Modify and combine two different frozen graphs generated using tensorflow object detection API for inference

I am working with TensorFlow object detection API, I have trained two different(SSD-mobilenet and FRCNN-inception-v2) models for my use case. Currently, my workflow is like this:

1. Take an input image, detect one particular object using SSD
   mobilenet.
2. Crop the input image with the bounding box generated from
   step 1 and then resize it to a fixed size(e.g. 200 X 300).
3. Feed this cropped and resized image to FRCNN-inception-V2 for detecting
   smaller objects inside the ROI.

Currently at the time of inferencing, when I load two separate frozen graphs and follow the steps, I am getting my desired results. But I need only a single frozen graph because of my deployment requirement. I am new to TensorFlow and wanted to combine both graphs with crop and resizing process in between them.

Thanks, @matt and @Vedanshu for responding, Here is the updated code that works fine for my requirement, Please give suggestions, if it needs any improvement as I am still learning it.

# Dependencies
import tensorflow as tf
import numpy as np
# load graphs using pb file path
def load_graph(pb_file):
graph = tf.Graph()
with graph.as_default():
od_graph_def = tf.GraphDef()
with tf.gfile.GFile(pb_file, 'rb') as fid:
serialized_graph = fid.read()
od_graph_def.ParseFromString(serialized_graph)
tf.import_graph_def(od_graph_def, name='') 
return graph
# returns tensor dictionaries from graph
def get_inference(graph, count=0):
with graph.as_default():
ops = tf.get_default_graph().get_operations()
all_tensor_names = {output.name for op in ops for output in op.outputs}
tensor_dict = {}
for key in ['num_detections', 'detection_boxes', 'detection_scores',
'detection_classes', 'detection_masks', 'image_tensor']:
tensor_name = key + ':0' if count == 0 else '_{}:0'.format(count)
if tensor_name in all_tensor_names:
tensor_dict[key] = tf.get_default_graph().\
get_tensor_by_name(tensor_name)
return tensor_dict
# renames while_context because there is one while function for every graph
# open issue at https://github.com/tensorflow/tensorflow/issues/22162  
def rename_frame_name(graphdef, suffix):
for n in graphdef.node:
if "while" in n.name:
if "frame_name" in n.attr:
n.attr["frame_name"].s = str(n.attr["frame_name"]).replace("while_context",
"while_context" + suffix).encode('utf-8')
if __name__ == '__main__':
# your pb file paths
frozenGraphPath1 = '...replace_with_your_path/some_frozen_graph.pb'
frozenGraphPath2 = '...replace_with_your_path/some_frozen_graph.pb'
# new file name to save combined model
combinedFrozenGraph = 'combined_frozen_inference_graph.pb'
# loads both graphs
graph1 = load_graph(frozenGraphPath1)
graph2 = load_graph(frozenGraphPath2)
# get tensor names from first graph
tensor_dict1 = get_inference(graph1)
with graph1.as_default():
# getting tensors to add crop and resize step
image_tensor = tensor_dict1['image_tensor']
scores = tensor_dict1['detection_scores'][0]
num_detections = tf.cast(tensor_dict1['num_detections'][0], tf.int32)
detection_boxes = tensor_dict1['detection_boxes'][0]
# I had to add NMS becuase my ssd model outputs 100 detections and hence it runs out of memory becuase of huge tensor shape
selected_indices = tf.image.non_max_suppression(detection_boxes, scores, 5, iou_threshold=0.5)
selected_boxes = tf.gather(detection_boxes, selected_indices)
# intermediate crop and resize step, which will be input for second model(FRCNN)
cropped_img = tf.image.crop_and_resize(image_tensor,
selected_boxes,
tf.zeros(tf.shape(selected_indices), dtype=tf.int32),
[300, 60] # resize to 300 X 60
)
cropped_img = tf.cast(cropped_img, tf.uint8, name='cropped_img')
gdef1 = graph1.as_graph_def()
gdef2 = graph2.as_graph_def()
g1name = "graph1"
g2name = "graph2"
# renaming while_context in both graphs
rename_frame_name(gdef1, g1name)
rename_frame_name(gdef2, g2name)
# This combines both models and save it as one
with tf.Graph().as_default() as g_combined:
x, y = tf.import_graph_def(gdef1, return_elements=['image_tensor:0', 'cropped_img:0'])
z, = tf.import_graph_def(gdef2, input_map={"image_tensor:0": y}, return_elements=['detection_boxes:0'])
tf.train.write_graph(g_combined, "./", combinedFrozenGraph, as_text=False)

You can load output of one graph into another using input_map in import_graph_def. Also you have to rename the while_context because there is one while function for every graph. Something like this:

def get_frozen_graph(graph_file):
"""Read Frozen Graph file from disk."""
with tf.gfile.GFile(graph_file, "rb") as f:
graph_def = tf.GraphDef()
graph_def.ParseFromString(f.read())
return graph_def
def rename_frame_name(graphdef, suffix):
# Bug reported at https://github.com/tensorflow/tensorflow/issues/22162#issuecomment-428091121
for n in graphdef.node:
if "while" in n.name:
if "frame_name" in n.attr:
n.attr["frame_name"].s = str(n.attr["frame_name"]).replace("while_context",
"while_context" + suffix).encode('utf-8')
...
l1_graph = tf.Graph()
with l1_graph.as_default():
trt_graph1 = get_frozen_graph(pb_fname1)
[tf_input1, tf_scores1, tf_boxes1, tf_classes1, tf_num_detections1] = tf.import_graph_def(trt_graph1, 
return_elements=['image_tensor:0', 'detection_scores:0', 'detection_boxes:0', 'detection_classes:0','num_detections:0'])
input1 = tf.identity(tf_input1, name="l1_input")
boxes1 = tf.identity(tf_boxes1[0], name="l1_boxes")  # index by 0 to remove batch dimension
scores1 = tf.identity(tf_scores1[0], name="l1_scores")
classes1 = tf.identity(tf_classes1[0], name="l1_classes")
num_detections1 = tf.identity(tf.dtypes.cast(tf_num_detections1[0], tf.int32), name="l1_num_detections")
...
# Make your output tensor 
tf_out = # your output tensor (here, crop the input image with the bounding box generated from step 1 and then resize it to a fixed size(e.g. 200 X 300).)
...
connected_graph = tf.Graph()
with connected_graph.as_default():
l1_graph_def = l1_graph.as_graph_def()
g1name = 'ved'
rename_frame_name(l1_graph_def, g1name)
tf.import_graph_def(l1_graph_def, name=g1name)
...
trt_graph2 = get_frozen_graph(pb_fname2)
g2name = 'level2'
rename_frame_name(trt_graph2, g2name)
[tf_scores, tf_boxes, tf_classes, tf_num_detections] = tf.import_graph_def(trt_graph2,
input_map={'image_tensor': tf_out},
return_elements=['detection_scores:0', 'detection_boxes:0', 'detection_classes:0','num_detections:0'])
#######
# Export the graph
with connected_graph.as_default():
print('\nSaving...')
cwd = os.getcwd()
path = os.path.join(cwd, 'saved_model')
shutil.rmtree(path, ignore_errors=True)
inputs_dict = {
"image_tensor": tf_input
}
outputs_dict = {
"detection_boxes_l1": tf_boxes_l1,
"detection_scores_l1": tf_scores_l1,
"detection_classes_l1": tf_classes_l1,
"max_num_detection": tf_max_num_detection,
"detection_boxes_l2": tf_boxes_l2,
"detection_scores_l2": tf_scores_l2,
"detection_classes_l2": tf_classes_l2
}
tf.saved_model.simple_save(
tf_sess_main, path, inputs_dict, outputs_dict
)
print('Ok')