EventNet is a neural network architecture that can efficiently process asynchronous event streams from event cameras in real-time. It uses a temporal coding function to recursively update the network's state as new events arrive, avoiding redundant computation. Experimental results show it can perform tasks like target motion estimation and ego-motion estimation at rates over 1000 Hz using only the new event data. Compared to frame-based and PointNet approaches, EventNet significantly reduces computation time by recursively updating representations rather than reprocessing all prior events.

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EventNet: Asynchronous Recursive Event Processing
Study 2020/09/18
2011136 Ryuta Shitomi
1
Yusuke Sekikawa, Kosuke Hara, and Hideo Saito, CVPR2019

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The event camera
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 High dynamic range
 Do not suffer from motion blur
 No latency
 Recently, frame-based approach have achieved impressive results in scenarios.

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The event data from event-based camera
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 When the new event occurred:
𝑒𝑖 = 𝑥𝑖, 𝑦𝑖, 𝑝𝑖, 𝑡 − 𝑡′
𝑡′
: time of the last event
 Asynchronous
 Sparse
 Many events (1 mega event per sec)
↓
Need a new learning approach to differ from frame-based
approach

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Frame-based approach for an event streams
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 Integration 𝛥𝑡 to create a frame and feed a neural
net.
 Disadvantages:
Cannot use of the sparseness of the data
Redundant computation
A. I. Maqueda, A. Loquercio, G. Gallego, N. N. Garcia, and D. Scaramuzza. Event-based vision meets deep
learning on steering prediction for self-driving cars. CoRR, abs/1804.01310, 2018.
𝜟𝒕

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Purpose and Contribution
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 Develop a novel neural network:
can process an extremely high-rate and variable length.
for real-time processing of an asynchronous event stream.
to output recursively using temporal information.
𝜏′
𝜏
𝑡
𝑝𝑜𝑙𝑎𝑟𝑖𝑡𝑦

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Problem Statement
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 Consider a sequence of events within a 𝜏 ms interval based on the latest time stamp 𝑡𝑗 of
the event:
𝒆𝑗 = 𝑒𝑖 𝑖 = 𝑗 − 𝑛 𝑗 + 1, … , 𝑗}
𝑒𝑖 = 𝑥𝑖, 𝑦𝑖, 𝑝𝑖, Δ𝑡𝑗,𝑖 Δ𝑡𝑗,𝑖 = 𝑡𝑗 − 𝑡𝑖
 Consider a neural network 𝑓 to estimate 𝑦𝑗 given event stream 𝒆𝒋
𝑦𝑗 = 𝑓 𝒆𝑗 𝑦𝑗: Task dependent
𝜏
𝑡
𝒆𝒋
𝜏
𝒆𝒋+𝟏

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Problem Statement
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 To realize a trainable neural network 𝑓 that satisfies following conditions:
1. End-to-End Trainable
2. Event-Wise Processing
3. Efficient Recursive Processing
4. Local Permutation Invariance
𝜏
𝑡
𝒆𝒋
𝜏
𝒆𝒋+𝟏

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Using PointNet architecture
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 PointNet solves the problem by approximating the function 𝑓:
𝑦𝑗 = 𝑓 𝒆𝑗 = 𝑔 max ℎ 𝑒𝑗−𝑛 𝑗 +1 , … , ℎ 𝑒𝑗
where ℎ: ℝ4
→ ℝ 𝐾
, 𝑔: ℝ 𝐾
→ ℝ
 Because of the max(∙) , the permutation of events does not change the output 𝑦𝑗.
MLP
 Problem:
A huge amount of computation of ℎ ∙ and max ∙ .
Should run on the event rate. (could be more than 1MEPS)
When a new j + 1 -th event arrives, Δtj,i = tj − ti changes to tj+1 − ti .
Many previous event has already been processed by h(∙).

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EventNet
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 Introduce the ”Temporal Coding” and “Recursive Processing” to overcome the difficulty.
The Δ𝑡 only changes as the new event is being received.
Split the ℎ(𝑒):
ℎ 𝑒𝑖 = 𝑐(ℎ 𝑒𝑖
−
, Δ𝑡𝑗,𝑖), 𝑒−
: (𝑥, 𝑦, 𝑝)
𝑐 ∙ :temporal coding function, ℎ: ℝ3
→ ℂ 𝐾
𝑦𝑗 become:
𝑓 𝒆𝑗 = 𝑔 max 𝑐 𝑧𝑗−𝑛 𝑗 +1, Δ𝑡𝑗,𝑗−𝑛 𝑗 +1 , … , 𝑐 𝑧𝑗, 0
where 𝑧𝑖 = ℎ 𝑒−
∈ ℂ 𝐾
𝑐 ∙ , max ∙ need to be computed for all events.
Temporal Coding

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EventNet
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• Assume the norm of each element of 𝑧𝑖 is less than 1 (by tanh).
• max ∙ , 𝑐 ∙ need to be recursive.
Example)
𝑠𝑗 = max 𝑠𝑗−1, 𝑐 𝑧𝑗, 0
𝑠𝑗+1 = max 𝑠𝑗, 𝑐 𝑧𝑗+1, 0
• Propose temporal coding function to satisfy this condition:
𝑎𝑗,𝑖 = 𝑐 𝑧𝑖, Δ𝑡𝑗,𝑖 = 𝑧𝑖 −
Δ𝑡𝑗,𝑖
𝜏
+
exp −i
2𝜋Δ𝑡𝑗,𝑖
𝜏
Recursive Processing
Encode the elapsed time

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Temporal Coding Function
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𝑎𝑗,𝑖 = 𝑐 𝑧𝑖, Δ𝑡𝑗,𝑖 = 𝑧𝑖 −
Δ𝑡𝑗,𝑖
𝜏
+
exp −i
2𝜋Δ𝑡𝑗,𝑖
𝜏
When the new event is received:
𝑎𝑗+1,𝑖 = 𝑐 𝑧𝑖, Δ𝑡𝑗+1,𝑖 = 𝑧𝑖 −
Δ𝑡𝑗,𝑖
𝜏
−
𝑡𝑗+1 − 𝑡𝑗
𝜏
+
exp −i
2𝜋(Δ𝑡𝑗,𝑖 − 𝑡𝑗+1 + 𝑡𝑗
𝜏
Recursive Processing
𝑡
|𝑎 𝑗,𝑖|
𝑒𝑗
New event is received
𝑡
|𝑎 𝑗,𝑖|
𝑒𝑗 𝑒𝑗+1
− 𝑡𝑗+1 − 𝑡𝑗

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Temporal Coding Function
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𝑡
|𝑎 𝑗,𝑖|
𝑒𝑗
New event is received
𝑡
|𝑎 𝑗+1,𝑖|
𝑒𝑗 𝑒𝑗+1
− 𝑡𝑗+1 − 𝑡𝑗
𝑎𝑗,𝑗−2 = max 𝑎𝑗,𝑗−5, … , 𝑎𝑗,𝑗
 when you new event is received.
𝑎𝑗+1,𝑗−2 = max 𝑐 𝑎𝑗,𝑗−2, 𝑡𝑗+1 − 𝑡𝑗 , 𝑎𝑗+1,𝑗+1
 Therefore, 𝑦𝑗+1:
𝑦𝑗+1 = 𝑓 𝒆𝒋+𝟏 = 𝑔 max 𝑐 𝑠𝑗, 𝑡𝑗+1 − 𝑡𝑗 , ℎ 𝑒𝑗+1
−
𝑒𝑗−2
𝑒𝑗−5
|𝑎 𝑗,𝑗−2|

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Overview of EventNet
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 First event is arrived:
𝒆0 = {𝑒0}
𝑧0 = ℎ(𝑒0
−
)
𝑦0 = 𝑓 𝒆0 = 𝑔 max 𝑐 𝑧0, 0
𝑡
𝑐(𝑧0, 0)
𝑗 = 0
𝑗 = 1
 Next event is arrived:
𝒆1 = {𝑒0, 𝑒1}
𝑠1 = max(𝑐 𝑧0, 𝑡1 − 𝑡0 , ℎ(𝑒1
−
)
𝑦1 = 𝑓 𝒆1 = 𝑔(𝑠1)
 Next event is arrived:
𝒆2 = {𝑒0, 𝑒1, 𝑒2}
𝑠2 = max 𝑐 𝑠1, 𝑡2 − 𝑡1 , ℎ 𝑒2
−
𝑦3 = 𝑔 𝑠2
𝑗 = 2

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EventNet Architecture
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 Create Look-up table(LUT) for decreasing inference time.
(There are only 𝑊 × 𝐻 × 2 patterns in inputs 𝑒−
)
ℎ(∙)
𝑔(∙)

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Experiments
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 ETHTED+ dataset for target motion estimation and semantic segmentation
 MVSEC dataset for ego-motion estimation

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Results for target motion estimation
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Results for ego-motion
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 Estimated ego motions are shown as yellow.
 Estimated at the rate of 1000 Hz.

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Compare the computation time [μs]
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 Using newly received event, PointNet need to process the all events again.