[FIXED] Tensorflow question: serialization of tensor in order to write it to tfrecords

Issue

def _bytes_feature(value):
    #Returns a bytes_list from a string / byte.
    if isinstance(value, type(tf.constant(0))): # if value ist tensor
        value = value.numpy() # get value of tensor
    return tf.train.Feature(bytes_list=tf.train.BytesList(value=[value]))


def _int64_feature(value):
    #Returns an int64_list from a bool / enum / int / uint.
    return tf.train.Feature(int64_list=tf.train.Int64List(value=[value]))


def write_tfrecords(data_list, output_file):
    """
        write data for nearest neighbor evaluation
    """
    total_samples = 0
    with tf.io.TFRecordWriter(output_file) as writer:    
        for image, label in tqdm(data_list):
            data = {
                "image": _bytes_feature(image.numpy().bytes()),
                "label": _int64_feature(label)
            }
            out = tf.train.Example(features=tf.train.Features(feature=data))
            writer.write(out.SerializeToString())
            total_samples += 1
    return total_samples

The image object at this line "image": _bytes_feature(image.numpy().bytes()), is a tensor.

I used numpy bytes here but when I decode the data I found that the shape of the array is missing. I need to specify the tensor array shape. This is how numpy.bytes() and numpy.frombuffer() works: Convert byte array back to numpy array

Is there a better way that the array shape can be serialized as well?

Solution

There are a few options:

1. tf.io.serialize_tensor() and tf.io.parse_tensor() for arbitrary tensors.
2. tf.io.encode_png() (or tf.io.encode_jpeg()) and tf.io.decode_image() for images.

As an example:

# generate a test image.
image = tf.random.uniform([24, 24, 3], 0.0, 1.0)                                    

# Create a tf.train.Example.                                                                                                                                                                                                                   
example = tf.train.Example(features=tf.train.Features(                          
    feature={                                                                   
        'image': _bytes_feature(tf.io.serialize_tensor(image)),         
    }))
                                                             
# Serialize and deserialize example (this mirrors writing and reading from a TFRecord)            
example = tf.train.Example.FromString(example.SerializeToString())              
# Decode the original image.
image = tf.io.parse_tensor(
    example.features.feature['image'].bytes_list.value[0], out_type=tf.float32)

In the first case, a tensor is converted to a serialized TensorProto which maintains shape information. In the second case, shape is maintained in the PNG/JPEG header.

Answered By - vasiliykarasev