**Category**: Object detection

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

**Attributes**:

*min_size (max_size)***Description**:*min_size (max_size)*is the minimum (maximum) box size (in pixels). For example,*min_size (max_size)*equal 15 means that the minimum (maximum) box size is 15.**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. For example,*aspect_ratio*equal "2.0,3.0" means that for the first box aspect_ratio is equal to 2.0 and for the second box is 3.0.**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

- false or 0 - each
**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. For example,*step*equal 85 means that the distance between neighborhood prior boxes centers is 85.**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. For example,*offset*equal 85 means that the shift of neighborhood prior boxes centers is 85.**Range of values**: floating point non-negative number**Type**: float**Default value**: None**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 the PriorBox layer is inferred in MXNet-like manner. In particular,*max_size*attribute is ignored.**Range of values**:- false -
*max_size*is ignored - true -
*max_size*is used

- false -
**Type**: boolean**Default value**: true**Required**:*no*

*fixed_ratio***Description**:*fixed_ratio*is an aspect ratio of a box. For example,*fixed_ratio*equal to 2.000000 means that the aspect ratio for the first box aspect ratio is 2.**Range of values**: a list of positive floating-point numbers**Type**:`float[]`

**Default value**: None**Required**:*no*

*fixed_size***Description**:*fixed_size*is an initial box size (in pixels). For example,*fixed_size*equal to 15 means that the initial box size is 15.**Range of values**: a list of positive floating-point numbers**Type**:`float[]`

**Default value**: None**Required**:*no*

*density***Description**:*density*is the square root of the number of boxes of each type. For example,*density*equal to 2 means that the first box generates four boxes of the same size and with the same shifted centers.**Range of values**: a list of positive floating-point numbers**Type**:`float[]`

**Default value**: None**Required**:*no*

**Inputs**:

**1**:`output_size`

- 1D tensor with two integer elements`[height, width]`

. Specifies the spatial size of generated grid with boxes. Required.**2**:`image_size`

- 1D tensor with two integer 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]`

with box coordinates. The`priors_per_point`

is the number of boxes generated per each grid element. The number depends on layer attribute values.

**Detailed description**:

*PriorBox* computes coordinates of prior boxes by following:

- First calculates
*center_x*and*center_y*of prior box:\[ W \equiv Width \quad Of \quad Image \]

\[ H \equiv Height \quad Of \quad Image \]

- If step equals 0:
\[ center_x=(w+0.5) \]

\[ center_y=(h+0.5) \]

- else:
\[ center_x=(w+offset)*step \]

\[ center_y=(h+offset)*step \]

\[ w \subset \left( 0, W \right ) \]

\[ h \subset \left( 0, H \right ) \]

- If step equals 0:
- Then, for each \( s \subset \left( 0, min_sizes \right ) \) calculates coordinates of prior boxes:
\[ xmin = \frac{\frac{center_x - s}{2}}{W} \]

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

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

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

**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>