Broadcast

Versioned name: Broadcast-1

Category: Data movement

Short description: Broadcast replicates data on the first input to fit a given shape on the second input.

Detailed description:

Broadcast takes the first tensor data and, following broadcasting rules that are specified by mode attribute and the 3rd input axes_mapping, builds a new tensor with shape matching the 2nd input tensor target_shape. target_shape input is a 1D integer tensor that represents required shape of the output.

Attribute mode and the 3rd input axes_mapping are relevant for cases when rank of the input data tensor doesn’t match the size of the target_shape input. They both define how axes from data shape are mapped to the output axes. If mode is set to numpy, it means that the standard one-directional numpy broadcasting rules are applied. These rules are described in Broadcast Rules For Elementwise Operations, when only one-directional broadcasting is applied: input tensor data is broadcasted to target_shape but not vice-versa.

In case if mode is set to explicit, then 3rd input axes_mapping comes to play. It contains a list of axis indices, each index maps an axis from the 1st input tensor data to axis in the output. The size of axis_mapping should match the rank of input data tensor, so all axes from data tensor should be mapped to axes of the output.

For example, axes_mapping = [1] enables broadcasting of a tensor with shape [C] to shape [N,C,H,W] by replication of initial tensor along dimensions 0, 2 and 3. Another example is broadcasting of tensor with shape [H,W] to shape [N,H,W,C] with axes_mapping = [1, 2]. Both examples requires mode set to explicit and providing mentioned axes_mapping input, because such operations cannot be expressed with axes_mapping set to numpy.

Attributes:

• mode

  • Description: specifies rules used for mapping of input tensor axes to output shape axes.

  • Range of values:

    • numpy - numpy broadcasting rules, aligned with ONNX Broadcasting. Description is available in ONNX docs.; only one-directional broadcasting is applied from data to target_shape. If this attribute value is used, then the 3rd input for the operation shouldn’t be provided.

    • explicit - mapping of the input data shape axes to output shape is provided as an explicit 3rd input.

  • Type: string

  • Default value: “numpy”

  • Required: no

Inputs:

• 1: data - source tensor of any type and shape that is being broadcasted. Required.

• 2: taget_shape - 1D integer tensor describing output shape. Required.

• 3: axes_mapping - 1D integer tensor describing a list of axis indices, each index maps an axis from the 1st input tensor data to axis in the output. The index values in this tensor should be sorted, that disables on-the-fly transpositions of input data tensor while the broadcasting. axes_mapping input is optional depending on mode value.

Outputs:

• 1: Output tensor with replicated content from the 1st tensor data and with shape matched target_shape.

Example

<layer ... type="Broadcast" ...>
    <data mode="numpy"/>
    <input>
        <port id="0">
            <dim>16</dim>
            <dim>1</dim>
            <dim>1</dim>
       </port>
        <port id="1">
            <dim>4</dim>   <!--The tensor contains 4 elements: [1, 16, 50, 50] -->
        </port>
        <!-- the 3rd input shouldn't be provided with mode="numpy" -->
    </input>
    <output>
        <port id="2">
            <dim>1</dim>
            <dim>16</dim>
            <dim>50</dim>
            <dim>50</dim>
        </port>
    </output>
</layer>

<layer ... type="Broadcast" ...>
    <data mode="explicit"/>
    <input>
        <port id="0">
            <dim>16</dim>
       </port>
        <port id="1">
            <dim>4</dim>   <!--The tensor contains 4 elements: [1, 16, 50, 50] -->
        </port>
        <port id="1">
            <dim>1</dim>   <!--The tensor contains 1 elements: [1] -->
        </port>
    </input>
    <output>
        <port id="2">
            <dim>1</dim>
            <dim>16</dim>
            <dim>50</dim>
            <dim>50</dim>
        </port>
    </output>
</layer>

<layer ... type="Broadcast" ...>
    <data mode="explicit"/>
    <input>
        <port id="0">
            <dim>50</dim>
            <dim>50</dim>
       </port>
        <port id="1">
            <dim>4</dim>   <!--The tensor contains 4 elements: [1, 50, 50, 16] -->
        </port>
        <port id="1">
            <dim>2</dim>   <!--The tensor contains 2 elements: [1, 2] -->
        </port>
    </input>
    <output>
        <port id="2">
            <dim>1</dim>
            <dim>50</dim>
            <dim>50</dim>
            <dim>16</dim>
        </port>
    </output>
</layer>