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Inference with existing models

MMDetection provides hundreds of pre-trained detection models in Model Zoo. This note will show how to inference, which means using trained models to detect objects on images.

In MMDetection, a model is defined by a configuration file and existing model parameters are saved in a checkpoint file.

To start with, we recommend Faster RCNN with this configuration file and this checkpoint file. It is recommended to download the checkpoint file to checkpoints directory.

High-level APIs for inference

MMDetection provides high-level Python APIs for inference on images. Here is an example of building the model and inference on given images or videos.

import cv2
import mmcv
from mmcv.transforms import Compose
from mmengine.utils import track_iter_progress
from mmdet.registry import VISUALIZERS
from mmdet.utils import register_all_modules
from mmdet.apis import init_detector, inference_detector

# Register all modules in mmdet into the registries
register_all_modules()

# Specify the path to model config and checkpoint file
config_file = 'configs/faster_rcnn/faster-rcnn_r50-fpn_1x_coco.py'
checkpoint_file = 'checkpoints/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth'

# Build the model from a config file and a checkpoint file
model = init_detector(config_file, checkpoint_file, device='cuda:0')

# Init visualizer
visualizer = VISUALIZERS.build(model.cfg.visualizer)
# The dataset_meta is loaded from the checkpoint and
# then pass to the model in init_detector
visualizer.dataset_meta = model.dataset_meta
# Ttest a single image and show the results

img = 'test.jpg'  # or img = mmcv.imread(img), which will only load it once
result = inference_detector(model, img)

# Show the results
img = mmcv.imread(img)
img = mmcv.imconvert(img, 'bgr', 'rgb')


visualizer.add_datasample(
    'result',
    img,
    data_sample=result,
    draw_gt=False,
    show=True)

# Test a video and show the results
# Build test pipeline
model.cfg.test_dataloader.dataset.pipeline[0].type = 'LoadImageFromNDArray'
test_pipeline = Compose(model.cfg.test_dataloader.dataset.pipeline)

# Init visualizer
visualizer = VISUALIZERS.build(model.cfg.visualizer)
# The dataset_meta is loaded from the checkpoint and
# then pass to the model in init_detector
visualizer.dataset_meta = model.dataset_meta

# The interval of show (s), 0 is block
wait_time = 1

video_reader = mmcv.VideoReader('video.mp4')

cv2.namedWindow('video', 0)

for frame in track_iter_progress(video_reader):
    result = inference_detector(model, frame, test_pipeline=test_pipeline)
    visualizer.add_datasample(
        name='video',
        image=frame,
        data_sample=result,
        draw_gt=False,
        show=False)
    frame = visualizer.get_image()
    mmcv.imshow(frame, 'video', wait_time)

cv2.destroyAllWindows()

A notebook demo can be found in demo/inference_demo.ipynb.

Note: inference_detector only supports single-image inference for now.

Demos

We also provide three demo scripts, implemented with high-level APIs and supporting functionality codes. Source codes are available here.

Image demo

This script performs inference on a single image.

python demo/image_demo.py \
    ${IMAGE_FILE} \
    ${CONFIG_FILE} \
    ${CHECKPOINT_FILE} \
    [--device ${GPU_ID}] \
    [--score-thr ${SCORE_THR}]

Examples:

python demo/image_demo.py demo/demo.jpg \
    configs/faster_rcnn/faster-rcnn_r50_fpn_1x_coco.py \
    checkpoints/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth \
    --device cpu

Webcam demo

This is a live demo from a webcam.

python demo/webcam_demo.py \
    ${CONFIG_FILE} \
    ${CHECKPOINT_FILE} \
    [--device ${GPU_ID}] \
    [--camera-id ${CAMERA-ID}] \
    [--score-thr ${SCORE_THR}]

Examples:

python demo/webcam_demo.py \
    configs/faster_rcnn/faster-rcnn_r50_fpn_1x_coco.py \
    checkpoints/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth

Video demo

This script performs inference on a video.

python demo/video_demo.py \
    ${VIDEO_FILE} \
    ${CONFIG_FILE} \
    ${CHECKPOINT_FILE} \
    [--device ${GPU_ID}] \
    [--score-thr ${SCORE_THR}] \
    [--out ${OUT_FILE}] \
    [--show] \
    [--wait-time ${WAIT_TIME}]

Examples:

python demo/video_demo.py demo/demo.mp4 \
    configs/faster_rcnn/faster-rcnn_r50_fpn_1x_coco.py \
    checkpoints/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth \
    --out result.mp4

Video demo with GPU acceleration

This script performs inference on a video with GPU acceleration.

python demo/video_gpuaccel_demo.py \
    ${VIDEO_FILE} \
    ${CONFIG_FILE} \
    ${CHECKPOINT_FILE} \
    [--device ${GPU_ID}] \
    [--score-thr ${SCORE_THR}] \
    [--nvdecode] \
    [--out ${OUT_FILE}] \
    [--show] \
    [--wait-time ${WAIT_TIME}]

Examples:

python demo/video_gpuaccel_demo.py demo/demo.mp4 \
    configs/faster_rcnn/faster-rcnn_r50_fpn_1x_coco.py \
    checkpoints/faster_rcnn_r50_fpn_1x_coco_20200130-047c8118.pth \
    --nvdecode --out result.mp4
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