Convert Image Files to OFRecord Datasets

In OFRecord Data Format and Loading and Preparing OFRecord Dataset, we learned how to convert other dataset formats to OFRecord separately and how to load OFRecord datasets.

In this article, we will explain how to make image files into OFRecord datasets. Also we provide relevant script for users to use directly or make modification base on that, which includes:

• Make OFRecord datasets based on MNIST dataset.
• How OFRecord Reader is encoded.
• Training on OFRecord dataset.

Make OFRecord Datasets Based on Image Files

We use MNIST Handwritten Digits dataset to produce an OFRecord format file.

we only take 50 pictures for demonstration. Please refer to img2ofrecord for relevant script and dataset. img2ofrecord.

• Download and unzip the relevant zip file

$ wget https://oneflow-static.oss-cn-beijing.aliyuncs.com/oneflow-tutorial-attachments/img2ofrecord.zip
$ unzip img2ofrecord.zip

• Change directory to corresponding path and run OFRecord production script img2ofrecord.py

$ cd ./img_to_ofrecord
$ python img2ofrecord.py --part_num=5 --save_dir=./dataset/ --img_format=.png --image_root=./images/train_set/

• The following output will display as the script runs.

The image root is:  ./images/train_set/
The amount of OFRecord data part is:  5
The directory of Labels is:  ./images/train_label/label.txt
The image format is:  .png
The OFRecord save directory is:  ./dataset/
Start Processing......
./images/train_set/00000030_3.png feature saved
./images/train_set/00000034_0.png feature saved
./images/train_set/00000026_4.png feature saved
./images/train_set/00000043_9.png feature saved
......
Process image successfully !!!

Thus far, we have created the OFRecord file and saved it under ./dataset.

Code Explanation

The hierarchy of code directory is:

img_to_ofrecord
├── images
    ├── train_set
        ├── 00000000_5.png
        ├── 00000001_0.png
        ├── 00000002_4.png
        ......
    ├── train_label
        ├── label.txt
├── img2ofrecord.py
├── lenet_train.py

• images directory holds the original training dataset and label file.

The label file is stored as json here in following format：

{"00000030_3.png": 3}
{"00000034_0.png": 0}
{"00000026_4.png": 4}
{"00000043_9.png": 9}
{"00000047_5.png": 5}
{"00000003_1.png": 1}
......

• img2ofrecord.py is the script which converts image files in train_set to OFRecord dataset.
• lenet_train.py is the script loading OFRecord we just made for training.

The command options of img2ofrecord.py are: - image_root specify the root directory of the image. - part_num specify the number of OFRecord files to generate. An error is reported if the number is greater than the total number of images. - label_dir specify the directory of the label. - img_format specify the format of the image. - save_dir specify the directory where the OFRecord file will be saved.

How OFRecord Reader is Encoded

The code associated with the encoding of OFRecord files is in img2ofrecord.py. The encoding process is as follows：

First, encoding the incoming image data.

def encode_img_file(filename, ext=".jpg"):
    img = cv2.imread(filename)
    encoded_data = cv2.imencode(ext, img)[1]
    return encoded_data.tostring()

The ext is the image encoding format. Currently, The format supported by ONEFLOW image encoding and decoding is consistent with that of OpenCV, which can be refered in cv::ImwriteFlags for details.

• JPEG, one of the most common lossy code formats. Please refer to JPEG.
• PNG, a common lossless bitmap encoding format. Please refer to Portable Network Graphics.
• TIFF, a extensible compressed encoding format. Please refer to Tagged Image File Format.

Second, data is converted to the form of Feature, serialized, and the data length is written to the file.

def ndarray2ofrecords(dsfile, dataname, encoded_data, labelname, encoded_label):
    topack = {dataname: bytes_feature(encoded_data),
              labelname: int32_feature(encoded_label)}
    ofrecord_features = ofrecord.OFRecord(feature=topack)
    serilizedBytes = ofrecord_features.SerializeToString()
    length = ofrecord_features.ByteSize()
    dsfile.write(struct.pack("q", length))
    dsfile.write(serilizedBytes)

Training on OFRecord Dataset

We run lenet_train.py. It will read the OFRecord dataset that we have just created and train it on the LeNet model.

The outputs of training script should like below:

[6.778578]
[2.0212684]
[1.3814741]
[0.47514156]
[0.13277876]
[0.16388433]
[0.03788032]
[0.01225162]
......

At this point, we have successfully completed the whole process of dataset production, reading and training.

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