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Title: Deformable Kernels: Adapting Effective Receptive Fields for Object Deformation

Paper content:
Convolutional networks are not aware of an object's geometric variations, which leads to inefficient utilization of model and data capacity.
To overcome this issue, recent works on deformation modeling seek to spatially reconfigure the data towards a common arrangement such that semantic recognition suffers less from deformation.
This is typically done by augmenting static operators with learned free-form sampling grids in the image space, dynamically tuned to the data and task for adapting the receptive field.
Yet adapting the receptive field does not quite reach the actual goal -- what really matters to the network is the *effective* receptive field (ERF), which reflects how much each pixel contributes.
It is thus natural to design other approaches to adapt the ERF directly during runtime.
In this work, we instantiate one possible solution as Deformable Kernels (DKs), a family of novel and generic convolutional operators for handling object deformations by directly adapting the ERF while leaving the receptive field untouched.
At the heart of our method is the ability to resample the original kernel space towards recovering the deformation of objects.
This approach is justified with theoretical insights that the ERF is strictly determined by data sampling locations and kernel values.
We implement DKs as generic drop-in replacements of rigid kernels and conduct a series of empirical studies whose results conform with our theories.
Over several tasks and standard base models, our approach compares favorably against prior works that adapt during runtime.
In addition, further experiments suggest a working mechanism orthogonal and complementary to previous works.
The rich diversity of object appearance in images arises from variations in object semantics and deformation.
Semantics describe the high-level abstraction of what we perceive, and deformation defines the geometric transformation tied to specific data (Gibson, 1950) .
Humans are remarkably adept at making abstractions of the world (Hudson & Manning, 2019) ; we see in raw visual signals, abstract semantics away from deformation, and form concepts.
Interestingly, modern convolutional networks follow an analogous process by making abstractions through local connectivity and weight sharing (Zhang, 2019) .
However, such a mechanism is an inefficient one, as the emergent representations encode semantics and deformation together, instead of as disjoint notions.
Though a convolution responds accordingly to each input, how it responds is primarily programmed by its rigid kernels, as in Figure 1(a, b) .
In effect, this consumes large model capacity and data modes .
We argue that the awareness of deformations emerges from adaptivity -the ability to adapt at runtime (Kanazawa et al., 2016; Jia et al., 2016; Li et al., 2019) .
Modeling of geometric transformations has been a constant pursuit for vision researchers over decades (Lowe et al., 1999; Lazebnik et al., 2006; Jaderberg et al., 2015; Dai et al., 2017) .
A basic idea is to spatially recompose data towards a common mode such that semantic recognition suffers less from deformation.
A recent work that is representative of this direction is Deformable Convolution (Dai et al., 2017; Zhu et al., 2019) .
As shown in Figure 1
(c), it augments the convolutions with free-form sampling grids in the data space.
It is previously justified as adapting receptive field, or what we phrase as the "theoretical receptive field", that defines which input pixels can contribute to the final output.
However, theoretical receptive field does not measure how much impact an input pixel actually has.
On the other hand, (Dai et al., 2017) reconfigure data towards common arrangement to counter the effects of geometric deformation.
(d) Our Deformable Kernels (DKs) instead resample kernels and, in effect, adapt kernel spaces while leaving the data untouched.
Note that
(b) and
(c) share kernel values but sample different data locations, while
(b) and
(d) share data locations but sample different kernel values.
Luo et al. (2016) propose to measure the effective receptive field (ERF), i.e. the partial derivative of the output with respect to the input data, to quantify the exact contribution of each raw pixel to the convolution.
Since adapting the theoretical receptive field is not the goal but a means to adapt the ERF, why not directly tune the ERF to specific data and tasks at runtime?
Toward this end, we introduce Deformable Kernels (DKs), a family of novel and generic convolutional operators for deformation modeling.
We aim to augment rigid kernels with the expressiveness to directly interact with the ERF of the computation during inference.
Illustrated in Figure 1 (d), DKs learn free-form offsets on kernel coordinates to deform the original kernel space towards specific data modality, rather than recomposing data.
This can directly adapt ERF while leaving receptive field untouched.
The design of DKs that is agnostic to data coordinates naturally leads to two variants -the global DK and the local DK, which behave differently in practice as we later investigate.
We justify our approach with theoretical results which show that ERF is strictly determined by data sampling locations and kernel values.
Used as a generic drop-in replacement of rigid kernels, DKs achieve empirical results coherent with our developed theory.
Concretely, we evaluate our operator with standard base models on image classification and object detection.
DKs perform favorably against prior works that adapt during runtime.
With both quantitative and qualitative analysis, we further show that DKs can work orthogonally and complementarily with previous techniques.
In this paper, we introduced Deformable Kernels (DKs) to adapt effective receptive fields (ERFs) of convolutional networks for object deformation.
We proposed to sample kernel values from the original kernel space.
This in effect samples the ERF in linear networks and also roughly generalizes to non-linear cases.
We instantiated two variants of DKs and validate our designs, showing connections to previous works.
Consistent improvements over them and compatibility with them were found, as illustrated in visualizations.
image patch kernel patch Figure 5 : Illustration of feed-forwarding through a 3×3 local Deformable Kernel from a 4×4 scope.
For each input patch, local DK first generates a group of kernel offsets {∆k} from input feature patch using the light-weight generator G (a 3×3 convolution of rigid kernel).
Given the original kernel weights W and the offset group {∆k}, DK samples a new set of kernel W using a bilinear sampler B. Finally, DK convolves the input feature map and the sampled kernels to complete the whole computation.

assistant: text: Don't deform your convolutions -- deform your kernels.