Storing multiple values in a tfrecord feature

I am trying to create the tfrecords using the above table. I need to get together the rle (Run Length Encoding) feature in the form of rle per class. Eg. the feature in the final tfrecord looks like

img_id: b'0002cc93b.jpg'
rle: [b'1 0'  b'29102 12 29346 24...'  b'1 0']

img_id: b'000a4bcdd.jpg'
rle: [b'131973 1 132228 4...'  b'1 0'  b'229501 11 229741 33...']

The rle feature should contain the rles for all the 3 masks for the corresponding image as strings and empty rle should be encoded as '1 0'

I tried using a list. But it gives the following error

TypeError: ['29102 12 29346 24 29602 24 29858 24 30114 24 30370 24 30626 24 30882 24 31139 23 31395 23 31651 23 has type list, but expected one of: bytes

I found an overall solution that suits my case.

The specific solution of storing multiple values in a feature.

Used pandas to replace empty rles in the df as'1 0'

A function to grab the rles corresponding to the image from the df.

def rle_class_1(image_id):
    temp_df = df['class_1_rle'][df['image_id'] == image_id]
    for rle in temp_df:
        rle_tensor = tf.constant(rle)
        return rle_tensor.numpy() 

Similar functions for class_2 and class_3.

Create tfrecord

paths_dict = dict(zip(file_ids, file_paths))

def _bytestring_feature(list_of_bytestrings):
    return tf.train.Feature(bytes_list=tf.train.BytesList(value=list_of_bytestrings))

def _int_feature(list_of_ints):
    return tf.train.Feature(int64_list=tf.train.Int64List(value=list_of_ints))

def image_bits_from_id(image_id):
    image = Image.open(paths_dict[image_id])
    image = tf.constant(image)
    image_bits = tf.image.encode_jpeg(image, optimize_size=True, chroma_downsampling=False)
    image_bits = image_bits.numpy()
    return image_bits

def create_tfrec_example(image_id):

    image = image_bits_from_id(image_id) 
    rle_1 = rle_class_1(image_id)
    rle_2 = rle_class_2(image_id)
    rle_3 = rle_class_3(image_id)
    
    feature = {
        'image': _bytestring_feature([image]),
        'img_id': _bytestring_feature([image_id.encode()]),
        'rle': _bytestring_feature([rle_1, rle_2, rle_3])
        }

    tfrec_example = tf.train.Example(features=tf.train.Features(feature=feature))
    return tfrec_example

Parse and view record

def parse_tfrecord_fn(example):
    features = {
        'image': tf.io.FixedLenFeature([], tf.string), 
        'img_id': tf.io.FixedLenFeature([], tf.string)
        }
    features['rle'] = tf.io.FixedLenFeature([3], tf.string)

    example = tf.io.parse_single_example(example, features)
    example["image"] = tf.io.decode_jpeg(example["image"], channels=3)
    return example

raw_dataset = tf.data.TFRecordDataset(TFREC[0])   # TFREC is a shard
parsed_dataset = raw_dataset.map(parse_tfrecord_fn)

for features in parsed_dataset.take(5):
    for key in features.keys():
        if key != "image":
            print(f"{key}: {features[key]}")

    print(f"Image shape: {features['image'].shape}")
    plt.figure(figsize=(7, 7))
    plt.imshow(features["image"].numpy())
    plt.show()

Output

img_id: b'0002cc93b.jpg'
rle: [b'29102 12 29346 24 29602 24 29858 24... '
 b'1 0' b'1 0']
Image plot