How to load a Keras model build with tensorflow backend in OpenCV

edit

Disclaimer, I posted the same question here and on Stackoverflow.

I'm trying to do deployment from Keras to opencv c++.

I trained a simple CNN with the mnist dataset (my example is a modified Keras example). After training I exposed tensorflow graph from Keras backend and saved the model and the graph.

tensorFlowSession = K.get_session()
tf.saved_model.simple_save(tensorFlowSession, newpath + "/TensorFlow", inputs={"x": x}, outputs={"y": y})
tf.train.write_graph(tensorFlowSession.graph_def,newpath + "/TensorFlow",  "trainGraph_def.pbtxt")

Then I tried to load the saved model using opencv in python, I started with opencv in python, however I experience a similar error using opencv in c++.

net = cv.dnn.readNet(newpath + '/TensorFlow/' + 'saved_model.pb', newpath + '/TensorFlow/' + 'trainGraph.pbtxt')

The problem is the opencv failed to load tensorflow graph, I get an error-

[libprotobuf ERROR /io/opencv/3rdparty/protobuf/src/google/protobuf/wire_format_lite.cc:629] String field 'tensorflow.FunctionDef.Node.ret' contains invalid UTF-8 data when parsing a protocol buffer. Use the 'bytes' type if you intend to send raw bytes.

Saving and loading a tensorflow graph using opencv should be rather straightforward, what am I missing here? See attached my code. Any help would be appreciated

'''Trains a simple convnet on the MNIST dataset.
Gets to 99.25% test accuracy after 12 epochs
(there is still a lot of margin for parameter tuning).
16 seconds per epoch on a GRID K520 GPU.
'''

from __future__ import print_function
import keras
from keras.datasets import mnist
from keras.models import Sequential
from keras.layers import Dense, Dropout, Flatten
from keras.layers import Conv2D, MaxPooling2D
from keras import backend as K
import tensorflow as tf
import datetime
import cv2 as cv
from pathlib import Path
import numpy as np
from os import listdir
from os.path import isfile, join
import os
os.environ['CUDA_VISIBLE_DEVICES'] = '-1'

batch_size = 128
num_classes = 10
epochs = 1

# input image dimensions
img_rows, img_cols = 28, 28

# the data, split between train and test sets
(x_train, y_train), (x_test, y_test) = mnist.load_data()

if K.image_data_format() == 'channels_first':
    x_train = x_train.reshape(x_train.shape[0], 1, img_rows, img_cols)
    x_test = x_test.reshape(x_test.shape[0], 1, img_rows, img_cols)
    input_shape = (1, img_rows, img_cols)
else:
    x_train = x_train.reshape(x_train.shape[0], img_rows, img_cols, 1)
    x_test = x_test.reshape(x_test.shape[0], img_rows, img_cols, 1)
    input_shape = (img_rows, img_cols, 1)

x_train = x_train.astype('float32')
x_test = x_test.astype('float32')
x_train /= 255
x_test /= 255
print('x_train shape:', x_train.shape)
print(x_train.shape[0], 'train samples')
print(x_test.shape[0], 'test samples')

# convert class vectors to binary class matrices
y_train = keras.utils.to_categorical(y_train, num_classes)
y_test = keras.utils.to_categorical(y_test, num_classes)

model = Sequential()
model.add(Conv2D(32, kernel_size=(3, 3),
                 activation='relu',
                 input_shape=input_shape))
model.add(Conv2D(64, (3, 3), activation='relu'))
model.add(MaxPooling2D(pool_size=(2, 2)))
model.add(Dropout(0.25))
model.add(Flatten())
model.add(Dense(128, activation='relu'))
model.add(Dropout(0.5))
model.add(Dense(num_classes, activation='softmax'))

model.compile(loss=keras.losses.categorical_crossentropy,
              optimizer=keras.optimizers.Adadelta(),
              metrics=['accuracy'])

model.fit(x_train, y_train,
          batch_size=batch_size,
          epochs=epochs,
          verbose=1,
          validation_data=(x_test ...