  When trained at a sufficient scale, self-supervised learning has exhibited a
notable ability to solve a wide range of visual or language understanding
tasks. In this paper, we investigate simple, yet effective approaches for
adapting the pre-trained foundation models to the downstream task of interest,
namely, open-vocabulary semantic segmentation. To this end, we make the
following contributions: (i) we introduce Fusioner, with a lightweight,
transformer-based fusion module, that pairs the frozen visual representation
with language concept through a handful of image segmentation data. As a
consequence, the model gains the capability of zero-shot transfer to segment
novel categories; (ii) without loss of generality, we experiment on a broad
range of self-supervised models that have been pre-trained with different
schemes, e.g. visual-only models (MoCo v3, DINO), language-only models (BERT),
visual-language model (CLIP), and show that, the proposed fusion approach is
effective to any pair of visual and language models, even those pre-trained on
a corpus of uni-modal data; (iii) we conduct thorough ablation studies to
analyze the critical components in our proposed Fusioner, while evaluating on
standard benchmarks, e.g. PASCAL-5i and COCO-20i , it surpasses existing
state-of-the-art models by a large margin, despite only being trained on frozen
visual and language features; (iv) to measure the model's robustness on
learning visual-language correspondence, we further evaluate on synthetic
dataset, named Mosaic-4, where images are constructed by mosaicking the samples
from FSS-1000. Fusioner demonstrates superior performance over previous models.
