Changing input shapes¶

Introduction (C++)¶

C++

OpenVINO™ provides capabilities to change model input shape during the runtime. It may be useful in case you would like to feed model an input that has different size than model input shape. In case you need to do this only once prepare a model with updated shapes via Model Optimizer for all the other cases follow instructions further.

Set a new input shape with reshape method¶

The ov::Model::reshape method updates input shapes and propagates them down to the outputs of the model through all intermediate layers. Example: Changing the batch size and spatial dimensions of input of a model with an image input:

Please see the code to achieve that:

model->reshape({8, 3, 448, 448});

Set a new batch size with set_batch method¶

Meaning of the model batch may vary depending on the model design. In order to change the batch dimension of the model, set the ov::Layout and call the ov::set_batch method.

// Mark up batch in the layout of the input(s) and reset batch to the new value
model->get_parameters()[0]->set_layout("N...");
ov::set_batch(model, new_batch);

ov::set_batch method is a high level API of ov::Model::reshape functionality, so all information about ov::Model::reshape method implications are applicable for ov::set_batch too, including the troubleshooting section.

Once the input shape of ov::Model is set, call the ov::Core::compile_model method to get an ov::CompiledModel object for inference with updated shapes.

There are other approaches to change model input shapes during the stage of IR generation or ov::Model creation.

Dynamic Shape Notice¶

Shape-changing functionality could be used to turn dynamic model input into a static one and vice versa. It is recommended to always set static shapes in case if the shape of data is not going to change from one inference to another. Setting static shapes avoids possible functional limitations, memory and run time overheads for dynamic shapes that vary depending on hardware plugin and model used. To learn more about dynamic shapes in OpenVINO please see a dedicated article.

Usage of Reshape Method¶

The primary method of the feature is ov::Model::reshape. It is overloaded to better serve two main use cases:

To change input shape of model with single input you may pass new shape into the method. Please see the example of adjusting spatial dimensions to the input image:

// Read an image and adjust models single input for image to fit
cv::Mat image = cv::imread("path/to/image");
model->reshape({1, 3, image.rows, image.cols});

To do the opposite - resize input image to the input shapes of the model, use the pre-processing API.

Otherwise, you can express reshape plan via mapping of input and its new shape:

map<ov::Output<ov::Node>, ov::PartialShape specifies input by passing actual input port
map<size_t, ov::PartialShape> specifies input by its index
map<string, ov::PartialShape> specifies input by its name

Port

Index

Tensor Name

Please find usage scenarios of reshape feature in our samples starting with Hello Reshape Sample.

Practically, some models are not ready to be reshaped. In this case, a new input shape cannot be set with the Model Optimizer or the ov::Model::reshape method.

Troubleshooting Reshape Errors¶

Operation semantics may impose restrictions on input shapes of the operation. Shape collision during shape propagation may be a sign that a new shape does not satisfy the restrictions. Changing the model input shape may result in intermediate operations shape collision.

Examples of such operations:

Reshape operation with a hard-coded output shape value
MatMul operation with the Const second input cannot be resized by spatial dimensions due to operation semantics

Model structure and logic should not change significantly after model reshaping.

The Global Pooling operation is commonly used to reduce output feature map of classification models output. Having the input of the shape [N, C, H, W], Global Pooling returns the output of the shape [N, C, 1, 1]. Model architects usually express Global Pooling with the help of the Pooling operation with the fixed kernel size [H, W]. During spatial reshape, having the input of the shape [N, C, H1, W1], Pooling with the fixed kernel size [H, W] returns the output of the shape [N, C, H2, W2], where H2 and W2 are commonly not equal to 1. It breaks the classification model structure. For example, publicly available Inception family models from TensorFlow* have this issue.
Changing the model input shape may significantly affect its accuracy. For example, Object Detection models from TensorFlow have resizing restrictions by design. To keep the model valid after the reshape, choose a new input shape that satisfies conditions listed in the pipeline.config file. For details, refer to the Tensorflow Object Detection API models resizing techniques.

How To Fix Non-Reshape-able Model¶

Some operators which prevent normal shape propagation can be fixed. To do so you can:

see if the issue can be fixed via changing the values of some operators input. E.g. most common problem of non-reshape-able models is a Reshape operator with hardcoded output shape. You can cut-off hard-coded 2nd input of Reshape and fill it in with relaxed values. For the following example on the picture Model Optimizer CLI should be:
```
mo --input_model path/to/model --input data[8,3,224,224],1:reshaped[2]->[0 -1]`
```

With 1:reshaped[2] we request to cut 2nd input (counting from zero, so 1: means 2nd inputs) of operation named reshaped and replace it with a Parameter with shape [2]. With ->[0 -1] we replace this new Parameter by a Constant operator which has value [0, -1]. Since Reshape operator has 0 and -1 as a specific values (see the meaning in the specification) it allows to propagate shapes freely without losing the intended meaning of Reshape.

transform model during Model Optimizer conversion on the back phase. See Model Optimizer extension article
transform OpenVINO Model during the runtime. See OpenVINO Runtime Transformations article
modify the original model with the help of original framework

Extensibility¶

OpenVINO provides a special mechanism that allows adding support of shape inference for custom operations. This mechanism is described in the Extensibility documentation

Introduction (Python)¶

Python

Set a new input shape with reshape method¶

The Model.reshape method updates input shapes and propagates them down to the outputs of the model through all intermediate layers. Example: Changing the batch size and spatial dimensions of input of a model with an image input:

Please see the code to achieve that:

Set a new batch size with set_batch method¶

Meaning of the model batch may vary depending on the model design. In order to change the batch dimension of the model, set the layout for inputs and call the set_batch method.

set_batch method is a high level API of Model.reshape functionality, so all information about Model.reshape method implications are applicable for set_batch too, including the troubleshooting section.

Once the input shape of Model is set, call the compile_model method to get a CompiledModel object for inference with updated shapes.

There are other approaches to change model input shapes during the stage of IR generation or Model creation.