PriorBox

Versioned name: PriorBox-1

Category: Object detection

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

Attributes:

Inputs:

Outputs:

Detailed description:

PriorBox computes coordinates of prior boxes by following:

  1. 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:

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

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

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

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

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

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

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

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

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

Example

<layer type="PriorBox" ...>
<data aspect_ratio="2.0" clip="0" density="" fixed_ratio="" fixed_size="" flip="1" 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>
</port>
<port id="1">
<dim>2</dim>
</port>
</input>
<output>
<port id="2">
<dim>2</dim>
<dim>16128</dim>
</port>
</output>
</layer>