DeformableConvolution¶

Versioned name: DeformableConvolution-1

Category: Convolution

Short description: Computes 2D deformable convolution of input and kernel tensors.

Detailed description: Deformable Convolution is similar to regular Convolution but its receptive field is deformed because of additional spatial offsets used during input sampling. More thorough explanation can be found in Deformable Convolutions Demystified and Deformable Convolutional Networks.

Output is calculated using the following formula:

\[y(p) = \displaystyle{\sum_{k = 1}^{K}}w_{k}x(p + p_{k} + {\Delta}p_{k})\]

Where

K is a number of sampling locations, e.g. for kernel 3x3 and dilation = 1, K = 9
\(x(p)\) and \(y(p)\) denote the features at location p from the input feature maps x and output feature maps y
\(w_{k}\) is the weight for k-th location.
\(p_{k}\) is pre-specified offset for the k-th location, e.g. K = 9 and \(p_{k} \in { (-1, -1),(-1, 0), . . . ,(1, 1) }\)
\({\Delta}p_{k}\) is the learnable offset for the k-th location.

Attributes:

strides
- Description: strides is a distance (in pixels) to slide the filter on the feature map over the (y,x) axes. For example, strides equal 2,1 means sliding the filter 2 pixel at a time over height dimension and 1 over width dimension.
- Range of values: integer values starting from 0
- Type: int[]
- Required: yes
pads_begin
- Description: pads_begin is a number of pixels to add to the beginning along each axis. For example, pads_begin equal 1,2 means adding 1 pixel to the top of the input and 2 to the left of the input.
- Range of values: integer values starting from 0
- Type: int[]
- Required: yes
- Note: the attribute is ignored when auto_pad attribute is specified.
pads_end
- Description: pads_end is a number of pixels to add to the ending along each axis. For example, pads_end equal 1,2 means adding 1 pixel to the bottom of the input and 2 to the right of the input.
- Range of values: integer values starting from 0
- Type: int[]
- Required: yes
- Note: the attribute is ignored when auto_pad attribute is specified.
dilations
- Description: dilations denotes the distance in width and height between elements (weights) in the filter. For example, dilation equal 1,1 means that all the elements in the filter are neighbors, so it is the same as for the usual convolution. dilation equal 2,2 means that all the elements in the filter are matched not to adjacent elements in the input matrix, but to those that are adjacent with distance 1.
- Range of values: integer value starting from 0
- Type: int[]
- Required: yes
auto_pad
- Description: auto_pad how the padding is calculated. Possible values:
  - explicit - use explicit padding values from pads_begin and pads_end.
  - same_upper - the input is padded to match the output size. In case of odd padding value an extra padding is added at the end.
  - same_lower - the input is padded to match the output size. In case of odd padding value an extra padding is added at the beginning.
  - valid - do not use padding.
- Type: string
- Default value: explicit
- Required: no
- Note: pads_begin and pads_end attributes are ignored when auto_pad is specified.
group
- Description: group is the number of groups which output and input should be split into. For example, group equal to 1 means that all filters are applied to the whole input (usual convolution), group equal to 2 means that both input and output channels are separated into two groups and the i-th output group is connected to the i-th input group channel. group equal to a number of output feature maps implies depth-wise separable convolution.
- Range of values: integer value starting from 1
- Type: int
- Default value: 1
- Required: no
deformable_group
- Description: deformable_group is the number of groups in which offsets input and output should be split into along the channel axis. Apply the deformable convolution using the i-th part of the offsets part on the i-th out.
- Range of values: integer value starting from 1
- Type: int
- Default value: 1
- Required: no

Inputs:

1: Input tensor of type T and rank 4. Layout is NCYX (number of batches, number of channels, spatial axes Y and X). Required.
2: Offsets tensor of type T and rank 4. Layout is NCYX (number of batches, deformable_group * kernel_Y * kernel_X * 2, spatial axes Y and X). Required.
3: Kernel tensor of type T and rank 4. Layout is OIYX (number of output channels, number of input channels, spatial axes Y and X). Required.

Outputs:

1: Output tensor of type T and rank 4. Layout is NOYX (number of batches, number of kernel output channels, spatial axes Y and X).

Types:

T: Any numeric type.

Example

2D DeformableConvolution (deformable_group=1)

<layer type="DeformableConvolution" ...>
    <data dilations="1,1" pads_begin="0,0" pads_end="0,0" strides="1,1" auto_pad="explicit"  group="1" deformable_group="1"/>
    <input>
        <port id="0">
            <dim>1</dim>
            <dim>4</dim>
            <dim>224</dim>
            <dim>224</dim>
        </port>
        <port id="1">
            <dim>1</dim>
            <dim>50</dim>
            <dim>220</dim>
            <dim>220</dim>
        </port>
        <port id="2">
            <dim>64</dim>
            <dim>4</dim>
            <dim>5</dim>
            <dim>5</dim>
        </port>
    </input>
    <output>
        <port id="2" precision="FP32">
            <dim>1</dim>
            <dim>64</dim>
            <dim>220</dim>
            <dim>220</dim>
        </port>
    </output>
</layer>

2D DeformableConvolution (deformable_group=4)

<layer type="DeformableConvolution" ...>
    <data dilations="1,1" pads_begin="0,0" pads_end="0,0" strides="1,1" auto_pad="explicit"  group="1" deformable_group="4"/>
    <input>
        <port id="0">
            <dim>1</dim>
            <dim>4</dim>
            <dim>224</dim>
            <dim>224</dim>
        </port>
        <port id="1">
            <dim>1</dim>
            <dim>200</dim>
            <dim>220</dim>
            <dim>220</dim>
        </port>
        <port id="2">
            <dim>64</dim>
            <dim>4</dim>
            <dim>5</dim>
            <dim>5</dim>
        </port>
    </input>
    <output>
        <port id="2" precision="FP32">
            <dim>1</dim>
            <dim>64</dim>
            <dim>220</dim>
            <dim>220</dim>
        </port>
    </output>
</layer>