image. It uses a convolutional neural network that can be trained from scratch or trained using transfer learning when a large number of training images are not available

The recommended input format for the Amazon SageMaker image classification algorithms is Apache MXNet RecordIO. However, you can also use raw images in .jpg or .png format. Refer to this discussion for a broad overview of efficient data preparation and loading for machine learning systems.


To maintain better interoperability with existing deep learning frameworks, this differs from the protobuf data formats commonly used by other Amazon SageMaker algorithms.

For more information on convolutional networks, see:

Input/Output Interface for the Image Classification Algorithm

The SageMaker Image Classification algorithm supports both RecordIO (application/x-recordio) and image (image/png, image/jpeg, and application/x-image) content types for training in file mode, and supports the RecordIO (application/x-recordio) content type for training in pipe mode. However, you can also train in pipe mode using the image files (image/png, image/jpeg, and application/x-image), without creating RecordIO files, by using the augmented manifest format.

Distributed training is supported for file mode and pipe mode. When using the RecordIO content type in pipe mode, you must set the S3DataDistributionType of the S3DataSource to FullyReplicated. The algorithm supports a fully replicated model where your data is copied onto each machine.

The algorithm supports image/png, image/jpeg, and application/x-image for inference.

Train with RecordIO Format

If you use the RecordIO format for training, specify both train and validation channels as values for the InputDataConfig parameter of the CreateTrainingJob request. Specify one RecordIO (.rec) file in the train channel and one RecordIO file in the validation channel. Set the content type for both channels to application/x-recordio.

Train with Image Format

If you use the Image format for training, specify train, validation, train_lst, and validation_lst channels as values for the InputDataConfig parameter of the CreateTrainingJob request. Specify the individual image data (.jpg or .png files) for the train and validation channels. Specify one .lst file in each of the train_lst and validation_lst channels. Set the content type for all four channels to application/x-image.


SageMaker reads the training and validation data separately from different channels, so you must store the training and validation data in different folders.

A .lst file is a tab-separated file with three columns that contains a list of image files. The first column specifies the image index, the second column specifies the class label index for the image, and the third column specifies the relative path of the image file. The image index in the first column must be unique across all of the images. The set of class label indices are numbered successively and the numbering should start with 0. For example, 0 for the cat class, 1 for the dog class, and so on for additional classes.

The following is an example of a .lst file:

5 1 your_image_directory/train_img_dog1.jpg 1000 0 your_image_directory/train_img_cat1.jpg 22 1 your_image_directory/train_img_dog2.jpg

For example, if your training images are stored in s3://<your_bucket>/train/class_dog, s3://<your_bucket>/train/class_cat, and so on, specify the path for your train channel as s3://<your_bucket>/train, which is the top-level directory for your data. In the .lst file, specify the relative path for an individual file named train_image_dog1.jpg in the class_dog class directory as class_dog/train_image_dog1.jpg. You can also store all your image files under one subdirectory inside the train directory. In that case, use that subdirectory for the relative path. For example, s3://<your_bucket>/train/your_image_directory.

Train with Augmented Manifest Image Format

The augmented manifest format enables you to do training in Pipe mode using image files without needing to create RecordIO files. You need to specify both train and validation channels as values for the InputDataConfig parameter of the CreateTrainingJob request. While using the format, an S3 manifest file needs to be generated that contains the list of images and their corresponding annotations. The manifest file format should be in JSON Lines format in which each line represents one sample. The images are specified using the 'source-ref' tag that points to the S3 location of the image. The annotations are provided under the "AttributeNames" parameter value as specified in the CreateTrainingJob request. It can also contain additional metadata under the metadata tag, but these are ignored by the algorithm. In the following example, the "AttributeNames" are contained in the list of image and annotation references ["source-ref", "class"]. The corresponding label value is "0" for the first image and “1” for the second image:

{"source-ref":"s3://image/filename1.jpg", "class":"0"} {"source-ref":"s3://image/filename2.jpg", "class":"1", "class-metadata": {"class-name": "cat", "type" : "groundtruth/image-classification"}}

The order of "AttributeNames" in the input files matters when training the ImageClassification algorithm. It accepts piped data in a specific order, with image first, followed by label. So the "AttributeNames" in this example are provided with "source-ref" first, followed by "class". When using the ImageClassification algorithm with Augmented Manifest, the value of the RecordWrapperType parameter must be "RecordIO".

Multi-label training is also supported by specifying a JSON array of values. The num_classes hyperparameter must be set to match the total number of classes. There are two valid label formats: multi-hot and class-id.

In the multi-hot format, each label is a multi-hot encoded vector of all classes, where each class takes the value of 0 or 1. In the following example, there are three classes. The first image is labeled with classes 0 and 2, while the second image is labeled with class 2 only:

{"image-ref": "s3://mybucket/sample01/image1.jpg", "class": "[1, 0, 1]"} {"image-ref": "s3://mybucket/sample02/image2.jpg", "class": "[0, 0, 1]"}

In the class-id format, each label is a list of the class ids, from [0, num_classes), which apply to the data point. The previous example would instead look like this:

{"image-ref": "s3://mybucket/sample01/image1.jpg", "class": "[0, 2]"} {"image-ref": "s3://mybucket/sample02/image2.jpg", "class": "[2]"}

The multi-hot format is the default, but can be explicitly set in the content type with the label-format parameter: "application/x-recordio; label-format=multi-hot". The class-id format, which is the format outputted by GroundTruth, must be set explicitly: "application/x-recordio; label-format=class-id".

For more information on augmented manifest files, see Provide Dataset Metadata to Training Jobs with an Augmented Manifest File.

Incremental Training

You can also seed the training of a new model with the artifacts from a model that you trained previously with SageMaker. Incremental training saves training time when you want to train a new model with the same or similar data. SageMaker image classification models can be seeded only with another built-in image classification model trained in SageMaker.

To use a pretrained model, in the CreateTrainingJob request, specify the ChannelName as "model" in the InputDataConfig parameter. Set the ContentType for the model channel to application/x-sagemaker-model. The input hyperparameters of both the new model and the pretrained model that you upload to the model channel must have the same settings for the num_layers, image_shape and num_classes input parameters. These parameters define the network architecture. For the pretrained model file, use the compressed model artifacts (in .tar.gz format) output by SageMaker. You can use either RecordIO or image formats for input data.

Inference with the Image Classification Algorithm

The generated models can be hosted for inference and support encoded .jpg and .png image formats as image/png, image/jpeg, and application/x-image content-type. The input image is resized automatically. The output is the probability values for all classes encoded in JSON format, or in JSON Lines text format for batch transform. The image classification model processes a single image per request and so outputs only one line in the JSON or JSON Lines format. The following is an example of a response in JSON Lines format:

accept: application/jsonlines {"prediction": [prob_0, prob_1, prob_2, prob_3, ...]}

For more details on training and inference, see the image classification sample notebook instances referenced in the introduction.

EC2 Instance Recommendation for the Image Classification Algorithm

For image classification, we support P2, P3, G4dn, and G5 instances. We recommend using GPU instances with more memory for training with large batch sizes. You can also run the algorithm on multi-GPU and multi-machine settings for distributed training. Both CPU (such as C4) and GPU (P2, P3, G4dn, or G5) instances can be used for inference.

Image Classification Sample Notebooks

For a sample notebook that uses the SageMaker image classification algorithm, see Build and Register an MXNet Image Classification Model via SageMaker Pipelines. For instructions how to create and access Jupyter notebook instances that you can use to run the example in SageMaker, see Amazon SageMaker Notebook Instances. Once you have created a notebook instance and opened it, select the SageMaker Examples tab to see a list of all the SageMaker samples. The example image classification notebooks are located in the Introduction to Amazon algorithms section. To open a notebook, click on its Use tab and select Create copy.