# PriorBox¶

Versioned name : PriorBox-8

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 the following rules:

1. First, it calculates center_x and center_y of a 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, it calculates coordinates of prior boxes for each $$s \subset \left( 0, min\_sizes \right )$$ :

$xmin = \frac{\frac{center_x - s}{2}}{W}$
$ymin = \frac{\frac{center_y - s}{2}}{H}$
$xmax = \frac{\frac{center_x + s}{2}}{W}$
$ymin = \frac{\frac{center_y + s}{2}}{H}$
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 : a 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 [4.0,2.0], meaning that the 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 the [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 the [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 a box to the top left corner respectively.

• 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

• min_max_aspect_ratios_order

• Description : min_max_aspect_ratios_order is a flag that denotes the order of output prior box. If set true, the output prior box is in [min, max, aspect_ratios] order, which is consistent with Caffe. Note that the order affects the weights order of the preceding convolution layer and does not affect the final detection results.

• Range of values :

• false - the output prior box is in [min, aspect_ratios, max] order

• true - the output prior box is in [min, max, aspect_ratios] order

• Type : boolean

• Default value : true

• 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]. 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>