PriorBox

Versioned name: PriorBox-1

Category: Object detection

Short description: PriorBox operation generates prior boxes of specified sizes and aspect ratios across all dimensions.

Detailed description:

PriorBox computes coordinates of prior boxes by following:

1. First calculates center_x and center_y of prior box:

\[\begin{split}W \equiv Width \quad Of \quad Image \\ H \equiv Height \quad Of \quad Image\end{split}\]

• If step equals 0:

\[\begin{split}center_x=(w+0.5) \\ center_y=(h+0.5)\end{split}\]

• else:

\[\begin{split}center_x=(w+offset)*step \\ center_y=(h+offset)*step \\ w \subset \left( 0, W \right ) \\ h \subset \left( 0, H \right )\end{split}\]

2. Then, for each \(s \subset \left( 0, min\_sizes \right )\) calculates coordinates of prior boxes:

\[xmin = \frac{\frac{center_x - s}{2}}{W}\]

\[ymin = \frac{\frac{center_y - s}{2}}{H}\]

\[xmax = \frac{\frac{center_x + s}{2}}{W}\]

\[ymin = \frac{\frac{center_y + s}{2}}{H}\]

3. If clip attribute is set to true, each output value is clipped between \(\left< 0, 1 \right>\).

Attributes:

• min_size (max_size)

  • Description: min_size (max_size) is the minimum (maximum) box size (in pixels).

  • Range of values: positive floating-point numbers

  • Type: float[]

  • Default value: []

  • Required: no

• aspect_ratio

  • Description: aspect_ratio is a variance of aspect ratios. Duplicate values are ignored.

  • Range of values: set of positive integer numbers

  • Type: float[]

  • Default value: []

  • Required: no

• flip

  • Description: flip is a flag that denotes that each aspect_ratio is duplicated and flipped. For example, flip equals 1 and aspect_ratio equals to [4.0,2.0] mean that aspect_ratio is equal to [4.0,2.0,0.25,0.5].

  • Range of values:

    • false or 0 - each aspect_ratio is flipped

    • true or 1 - each aspect_ratio is not flipped

  • Type: boolean

  • Default value: false

  • Required: no

• clip

  • Description: clip is a flag that denotes if each value in the output tensor should be clipped to [0,1] interval.

  • Range of values:

    • false or 0 - clipping is not performed

    • true or 1 - each value in the output tensor is clipped to [0,1] interval.

  • Type: boolean

  • Default value: false

  • Required: no

• step

  • Description: step is a distance between box centers.

  • Range of values: floating-point non-negative number

  • Type: float

  • Default value: 0

  • Required: no

• offset

  • Description: offset is a shift of box respectively to top left corner.

  • Range of values: floating-point non-negative number

  • Type: float

  • Required: yes

• variance

  • Description: variance denotes a variance of adjusting bounding boxes. The attribute could contain 0, 1 or 4 elements.

  • Range of values: floating-point positive numbers

  • Type: float[]

  • Default value: []

  • Required: no

• scale_all_sizes

  • Description: scale_all_sizes is a flag that denotes type of inference. For example, scale_all_sizes equals 0 means that max_size attribute is ignored.

  • Range of values:

    • false - max_size is ignored

    • true - max_size is used

  • Type: boolean

  • Default value: true

  • Required: no

• fixed_ratio

  • Description: fixed_ratio is an aspect ratio of a box.

  • Range of values: a list of positive floating-point numbers

  • Type: float[]

  • Default value: []

  • Required: no

• fixed_size

  • Description: fixed_size is an initial box size (in pixels).

  • Range of values: a list of positive floating-point numbers

  • Type: float[]

  • Default value: []

  • Required: no

• density

  • Description: density is the square root of the number of boxes of each type.

  • Range of values: a list of positive floating-point numbers

  • Type: float[]

  • Default value: []

  • Required: no

Inputs:

• 1: output_size - 1D tensor of type T_INT with two elements [height, width]. Specifies the spatial size of generated grid with boxes. Required.

• 2: image_size - 1D tensor of type T_INT with two elements [image_height, image_width] that specifies shape of the image for which boxes are generated. Required.

Outputs:

• 1: 2D tensor of shape [2, 4 * height * width * priors_per_point] and type T_OUT with box coordinates. The priors_per_point is the number of boxes generated per each grid element. The number depends on operation attribute values.

Types

• T_INT: any supported integer type.

• T_OUT: supported floating-point type.

Example

<layer type="PriorBox" ...>
    <data aspect_ratio="2.0" clip="false" density="" fixed_ratio="" fixed_size="" flip="true" max_size="38.46" min_size="16.0" offset="0.5" step="16.0" variance="0.1,0.1,0.2,0.2"/>
    <input>
        <port id="0">
            <dim>2</dim>        <!-- values: [24, 42] -->
        </port>
        <port id="1">
            <dim>2</dim>        <!-- values: [384, 672] -->
        </port>
    </input>
    <output>
        <port id="2">
            <dim>2</dim>
            <dim>16128</dim>
        </port>
    </output>
</layer>