TIP
To get the most of this tutorial, we suggest using this Colab Version. This will allow you to experiment with the information presented below.
For this tutorial, we will be finetuning a pre-trained Mask R-CNN model in the Penn-Fudan Database for Pedestrian Detection and Segmentation. It contains 170 images with 345 instances of pedestrians, and we will use it to illustrate how to use the new features in torchvision in order to train an instance segmentation model on a custom dataset.
Defining the Dataset
The reference scripts for training object detection, instance segmentation and person keypoint detection allows for easily supporting adding new custom datasets. The dataset should inherit from the standard torch.utils.data.Dataset class, and implement __len__ and __getitem__.
The only specificity that we require is that the dataset __getitem__ should return:
- image: a PIL Image of size
(H, W) - target: a dict containing the following fields
boxes (FloatTensor[N, 4]): the coordinates of theNbounding boxes in[x0, y0, x1, y1]format, ranging from0toWand0toHlabels (Int64Tensor[N]): the label for each bounding boximage_id (Int64Tensor[1]): an image identifier. It should be unique between all the images in the dataset, and is used during evaluationarea (Tensor[N]): The area of the bounding box. This is used during evaluation with the COCO metric, to separate the metric scores between small, medium and large boxes.iscrowd (UInt8Tensor[N]): instances with iscrowd=True will be ignored during evaluation.- (optionally)
masks (UInt8Tensor[N, H, W]): The segmentation masks for each one of the objects - (optionally)
keypoints (FloatTensor[N, K, 3]): For each one of the N objects, it contains the K keypoints in[x, y, visibility]format, defining the object. visibility=0 means that the keypoint is not visible. Note that for data augmentation, the notion of flipping a keypoint is dependent on the data representation, and you should probably adaptreferences/detection/transforms.pyfor your new keypoint representation
If your model returns the above methods, they will make it work for both training and evaluation, and will use the evaluation scripts from pycocotools.
Additionally, if you want to use aspect ratio grouping during training (so that each batch only contains images with similar aspect ratio), then it is recommended to also implement a get_height_and_width method, which returns the height and the width of the image. If this method is not provided, we query all elements of the dataset via __getitem__ , which loads the image in memory and is slower than if a custom method is provided.
Writing a custom dataset for PennFudan
Let’s write a dataset for the PennFudan dataset. After downloading and extracting the zip file, we have the following folder structure:
PennFudanPed/
PedMasks/
FudanPed00001_mask.png
FudanPed00002_mask.png
FudanPed00003_mask.png
FudanPed00004_mask.png
...
PNGImages/
FudanPed00001.png
FudanPed00002.png
FudanPed00003.png
FudanPed00004.png
Here is one example of a pair of images and segmentation masks
So each image has a corresponding segmentation mask, where each color correspond to a different instance. Let’s write a torch.utils.data.Dataset class for this dataset.
import os
import numpy as np
import torch
from PIL import Image
class PennFudanDataset(object):
def __init__(self, root, transforms):
self.root = root
self.transforms = transforms
# load all image files, sorting them to
# ensure that they are aligned
self.imgs = list(sorted(os.listdir(os.path.join(root, "PNGImages"))))
self.masks = list(sorted(os.listdir(os.path.join(root, "PedMasks"))))
def __getitem__(self, idx):
# load images ad masks
img_path = os.path.join(self.root, "PNGImages", self.imgs[idx])
mask_path = os.path.join(self.root, "PedMasks", self.masks[idx])
img = Image.open(img_path).convert("RGB")
# note that we haven't converted the mask to RGB,
# because each color corresponds to a different instance
# with 0 being background
mask = Image.open(mask_path)
# convert the PIL Image into a numpy array
mask = np.array(mask)
# instances are encoded as different colors
obj_ids = np.unique(mask)
# first id is the background, so remove it
obj_ids = obj_ids[1:]
# split the color-encoded mask into a set
# of binary masks
masks = mask == obj_ids[:, None, None]
# get bounding box coordinates for each mask
num_objs = len(obj_ids)
boxes = []
for i in range(num_objs):
pos = np.where(masks[i])
xmin = np.min(pos[1])
xmax = np.max(pos[1])
ymin = np.min(pos[0])
ymax = np.max(pos[0])
boxes.append([xmin, ymin, xmax, ymax])
# convert everything into a torch.Tensor
boxes = torch.as_tensor(boxes, dtype=torch.float32)
# there is only one class
labels = torch.ones((num_objs,), dtype=torch.int64)
masks = torch.as_tensor(masks, dtype=torch.uint8)
image_id = torch.tensor([idx])
area = (boxes[:, 3] - boxes[:, 1]) * (boxes[:, 2] - boxes[:, 0])
# suppose all instances are not crowd
iscrowd = torch.zeros((num_objs,), dtype=torch.int64)
target = {}
target["boxes"] = boxes
target["labels"] = labels
target["masks"] = masks
target["image_id"] = image_id
target["area"] = area
target["iscrowd"] = iscrowd
if self.transforms is not None:
img, target = self.transforms(img, target)
return img, target
def __len__(self):
return len(self.imgs)
That’s all for the dataset. Now let’s define a model that can perform predictions on this dataset.
Defining your model
In this tutorial, we will be using Mask R-CNN, which is based on top of Faster R-CNN. Faster R-CNN is a model that predicts both bounding boxes and class scores for potential objects in the image.
Mask R-CNN adds an extra branch into Faster R-CNN, which also predicts segmentation masks for each instance.
There are two common situations where one might want to modify one of the available models in torchvision modelzoo. The first is when we want to start from a pre-trained model, and just finetune the last layer. The other is when we want to replace the backbone of the model with a different one (for faster predictions, for example).
Let’s go see how we would do one or another in the following sections.
1 – Finetuning from a pretrained model
Let’s suppose that you want to start from a model pre-trained on COCO and want to finetune it for your particular classes. Here is a possible way of doing it:
import torchvision from torchvision.models.detection.faster_rcnn import FastRCNNPredictor # load a model pre-trained pre-trained on COCO model = torchvision.models.detection.fasterrcnn_resnet50_fpn(pretrained=True) # replace the classifier with a new one, that has # num_classes which is user-defined num_classes = 2 # 1 class (person) + background # get number of input features for the classifier in_features = model.roi_heads.box_predictor.cls_score.in_features # replace the pre-trained head with a new one model.roi_heads.box_predictor = FastRCNNPredictor(in_features, num_classes)
2 – Modifying the model to add a different backbone
import torchvision
from torchvision.models.detection import FasterRCNN
from torchvision.models.detection.rpn import AnchorGenerator
# load a pre-trained model for classification and return
# only the features
backbone = torchvision.models.mobilenet_v2(pretrained=True).features
# FasterRCNN needs to know the number of
# output channels in a backbone. For mobilenet_v2, it's 1280
# so we need to add it here
backbone.out_channels = 1280
# let's make the RPN generate 5 x 3 anchors per spatial
# location, with 5 different sizes and 3 different aspect
# ratios. We have a Tuple[Tuple[int]] because each feature
# map could potentially have different sizes and
# aspect ratios
anchor_generator = AnchorGenerator(sizes=((32, 64, 128, 256, 512),),
aspect_ratios=((0.5, 1.0, 2.0),))
# let's define what are the feature maps that we will
# use to perform the region of interest cropping, as well as
# the size of the crop after rescaling.
# if your backbone returns a Tensor, featmap_names is expected to
# be [0]. More generally, the backbone should return an
# OrderedDict[Tensor], and in featmap_names you can choose which
# feature maps to use.
roi_pooler = torchvision.ops.MultiScaleRoIAlign(featmap_names=[0],
output_size=7,
sampling_ratio=2)
# put the pieces together inside a FasterRCNN model
model = FasterRCNN(backbone,
num_classes=2,
rpn_anchor_generator=anchor_generator,
box_roi_pool=roi_pooler)
An Instance segmentation model for PennFudan Dataset
In our case, we want to fine-tune from a pre-trained model, given that our dataset is very small, so we will be following approach number 1.
Here we want to also compute the instance segmentation masks, so we will be using Mask R-CNN:
import torchvision
from torchvision.models.detection.faster_rcnn import FastRCNNPredictor
from torchvision.models.detection.mask_rcnn import MaskRCNNPredictor
def get_model_instance_segmentation(num_classes):
# load an instance segmentation model pre-trained pre-trained on COCO
model = torchvision.models.detection.maskrcnn_resnet50_fpn(pretrained=True)
# get number of input features for the classifier
in_features = model.roi_heads.box_predictor.cls_score.in_features
# replace the pre-trained head with a new one
model.roi_heads.box_predictor = FastRCNNPredictor(in_features, num_classes)
# now get the number of input features for the mask classifier
in_features_mask = model.roi_heads.mask_predictor.conv5_mask.in_channels
hidden_layer = 256
# and replace the mask predictor with a new one
model.roi_heads.mask_predictor = MaskRCNNPredictor(in_features_mask,
hidden_layer,
num_classes)
return model
That’s it, this will make model be ready to be trained and evaluated on your custom dataset.
Putting everything together
In references/detection/, we have a number of helper functions to simplify training and evaluating detection models. Here, we will use references/detection/engine.py, references/detection/utils.py and references/detection/transforms.py. Just copy them to your folder and use them here.
Let’s write some helper functions for data augmentation / transformation:
import transforms as T
def get_transform(train):
transforms = []
transforms.append(T.ToTensor())
if train:
transforms.append(T.RandomHorizontalFlip(0.5))
return T.Compose(transforms)
Let’s now write the main function which performs the training and the validation:
from engine import train_one_epoch, evaluate
import utils
def main():
# train on the GPU or on the CPU, if a GPU is not available
device = torch.device('cuda') if torch.cuda.is_available() else torch.device('cpu')
# our dataset has two classes only - background and person
num_classes = 2
# use our dataset and defined transformations
dataset = PennFudanDataset('PennFudanPed', get_transform(train=True))
dataset_test = PennFudanDataset('PennFudanPed', get_transform(train=False))
# split the dataset in train and test set
indices = torch.randperm(len(dataset)).tolist()
dataset = torch.utils.data.Subset(dataset, indices[:-50])
dataset_test = torch.utils.data.Subset(dataset_test, indices[-50:])
# define training and validation data loaders
data_loader = torch.utils.data.DataLoader(
dataset, batch_size=2, shuffle=True, num_workers=4,
collate_fn=utils.collate_fn)
data_loader_test = torch.utils.data.DataLoader(
dataset_test, batch_size=1, shuffle=False, num_workers=4,
collate_fn=utils.collate_fn)
# get the model using our helper function
model = get_model_instance_segmentation(num_classes)
# move model to the right device
model.to(device)
# construct an optimizer
params = [p for p in model.parameters() if p.requires_grad]
optimizer = torch.optim.SGD(params, lr=0.005,
momentum=0.9, weight_decay=0.0005)
# and a learning rate scheduler
lr_scheduler = torch.optim.lr_scheduler.StepLR(optimizer,
step_size=3,
gamma=0.1)
# let's train it for 10 epochs
num_epochs = 10
for epoch in range(num_epochs):
# train for one epoch, printing every 10 iterations
train_one_epoch(model, optimizer, data_loader, device, epoch, print_freq=10)
# update the learning rate
lr_scheduler.step()
# evaluate on the test dataset
evaluate(model, data_loader_test, device=device)
print("That's it!")
You should get as output for the first epoch:
Epoch: [0] [ 0/60] eta: 0:01:18 lr: 0.000090 loss: 2.5213 (2.5213) loss_classifier: 0.8025 (0.8025) loss_box_reg: 0.2634 (0.2634) loss_mask: 1.4265 (1.4265) loss_objectness: 0.0190 (0.0190) loss_rpn_box_reg: 0.0099 (0.0099) time: 1.3121 data: 0.3024 max mem: 3485 Epoch: [0] [10/60] eta: 0:00:20 lr: 0.000936 loss: 1.3007 (1.5313) loss_classifier: 0.3979 (0.4719) loss_box_reg: 0.2454 (0.2272) loss_mask: 0.6089 (0.7953) loss_objectness: 0.0197 (0.0228) loss_rpn_box_reg: 0.0121 (0.0141) time: 0.4198 data: 0.0298 max mem: 5081 Epoch: [0] [20/60] eta: 0:00:15 lr: 0.001783 loss: 0.7567 (1.1056) loss_classifier: 0.2221 (0.3319) loss_box_reg: 0.2002 (0.2106) loss_mask: 0.2904 (0.5332) loss_objectness: 0.0146 (0.0176) loss_rpn_box_reg: 0.0094 (0.0123) time: 0.3293 data: 0.0035 max mem: 5081 Epoch: [0] [30/60] eta: 0:00:11 lr: 0.002629 loss: 0.4705 (0.8935) loss_classifier: 0.0991 (0.2517) loss_box_reg: 0.1578 (0.1957) loss_mask: 0.1970 (0.4204) loss_objectness: 0.0061 (0.0140) loss_rpn_box_reg: 0.0075 (0.0118) time: 0.3403 data: 0.0044 max mem: 5081 Epoch: [0] [40/60] eta: 0:00:07 lr: 0.003476 loss: 0.3901 (0.7568) loss_classifier: 0.0648 (0.2022) loss_box_reg: 0.1207 (0.1736) loss_mask: 0.1705 (0.3585) loss_objectness: 0.0018 (0.0113) loss_rpn_box_reg: 0.0075 (0.0112) time: 0.3407 data: 0.0044 max mem: 5081 Epoch: [0] [50/60] eta: 0:00:03 lr: 0.004323 loss: 0.3237 (0.6703) loss_classifier: 0.0474 (0.1731) loss_box_reg: 0.1109 (0.1561) loss_mask: 0.1658 (0.3201) loss_objectness: 0.0015 (0.0093) loss_rpn_box_reg: 0.0093 (0.0116) time: 0.3379 data: 0.0043 max mem: 5081 Epoch: [0] [59/60] eta: 0:00:00 lr: 0.005000 loss: 0.2540 (0.6082) loss_classifier: 0.0309 (0.1526) loss_box_reg: 0.0463 (0.1405) loss_mask: 0.1568 (0.2945) loss_objectness: 0.0012 (0.0083) loss_rpn_box_reg: 0.0093 (0.0123) time: 0.3489 data: 0.0042 max mem: 5081 Epoch: [0] Total time: 0:00:21 (0.3570 s / it) creating index... index created! Test: [ 0/50] eta: 0:00:19 model_time: 0.2152 (0.2152) evaluator_time: 0.0133 (0.0133) time: 0.4000 data: 0.1701 max mem: 5081 Test: [49/50] eta: 0:00:00 model_time: 0.0628 (0.0687) evaluator_time: 0.0039 (0.0064) time: 0.0735 data: 0.0022 max mem: 5081 Test: Total time: 0:00:04 (0.0828 s / it) Averaged stats: model_time: 0.0628 (0.0687) evaluator_time: 0.0039 (0.0064) Accumulating evaluation results... DONE (t=0.01s). Accumulating evaluation results... DONE (t=0.01s). IoU metric: bbox Average Precision (AP) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.606 Average Precision (AP) @[ IoU=0.50 | area= all | maxDets=100 ] = 0.984 Average Precision (AP) @[ IoU=0.75 | area= all | maxDets=100 ] = 0.780 Average Precision (AP) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.313 Average Precision (AP) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.582 Average Precision (AP) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.612 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 1 ] = 0.270 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 10 ] = 0.672 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.672 Average Recall (AR) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.650 Average Recall (AR) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.755 Average Recall (AR) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.664 IoU metric: segm Average Precision (AP) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.704 Average Precision (AP) @[ IoU=0.50 | area= all | maxDets=100 ] = 0.979 Average Precision (AP) @[ IoU=0.75 | area= all | maxDets=100 ] = 0.871 Average Precision (AP) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.325 Average Precision (AP) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.488 Average Precision (AP) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.727 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 1 ] = 0.316 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 10 ] = 0.748 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.749 Average Recall (AR) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.650 Average Recall (AR) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.673 Average Recall (AR) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.758
So after one epoch of training, we obtain a COCO-style mAP of 60.6, and a mask mAP of 70.4.
After training for 10 epochs, I got the following metrics
IoU metric: bbox Average Precision (AP) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.799 Average Precision (AP) @[ IoU=0.50 | area= all | maxDets=100 ] = 0.969 Average Precision (AP) @[ IoU=0.75 | area= all | maxDets=100 ] = 0.935 Average Precision (AP) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.349 Average Precision (AP) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.592 Average Precision (AP) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.831 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 1 ] = 0.324 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 10 ] = 0.844 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.844 Average Recall (AR) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.400 Average Recall (AR) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.777 Average Recall (AR) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.870 IoU metric: segm Average Precision (AP) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.761 Average Precision (AP) @[ IoU=0.50 | area= all | maxDets=100 ] = 0.969 Average Precision (AP) @[ IoU=0.75 | area= all | maxDets=100 ] = 0.919 Average Precision (AP) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.341 Average Precision (AP) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.464 Average Precision (AP) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.788 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 1 ] = 0.303 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets= 10 ] = 0.799 Average Recall (AR) @[ IoU=0.50:0.95 | area= all | maxDets=100 ] = 0.799 Average Recall (AR) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.400 Average Recall (AR) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = 0.769 Average Recall (AR) @[ IoU=0.50:0.95 | area= large | maxDets=100 ] = 0.818
But what do the predictions look like? Let’s take one image in the dataset and verify
The trained model predicts 9 instances of person in this image, let’s see a couple of them:
The results look pretty good!
Wrapping up
In this tutorial, you have learned how to create your own training pipeline for instance segmentation models, on a custom dataset. For that, you wrote a torch.utils.data.Dataset class that returns the images and the ground truth boxes and segmentation masks. You also leveraged a Mask R-CNN model pre-trained on COCO train2017 in order to perform transfer learning on this new dataset.
For a more complete example, which includes multi-machine / multi-gpu training, check references/detection/train.py, which is present in the torchvision repo.
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