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Title: Correctness Verification of Neural Network

Paper content:
We present the first verification that a neural network for perception tasks produces
a correct output within a specified tolerance for every input of interest.
We define
correctness relative to a specification which identifies 1) a state space consisting of
all relevant states of the world and 2) an observation process that produces neural
network inputs from the states of the world.
Tiling the state and input spaces with
a finite number of tiles, obtaining ground truth bounds from the state tiles and
network output bounds from the input tiles, then comparing the ground truth and
network output bounds delivers an upper bound on the network output error for
any input of interest.
Results from two case studies highlight the ability of our
technique to deliver tight error bounds for all inputs of interest and show how the
error bounds vary over the state and input spaces.
Neural networks are now recognized as powerful function approximators with impressive performance across a wide range of applications, especially perception tasks (e.g. vision, speech recognition).
Current techniques, however, provide no correctness guarantees on such neural perception systemsthere is currently no way to verify that a neural network provides correct outputs (within a specified tolerance) for all inputs of interest.
The closest the field has come is robustness verification, which aims to verify if the network prediction is stable for all inputs in some neighborhood around a selected input point.
But robustness verification does not verify for all inputs of interest -it only verifies around local regions.
Besides, it does not guarantee that the output, even if stable, is actually correct -there is no specification that defines the correct output for any input except for the manually-labeled center point of each region.
We present the first correctness verification of neural networks for perception -the first verification that a neural network produces a correct output within a specified tolerance for every input of interest.
Neural networks are often used to predict some property of the world given an observation such as an image or audio recording.
We therefore define correctness relative to a specification which identifies
1) a state space consisting of all relevant states of the world and
2) an observation process that produces neural network inputs from the states of the world.
Then the inputs of interest are all inputs that can be observed from the state space via the observation process.
We define the set of inputs of interest as the feasible input space.
Because the quantity of interest that the network predicts is some property of the state of the world, the state defines the ground truth output (and therefore defines the correct output for each input to the neural network).
We present Tiler, the algorithm for correctness verification of neural networks.
Evaluating the correctness of the network on a single state is straightforward -use the observation process to obtain the possible inputs for that state, use the neural network to obtain the possible outputs, then compare the outputs to the ground truth from the state.
To do correctness verification, we generalize this idea to work with tiled state and input spaces.
We cover the state and input spaces with a finite number of tiles: each state tile comprises a set of states; each input tile is the image of the corresponding state tile under the observation process.
The state tiles provide ground truth bounds for the corresponding input tiles.
We use recently developed techniques from the robustness verification literature to obtain network output bounds for each input tile (Xiang et al., 2018; Gehr et al., 2018; Weng et al., 2018; Bastani et al., 2016; Lomuscio and Maganti, 2017; Tjeng et al., 2019) .
A comparison of the ground truth and output bounds delivers an error upper bound for that region of the state space.
The error bounds for all the tiles jointly provide the correctness verification result.
We present two case studies.
The first involves a world with a (idealized) fixed road and a camera that can vary its horizontal offset and viewing angle with respect to the centerline of the road (Section 5).
The state of the world is therefore characterized by the offset δ and the viewing angle θ.
A neural network takes the camera image as input and predicts the offset and the viewing angle.
The state space includes the δ and θ of interest.
The observation process is the camera imaging process, which maps camera positions to images.
This state space and the camera imaging process provide the specification.
The feasible input space is the set of camera images that can be observed from all camera positions of interest.
For each image, the camera positions of all the states that can produce the image give the possible ground truths.
We tile the state space using a grid on (δ, θ).
Each state tile gives a bound on the ground truth of δ and θ.
We then apply the observation process to project each state tile into the image space.
We compute a bounding box for each input tile and apply techniques from robustness verification (Tjeng et al., 2019) to obtain neural network output bounds for each input tile.
Comparing the ground truth bounds and the network output bounds gives upper bounds on network prediction error for each tile.
We verify that our trained neural network provides good accuracy across the majority of the state space of interest and bound the maximum error the network will ever produce on any feasible input.
The second case study verifies a neural network that classifies a LiDAR measurement of a sign in an (idealized) scene into one of three shapes (Section 6).
The state space includes the position of the LiDAR sensor and the shape of the sign.
We tile the state space, project each tile into the input space via the LiDAR observation process, and again apply techniques from robustness verification to verify the network, including identifying regions of the input space where the network may deliver an incorrect classification.
The techniques presented in this paper work with specifications provided by the combination of a state space of the world and an observation process that converts states into neural network inputs.
Results from the case studies highlight how well the approach works for a state space characterized by several attributes and a camera imaging or LiDAR measurement observation process.
We anticipate that the technique will also work well for other problems that have a low dimensional state space (but potentially a high dimensional input space).
For higher dimensional state spaces, the framework makes it possible to systematically target specific regions of the input space to verify.
Potential applications include targeted verification, directed testing, and the identification of illegal inputs for which the network is not expected to work on.

assistant: text: We present the first verification that a neural network for perception tasks produces a correct output within a specified tolerance for every input of interest.