Optimize Preprocessing


When your input data don’t perfectly fit to Neural Network model input tensor - this means that additional operations/steps are needed to transform your data to format expected by model. These operations are known as “preprocessing”.


Consider the following standard example: deep learning model expects input with shape {1, 3, 224, 224}, FP32 precision, RGB color channels order, and requires data normalization (subtract mean and divide by scale factor). But you have just a 640x480 BGR image (data is {480, 640, 3}). This means that we need some operations which will:

  • Convert U8 buffer to FP32

  • Transform to planar format: from {1, 480, 640, 3} to {1, 3, 480, 640}

  • Resize image from 640x480 to 224x224

  • Make BGR->RGB conversion as model expects RGB

  • For each pixel, subtract mean values and divide by scale factor


Even though all these steps can be relatively easy implemented manually in application’s code before actual inference, it is possible to do it with Preprocessing API. Reasons to use this API are:

  • Preprocessing API is easy to use

  • Preprocessing steps will be integrated into execution graph and will be performed on selected device (CPU/GPU/VPU/etc.) rather than always being executed on CPU. This will improve selected device utilization which is always good.

Preprocessing API

Intuitively, Preprocessing API consists of the following parts:

  1. Tensor: Declare user’s data format, like shape, layout, precision, color format of actual user’s data

  2. Steps: Describe sequence of preprocessing steps which need to be applied to user’s data

  3. Model: Specify Model’s data format. Usually, precision and shape are already known for model, only additional information, like layout can be specified

Note: All model’s graph modification shall be performed after model is read from disk and before it is being loaded on actual device.

PrePostProcessor object

ov::preprocess::PrePostProcessor class allows specifying preprocessing and postprocessing steps for model read from disk.

ov::Core core;
std::shared_ptr<ov::Model> model = core.read_model(model_path);
ov::preprocess::PrePostProcessor ppp(model);
from openvino.preprocess import PrePostProcessor
from openvino.runtime import Core

core = Core()
model = core.read_model(model=xml_path)
ppp = PrePostProcessor(model)

Declare user’s data format

To address particular input of model/preprocessor, use ov::preprocess::PrePostProcessor::input(input_name) method

ov::preprocess::InputInfo& input = ppp.input(input_name);
  .set_shape({1, 480, 640, 3})
from openvino.preprocess import ColorFormat
from openvino.runtime import Layout, Type
ppp.input(input_name).tensor() \
        .set_element_type(Type.u8) \
        .set_shape([1, 480, 640, 3]) \
        .set_layout(Layout('NHWC')) \

Here we’ve specified all information about user’s input:

  • Precision is U8 (unsigned 8-bit integer)

  • Data represents tensor with {1,480,640,3} shape

  • Layout is “NHWC”. It means that ‘height=480, width=640, channels=3’

  • Color format is BGR

Declare model’s layout

Model’s input already has information about precision and shape. Preprocessing API is not intended to modify this. The only thing that may be specified is input’s data layout

// `model's input` already `knows` it's shape and data type, no need to specify them here
# `model's input` already `knows` it's shape and data type, no need to specify them here

Now, if model’s input has {1,3,224,224} shape, preprocessing will be able to identify that model’s height=224, width=224, channels=3. Height/width information is necessary for ‘resize’, and channels is needed for mean/scale normalization

Preprocessing steps

Now we can define sequence of preprocessing steps:

  .mean({100.5, 101, 101.5})
  .scale({50., 51., 52.});
  // Not needed, such conversion will be added implicitly
  // .convert_layout("NCHW");
from openvino.preprocess import ResizeAlgorithm
ppp.input(input_name).preprocess() \
    .convert_element_type(Type.f32) \
    .convert_color(ColorFormat.RGB) \
    .resize(ResizeAlgorithm.RESIZE_LINEAR) \
    .mean([100.5, 101, 101.5]) \
    .scale([50., 51., 52.])
# .convert_layout(Layout('NCHW')); # Not needed, such conversion will be added implicitly


  • Convert U8 to FP32 precision

  • Convert current color format (BGR) to RGB

  • Resize to model’s height/width. Note that if model accepts dynamic size, e.g. {?, 3, ?, ?}, resize will not know how to resize the picture, so in this case you should specify target height/width on this step. See also ov::preprocess::PreProcessSteps::resize()

  • Subtract mean from each channel. On this step, color format is RGB already, so 100.5 will be subtracted from each Red component, and 101.5 will be subtracted from Blue one.

  • Divide each pixel data to appropriate scale value. In this example each Red component will be divided by 50, Green by 51, Blue by 52 respectively

  • Note: last convert_layout step is commented out as it is not necessary to specify last layout conversion. PrePostProcessor will do such conversion automatically

Integrate steps into model

We’ve finished with preprocessing steps declaration, now it is time to build it. For debugging purposes it is possible to print PrePostProcessor configuration on screen:

std::cout << "Dump preprocessor: " << ppp << std::endl;
model = ppp.build();
print(f'Dump preprocessor: {ppp}')
model = ppp.build()

After this, model will accept U8 input with {1, 480, 640, 3} shape, with BGR channels order. All conversion steps will be integrated into execution graph. Now you can load model on device and pass your image to model as is, without any data manipulation on application’s side