PackDet: Packed Long-Head Object Detector
State-of-the-art object detectors exploit multi-branch structure and predict objects at several different scales, although substantially boosted accuracy is acquired, low efficiency is inevitable as fragmented structure is hardware unfriendly. To solve this issue, we propose a packing operator (PackOp) to combine all head branches together at spatial. Packed features are computationally more efficient and allow to use cross-head group normalization (GN) at handy, leading to notable accuracy improvement against the common head-separate GN. All of these are only at the cost of less than 5.7% relative increase on runtime memory and introduction of a few noisy training samples, however, whose side-effects could be diminished by good packing patterns design. With PackOp, we propose a new anchor-free one-stage detector, PackDet, which features a single deeper/longer but narrower head compared to the existing methods: multiple shallow but wide heads. Our best models on COCO test-dev achieve better speed-accuracy balance: 35.1%, 42.3%, 44.0%, 47.4% AP with 22.6, 16.9, 12.4, 4.7 FPS using MobileNet-v2, ResNet-50, ResNet-101, and ResNeXt-101-DCN backbone, respectively. Codes will be released."