Mask R-CNN

Like Faster R-CNN, but for instance segmentation

Mask R-CNN is the most used architecture for instance segmentation. It is almost built the same way as Faster R-CNN. The major difference: there is an extra head that predicts masks inside the predicted bounding boxes.

Also, the authors replaced the RoI pool layer with the RoI align layer. RoI pool mappings are often a bit noisy. The difference is so small that it is negligible for object detection, but not when you want to create pixel-perfect masks for instance segmentation.

Image taken from the original paper

Hyperparameters

Typically, the following hyperparameters are tweaked when using Faster R-CNN:

โ€ŒBackbone network

โ€ŒSpecifying the architecture for the network on which Faster R-CNN is built

Mostly, the backbone network is a ResNet variation.

โ€‹IoU thresholds for RPNโ€‹

These thresholds are used to decide if an anchor box generated contains an object or is part of the background.

Everything that is above the upper IoU threshold of the proposed anchor box and ground truth label will be classified as an object and forwarded. Everything below the lower threshold will be classified as background and the network will be penalized. For all the anchor boxes with an IoU between the thresholds, we're not sure if it's for- or background and we'll just ignore them.

Empirically, setting the lower bound to 0.3 and the upper to 0.7 leads to robust results.

Number of convolution filters in the ROI box headโ€‹

โ€ŒHow many convolution filters the final layer to make the classification contains. To a certain degree, increasing the number of filters will enable the network to learn more complex features, but the effect vanishes if you add too many filters and the network will perform worse (see the original ResNet paper to understand why you cannot endlessly chain convolution filters).

A good default value is 4 conv filters.

Number of fully connected layers in the ROI box headโ€‹

โ€ŒHow many fully connected layers (FC) the last part of the network contains. Increasing the number of FCs can increase performance for a computational cost, but you might overfit the sub-network if you add too many.

Often, 2 FCs are used as starting point.

โ€‹NMS number of proposals

โ€ŒPre NMSโ€‹

โ€ŒThe maximum of proposals that are taken into consideration by NMS. The proposals are sorted descending after confidence and only the ones with the highest confidence are chosen.

โ€ŒPost NMSโ€‹

โ€ŒThe maximum of proposals that will be forwarded to the ROI box head. Again, the proposals are sorted descending after confidence and only the ones with the highest confidence are chosen.

โ€ŒConfig for training

โ€ŒLow numbers of NMS proposals in training will result in a lower recall, but higher precision. Vice versa.

The default for post NMS number of proposals in training is 1000, pre NMS 4000.

โ€ŒConfig for testing

โ€ŒHere, the number of NMS proposals for the simple forward pass, e.g., inference, is defined. Less NMS proposals will increase inference speed, but higher numbers yield greater performance.

If you don't need super-fast inference, a good default is 1000 for post NMS number of proposals and 2000 for pre NMS.

โ€‹

Code implementation

PyTorch
PyTorch
# import necessary libraries
from PIL import Image
import matplotlib.pyplot as plt
import torch
import torchvision.transforms as T
import torchvision
import torch
import numpy as np
import cv2
import random
import time
import os
โ€‹
โ€‹
โ€‹
โ€‹
# These are the classes that are available in the COCO-Dataset
COCO_INSTANCE_CATEGORY_NAMES = [
'__background__', 'person', 'bicycle', 'car', 'motorcycle', 'airplane', 'bus',
'train', 'truck', 'boat', 'traffic light', 'fire hydrant', 'N/A', 'stop sign',
'parking meter', 'bench', 'bird', 'cat', 'dog', 'horse', 'sheep', 'cow',
'elephant', 'bear', 'zebra', 'giraffe', 'N/A', 'backpack', 'umbrella', 'N/A', 'N/A',
'handbag', 'tie', 'suitcase', 'frisbee', 'skis', 'snowboard', 'sports ball',
'kite', 'baseball bat', 'baseball glove', 'skateboard', 'surfboard', 'tennis racket',
'bottle', 'N/A', 'wine glass', 'cup', 'fork', 'knife', 'spoon', 'bowl',
'banana', 'apple', 'sandwich', 'orange', 'broccoli', 'carrot', 'hot dog', 'pizza',
'donut', 'cake', 'chair', 'couch', 'potted plant', 'bed', 'N/A', 'dining table',
'N/A', 'N/A', 'toilet', 'N/A', 'tv', 'laptop', 'mouse', 'remote', 'keyboard', 'cell phone',
'microwave', 'oven', 'toaster', 'sink', 'refrigerator', 'N/A', 'book',
'clock', 'vase', 'scissors', 'teddy bear', 'hair drier', 'toothbrush'
]
โ€‹
# get the pretrained model from torchvision.models
# Note: pretrained=True will get the pretrained weights for the model.
# model.eval() to use the model for inference
model = torchvision.models.detection.maskrcnn_resnet50_fpn(pretrained=True)
model.eval()
โ€‹
# random_colour_masks() function to fill the predicted-mask with colors
# get_predictions() to return the final predictions from the model
# instance_segmentation_api() to overlay the colored mask over the original image and plot it
โ€‹
def random_colour_masks(image):
"""
random_colour_masks
parameters:
- image - predicted masks
method:
- the masks of each predicted object is given random colour for visualization
"""
colours = [[0, 255, 0],[0, 0, 255],[255, 0, 0],[0, 255, 255],[255, 255, 0],[255, 0, 255],[80, 70, 180],[250, 80, 190],[245, 145, 50],[70, 150, 250],[50, 190, 190]]
r = np.zeros_like(image).astype(np.uint8)
g = np.zeros_like(image).astype(np.uint8)
b = np.zeros_like(image).astype(np.uint8)
r[image == 1], g[image == 1], b[image == 1] = colours[random.randrange(0,10)]
coloured_mask = np.stack([r, g, b], axis=2)
return coloured_mask
โ€‹
def get_prediction(img_path, threshold):
"""
get_prediction
parameters:
- img_path - path of the input image
method:
- Image is obtained from the image path
- the image is converted to image tensor using PyTorch's Transforms
- image is passed through the model to get the predictions
- masks, classes and bounding boxes are obtained from the model and soft masks are made binary(0 or 1) on masks
ie: eg. segment of cat is made 1 and rest of the image is made 0
โ€‹
"""
img = Image.open(img_path)
transform = T.Compose([T.ToTensor()])
img = transform(img)
pred = model([img])
pred_score = list(pred[0]['scores'].detach().numpy())
pred_t = [pred_score.index(x) for x in pred_score if x>threshold][-1]
masks = (pred[0]['masks']>0.5).squeeze().detach().cpu().numpy()
pred_class = [COCO_INSTANCE_CATEGORY_NAMES[i] for i in list(pred[0]['labels'].numpy())]
pred_boxes = [[(i[0], i[1]), (i[2], i[3])] for i in list(pred[0]['boxes'].detach().numpy())]
masks = masks[:pred_t+1]
pred_boxes = pred_boxes[:pred_t+1]
pred_class = pred_class[:pred_t+1]
return masks, pred_boxes, pred_class
โ€‹
โ€‹
def instance_segmentation_api(img_path, threshold=0.5, rect_th=3, text_size=3, text_th=3):
"""
instance_segmentation_api
parameters:
- img_path - path to input image
method:
- prediction is obtained by get_prediction
- each mask is given random color
- each mask is added to the image in the ration 1:0.8 with opencv
- final output is displayed
"""
masks, boxes, pred_cls = get_prediction(img_path, threshold)
img = cv2.imread(img_path)
img = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)
for i in range(len(masks)):
rgb_mask = random_colour_masks(masks[i])
img = cv2.addWeighted(img, 1, rgb_mask, 0.5, 0)
cv2.rectangle(img, boxes[i][0], boxes[i][1],color=(0, 255, 0), thickness=rect_th)
cv2.putText(img,pred_cls[i], boxes[i][0], cv2.FONT_HERSHEY_SIMPLEX, text_size, (0,255,0),thickness=text_th)
plt.figure(figsize=(20,30))
plt.imshow(img)
plt.xticks([])
plt.yticks([])
plt.show()
# We will use the following colors to fill the pixels
colours = [[0, 255, 0],
[0, 0, 255],
[255, 0, 0],
[0, 255, 255],
[255, 255, 0],
[255, 0, 255],
[80, 70, 180],
[250, 80, 190],
[245, 145, 50],
[70, 150, 250],
[50, 190, 190]]
โ€‹
#Testing on Image
โ€‹
instance_segmentation_api('/content/Hasty_Founders.jpg', 0.75)

In Model playground, after creating a split for Instance segmentation/Object detection tweak the Hyper-parameters of Mask R-CNN:

  • Backbone network: The Backbone is the Conv Net architecture that is to be used in the first step of Mask R-CNN. Here it's ResNet50.

  • IoU Thresholds: IoU thresholds for considering objects as background or foreground. Objects with IoU in between are ignored. The closer the predicted bounding box values are to the actual bounding box values the greater the intersection, and the greater the IoU value.

  • Number of Fully Connected Layers: Number of hidden layers in the box predictor

NMS: In Object Detection, the objects in the image can be of different sizes and shapes, and to capture each of these perfectly, the algorithms that are used create multiple bounding boxes. (left image). Ideally, for each object in the image, we must have a single bounding box. To select the best bounding box, from the multiple predicted bounding boxes, these object detection algorithms use non-max suppression. This technique is used to โ€œsuppressโ€ the less likely bounding boxes and keep only the best one. It takes into account the IoU Thresholds

  • Post NMS parameters: This step is different for training and testing. In training, we select top N (default = 2000) proposals based on their corresponding objectness score from all proposals of all images in an entire batch. In testing, N proposals are selected for each image in the batch and kept separately.

  • Pre NMS parameters: Selection of top k anchors based on their corresponding objectness score.

  • The depth of the resnet model: The depth variant of resnet to use as the backbone feature extractor

  • Weights: Weights to use for model initialization (R50-FPN COCO)

  • Pooler Resolution: Size to pool proposals before feeding them to the box predictor. ROI Align/Pool gives a constant output of P X P irrespective of the proposal size, where P is Pooler Resolution.

  • Pooler Sampling Ratio: Number of bins for each sampling point

Further resources