TCAM: Temporal Class Activation Maps for Object Localization in Weakly-Labeled Unconstrained Videos

Abstract

Weakly supervised video object localization (WSVOL) allows locating object in videos using only global video tags such as object classes. State-of-art methods rely on multiple independent stages, where initial spatio-temporal proposals are generated using visual and motion cues, and then prominent objects are identified and refined. The localization involves solving an optimization problem over one or more videos, and video tags are typically used for video clustering. This process requires a model per video or per class making for costly inference. Moreover, localized regions are not necessary discriminant because these methods rely on unsupervised motion methods like optical flow, or discarded video tags from optimization. In this paper, we leverage the successful class activation mapping (CAM) methods, designed for WSOL based on still images. A new Temporal CAM (TCAM) method is introduced for training a discriminant deep learning (DL) model to exploit spatio-temporal information in videos, using an CAM-Temporal Max Pooling (CAM-TMP) aggregation mechanism over consecutive CAMs. In particular, activations of regions of interest (ROIs) are collected from CAMs produced by a pretrained CNN classifier, and generate pixel-wise pseudo-labels for training a decoder. In addition, a global unsupervised size constraint, and local constraint such as CRF are used to yield more accurate CAMs. Inference over single independent frames allows parallel processing of a clip of frames, and real-time localization. Extensive experiments <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> on two challenging YouTube-Objects datasets with unconstrained videos indicate that CAM methods (trained on independent frames) can yield decent localization accuracy. Our proposed TCAM method achieves a new state-of-art in WSVOL accuracy, and visual results suggest that it can be adapted for subsequent tasks, such as object detection and tracking.