If your model has only one input, then simple <code>ov::preprocess::PrePostProcessor::input()</code> will get a reference to preprocessing builder for this input (tensor, steps, model):
Typical data normalization includes 2 operations for each data item: subtract mean value and divide to standard deviation. This can be done with the following code:
In Computer Vision area normalization is usually done separately for R, G, B values. To do this, [layout with 'C' dimension](./layout_overview.md) shall be defined. Example:
Transposing of matrices/tensors is a typical operation in Deep Learning - you may have a BMP image 640x480 which is an array of `{480, 640, 3}` elements, but Deep Learning model can require input with shape `{1, 3, 480, 640}`
Using [layout](./layout_overview.md) of user's tensor and layout of original model conversion can be done implicitly
Resizing of image is a typical preprocessing step for computer vision tasks. With preprocessing API this step can also be integrated into execution graph and performed on target device.
To resize the input image, it is needed to define `H` and `W` dimensions of [layout](./layout_overview.md)
Typical use case is to reverse color channels from RGB to BGR and wise versa. To do this, specify source color format in `tensor` section and perform `convert_color` preprocessing operation. In example below, user has `BGR` image and needs to convert it to `RGB` as required for model's input
Preprocessing also support YUV-family source color formats, i.e. NV12 and I420.
In advanced cases such YUV images can be splitted into separate planes, e.g. for NV12 images Y-component may come from one source and UV-component comes from another source. Concatenating such components in user's application manually is not a perfect solution from performance and device utilization perspectives, so there is a way to use Preprocessing API. For such cases there is `NV12_TWO_PLANES` and `I420_THREE_PLANES` source color formats, which will split original `input` to 2 or 3 inputs
In this example, original `input` is being split to `input/y` and `input/uv` inputs. You can fill `input/y` from one source, and `input/uv` from another source. Color conversion to `RGB` will be performed using these sources, it is more optimal as there will be no additional copies of NV12 buffers.
Preprocessing API also allows adding custom preprocessing steps into execution graph. Custom step is a function which accepts current 'input' node and returns new node after adding preprocessing step
> **Note:** Custom preprocessing function shall only insert node(s) after input, it will be done during model compilation. This function will NOT be called during execution phase. This may look not trivial and require some knowledge of [OpenVINO™ operations](../ops/opset.md)
If there is a need to insert some additional operations to execution graph right after input, like some specific crops and/or resizes - Preprocessing API can be a good choice to implement this
Postprocessing steps can be added to model outputs. As for preprocessing, these steps will be also integrated into graph and executed on selected device.
Comparing to preprocessing, there is not so much operations needed to do in post-processing stage, so right now only following postprocessing operations are supported:
- Convert [layout](./layout_overview.md)
- Convert element type
- Custom operations
Usage of these operations is similar to Preprocessing. Some example is shown below:
