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Monocular 3D traffic line detection jointly tackles the detection of lane markings and regression of their 3D location. The greatest challenge is the exact estimation of various line shapes in the world, which highly depends on the chosen representation. While anchor-based and grid-based line representations have been proposed, all suffer from the same limitation, the necessity of discretizing the 3D space. To address this limitation, we present an anchor-free parametric lane representation, which defines traffic lines as continuous curves in 3D space. Choosing splines as our representation, we show their superiority over polynomials of different degrees that were proposed in previous 2D lane detection approaches. Our continuous representation allows us to model even complex lane shapes at any position in the 3D space, while implicitly enforcing smoothness constraints. Our model is validated on a synthetic 3D lane dataset including a variety of scenes in terms of complexity of road shape and illumination. We outperform the state-of-the-art in nearly all geometric performance metrics and achieve a great leap in the detection rate. In contrast to discrete representations, our parametric model requires no post-processing achieving highest processing speed. Additionally, we provide a thorough analysis over different parametric representations for 3D lane detection.
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