Add two operators to support TFLite SSD Mobilenet models

I downloaded TF SSD quantized model ssd_mobilenet_v1_quantized_coco from
Tensorflow Model Zoo

The zip file contains tflite_graph.pb
I used tflite_convert util to convert tflite_graph.pb to model.tflite file

tflite_convert \
--graph_def_file=/home/ubuntu/ssd_mobilenet_v1_quantized_300x300_coco14_sync_2018_07_18/tflite_graph.pb \
--output_file=/home/ubuntu/ssd_mobilenet_v1_quantized_300x300_coco14_sync_2018_07_18/model.tflite \
--output_format=TFLITE \
--input_arrays="normalized_input_image_tensor" \
--input_shapes="1,300,300,3" \
--inference_type=QUANTIZED_UINT8 \
--mean_values=128 \
--std_dev_values=128 \
--output_arrays="TFLite_Detection_PostProcess,TFLite_Detection_PostProcess:1,TFLite_Detection_PostProcess:2,TFLite_Detection_PostProcess:3" \

SSD Mobilenet model.tflite Summary:

### Ops:
# id:   name    -   count
 3: CONV_2D  - 34
14: LOGISTIC - 1
22: RESHAPE - 13
32: CUSTOM - 1

When I tried to compile model.tflite using TVM relay I got the following error

Traceback (most recent call last):
  File "./", line 62, in <module>
    dtype_dict={input_tensor: input_dtype})
  File "/usr/local/lib/python3.5/dist-packages/tvm-0.6.dev0-py3.5-linux-x86_64.egg/tvm/relay/frontend/", line 709, in from_tflite
  File "/usr/local/lib/python3.5/dist-packages/tvm-0.6.dev0-py3.5-linux-x86_64.egg/tvm/relay/frontend/", line 84, in check_unsupported_ops
    raise tvm.error.OpNotImplemented(msg.format(ops))

tvm.error.OpNotImplemented: The following operators are not supported in frontend TFLite: 'LOGISTIC', 'CUSTOM'

Relay does not support 2 operators - 1 built-in and 1 custom
built-in - LOGISTIC (op_code_id = 14)
custom - TFLite_Detection_PostProcess (op_code_id = 32)

Looks like tf.sigmoid op in tflite_graph.pb is mapped to tflite LOGISTIC op.
LOGISTIC is generalized sigmoid.

TFLite_Detection_PostProcess is custom op which produces final 4 outputs - (classes, scores, bboxes and num_outputs)


My first questions are:

  1. Do you think it makes sense to add TFLite built-it LOGISTIC operator to relay? It should be similar to tf.sigmoid

  2. The Same question for custom operator TFLite_Detection_PostProcess.
    This operator is the last operator in TFLite graph in all SSD models from TF model zoo.

BTW, custom operator TFLite_Detection_PostProcess is optional. which converts checkpoint to TFLite compatible pb file has parameter add_postprocessing_op=true/false.
It is possible to create tflite_graph.pb without TFLite_Detection_PostProcess
in that case the model output will be

the graph has two outputs:
   raw_outputs/box_encodings: a float32 tensor of shape [1, num_anchors, 4]
    containing the encoded box predictions.
   raw_outputs/class_predictions: a float32 tensor of shape
    [1, num_anchors, num_classes] containing the class scores for each anchor
    after applying score conversion.


I also tried to compile SSD Resnet 50 model (ssd_resnet_50_fpn_coco)
It complains about the same two operators plus two additional operators: MUL and PAD
SSD Resnet 50 tflite summary:

 0: ADD - 58
 3: CONV_2D - 110
14: LOGISTIC - 1
17: MAX_POOL_2D - 4
18: MUL - 42
22: RESHAPE - 14
32: CUSTOM - 1
34: PAD - 1

Visualisation of TFLite graphs are below:

TFLite model graph for ssd_mobilenet_v1_quantized_coco (quantized, Uint8):

I also tried to compile non-quantized tflite model. The same 2 operators are missing.
TFLite model graph for ssd_mobilenet_v1_coco_2018_01_28 (non-quantized, float):

SSD Resnet 50 FPN COCO model graph

@FrozenGene @tqchen What you think?

For Custom Op, maybe I prefer letting users add_postprocessing_op= false.

@apivovarov Hi, if add_postprocessing_op=false, how can we get bbox x,y,h,w? I follow your step,
but get some strange box data,like [ -1.1977348 0.29819602 -1.3088214 -0.08522664]

We need to implement TFLite_Detection_PostProcess in python numpy and run it after getting model output.
Original implementation is here:

anchors tensors values (Constants) [1917,4] float32 can be extracted from tflite_graph.pb or tflite_graph.pbtxt files

node {
  name: "anchors"
  op: "Const"
  attr {
    key: "dtype"
    value {
      type: DT_FLOAT
  attr {
    key: "value"
    value {
      tensor {
        dtype: DT_FLOAT
        tensor_shape {
          dim {
            size: 1917
          dim {
            size: 4
        tensor_content: "6\224\327<6\224\327<\315\31..

I already extracted anchors for ssd_mobilenet_v1_coco_2018_01_28 and saved values to anchors.npy

Get it. Thanks for your anchor data.

Hi @apivovarov : I am also trying to remove custom detection_postprocess op and add my logic. If you can help clear my doubts, it would be great! TIA.

  1. What is the use of anchors constant? What does it do?
  2. I am trying to implement in cpp, how should i use (which function to call to get location, classes, score, num) ? In the file, there are many functions.

Can you share some sample code how to use this class? I’m a beginner to tflite and any hint would be appreciated. Thanks.

Now that the PR linked has been merged, you maybe able to load the model directly.

Hi @ramana-arm Thank you for replying. I see the link which you attach but i could not find any link to new model. Can you elaborate your answer please?

I meant you could construct the model with post_processing_custom_ops = true ?

I found that quantized mobilenet_v1 TFLite model is different from non-quantized one.

non-quantized uses RELU6 fused_activation_function

but quantized uses NONE - which is actually not true because conv2 output tensors min/max is 0/6 - so, they actually use RELU6

Why quantized model metadata says 'fused_activation_function': 'NONE' ???

non-quantized model:

quantized model:


Could tell me how you generated the anchors from the .pb and the .pbtx files?

I have found some leads on the python version to decode the encoded boxes. It looks like this function can be used to understand the same : models/ at master · tensorflow/models · GitHub