
OWLv2
Overview
OWLv2 was proposed in Scaling Open-Vocabulary Object Detection by Matthias Minderer, Alexey Gritsenko, Neil Houlsby. OWLv2 scales up OWL-ViT using self-training, which uses an existing detector to generate pseudo-box annotations on image-text pairs. This results in large gains over the previous state-of-the-art for zero-shot object detection.
The abstract from the paper is the following:
Open-vocabulary object detection has benefited greatly from pretrained vision-language models, but is still limited by the amount of available detection training data. While detection training data can be expanded by using Web image-text pairs as weak supervision, this has not been done at scales comparable to image-level pretraining. Here, we scale up detection data with self-training, which uses an existing detector to generate pseudo-box annotations on image-text pairs. Major challenges in scaling self-training are the choice of label space, pseudo-annotation filtering, and training efficiency. We present the OWLv2 model and OWL-ST self-training recipe, which address these challenges. OWLv2 surpasses the performance of previous state-of-the-art open-vocabulary detectors already at comparable training scales (~10M examples). However, with OWL-ST, we can scale to over 1B examples, yielding further large improvement: With an L/14 architecture, OWL-ST improves AP on LVIS rare classes, for which the model has seen no human box annotations, from 31.2% to 44.6% (43% relative improvement). OWL-ST unlocks Web-scale training for open-world localization, similar to what has been seen for image classification and language modelling.

 OWLv2 high-level overview. Taken from the original paper. 
This model was contributed by nielsr.
The original code can be found here.
Usage example
OWLv2 is, just like its predecessor OWL-ViT, a zero-shot text-conditioned object detection model. OWL-ViT uses CLIP as its multi-modal backbone, with a ViT-like Transformer to get visual features and a causal language model to get the text features. To use CLIP for detection, OWL-ViT removes the final token pooling layer of the vision model and attaches a lightweight classification and box head to each transformer output token. Open-vocabulary classification is enabled by replacing the fixed classification layer weights with the class-name embeddings obtained from the text model. The authors first train CLIP from scratch and fine-tune it end-to-end with the classification and box heads on standard detection datasets using a bipartite matching loss. One or multiple text queries per image can be used to perform zero-shot text-conditioned object detection.
[Owlv2ImageProcessor] can be used to resize (or rescale) and normalize images for the model and [CLIPTokenizer] is used to encode the text. [Owlv2Processor] wraps [Owlv2ImageProcessor] and [CLIPTokenizer] into a single instance to both encode the text and prepare the images. The following example shows how to perform object detection using [Owlv2Processor] and [Owlv2ForObjectDetection].
thon

import requests
from PIL import Image
import torch
from transformers import Owlv2Processor, Owlv2ForObjectDetection
processor = Owlv2Processor.from_pretrained("google/owlv2-base-patch16-ensemble")
model = Owlv2ForObjectDetection.from_pretrained("google/owlv2-base-patch16-ensemble")
url = "http://images.cocodataset.org/val2017/000000039769.jpg"
image = Image.open(requests.get(url, stream=True).raw)
texts = [["a photo of a cat", "a photo of a dog"]]
inputs = processor(text=texts, images=image, return_tensors="pt")
outputs = model(**inputs)
Target image sizes (height, width) to rescale box predictions [batch_size, 2]
target_sizes = torch.Tensor([image.size[::-1]])
Convert outputs (bounding boxes and class logits) to Pascal VOC Format (xmin, ymin, xmax, ymax)
results = processor.post_process_object_detection(outputs=outputs, target_sizes=target_sizes, threshold=0.1)
i = 0  # Retrieve predictions for the first image for the corresponding text queries
text = texts[i]
boxes, scores, labels = results[i]["boxes"], results[i]["scores"], results[i]["labels"]
for box, score, label in zip(boxes, scores, labels):
     box = [round(i, 2) for i in box.tolist()]
     print(f"Detected {text[label]} with confidence {round(score.item(), 3)} at location {box}")
Detected a photo of a cat with confidence 0.614 at location [341.67, 17.54, 642.32, 278.51]
Detected a photo of a cat with confidence 0.665 at location [6.75, 38.97, 326.62, 354.85]

Resources

A demo notebook on using OWLv2 for zero- and one-shot (image-guided) object detection can be found here.
Zero-shot object detection task guide

The architecture of OWLv2 is identical to OWL-ViT, however the object detection head now also includes an objectness classifier, which predicts the (query-agnostic) likelihood that a predicted box contains an object (as opposed to background). The objectness score can be used to rank or filter predictions independently of text queries.
Usage of OWLv2 is identical to OWL-ViT with a new, updated image processor ([Owlv2ImageProcessor]).

Owlv2Config
[[autodoc]] Owlv2Config
    - from_text_vision_configs
Owlv2TextConfig
[[autodoc]] Owlv2TextConfig
Owlv2VisionConfig
[[autodoc]] Owlv2VisionConfig
Owlv2ImageProcessor
[[autodoc]] Owlv2ImageProcessor
    - preprocess
    - post_process_object_detection
    - post_process_image_guided_detection
Owlv2Processor
[[autodoc]] Owlv2Processor
Owlv2Model
[[autodoc]] Owlv2Model
    - forward
    - get_text_features
    - get_image_features
Owlv2TextModel
[[autodoc]] Owlv2TextModel
    - forward
Owlv2VisionModel
[[autodoc]] Owlv2VisionModel
    - forward
Owlv2ForObjectDetection
[[autodoc]] Owlv2ForObjectDetection
    - forward
    - image_guided_detection