Content tagged computer-vision

What is it about?

Semantic segmentation is a process of dividing an image into sets of pixels sharing similar properties and assigning to each of these sets one of the pre-defined labels. Ideally, you would like to get a picture such as the one below. It's a result of blending color-coded class labels with the original image. This sample comes from the CityScapes dataset.

Segmented Image
Segmented Image

Segmentation Classes
Segmentation Classes

How is it done?

Figuring out object boundaries in an image is hard. There's a variety of "classical" approaches taking into account colors and gradients that obtained encouraging results, see this paper by Shi and Malik for example. However, in 2015 and 2016, Long, Shelhamer, and Darrell presented a method using Fully Convolutional Networks that significantly improved the accuracy (the mean intersection over union metric) and the inference speed. My goal was to replicate their architecture and use it to segment road scenes.

A fully convolutional network differs from a regular convolutional network in the fact that it has the final fully-connected classifier stripped off. Its goal is to take an image as an input and produce an equally-sized output in which each pixel is represented by a softmax distribution describing the probability of this pixel belonging to a given class. I took this picture from one of the papers mentioned above:

Fully Convolutional Network
Fully Convolutional Network

For the results presented in this post, I used the pre-trained VGG16 network provided by Udacity for the beta test of their Advanced Deep Learning Capstone. I took layers 3, 4, and 7 and combined them in the manner described in the picture below, which, again, is taken from one of the papers by Long et al.

Upscaling and merging
Upscaling and merging

First, I used a 1x1 convolutions on top of each extracted layer to act as a local classifier. After doing that, these partial results are still 32, 16, and 8 times smaller than the input image, so I needed to upsample them (see below). Finally, I used a weighted addition to obtain the result. The authors of the original paper report that without weighting the learning process diverges.

Learnable Upsampling

Upsampling is done by applying a process called transposed convolution. I will not describe it here because this post over at cv-tricks.com does a great job of doing that. I will just say that transposed convolutions (just like the regular ones) use learnable weights to produce output. The trick here is the initialization of those weights. You don't use the truncated normal distribution, but you initialize the weights in such a way that the convolution operation performs a bilinear interpolation. It's easy and interesting to test whether the implementation works correctly. When fed an image, it should produce the same image but n times larger.

 1 img = cv2.imread(sys.argv[1])
 2 print('Original size:', img.shape)
 3 
 4 imgs = np.zeros([1, *img.shape], dtype=np.float32)
 5 imgs[0,:,:,:] = img
 6 
 7 img_input = tf.placeholder(tf.float32, [None, *img.shape])
 8 upscale = upsample(img_input, 3, 8, 'upscaled')
 9 
10 with tf.Session() as sess:
11     sess.run(tf.global_variables_initializer())
12     upscaled = sess.run(upscale, feed_dict={img_input: imgs})
13 
14 print('Upscaled:', upscaled.shape[1:])
15 cv2.imwrite(sys.argv[2], upscaled[0,:, :, :])

Where upsample is defined here.

Datasets

I was mainly interested in road scenes, so I played with the KITTI Road and CityScapes datasets. The first one has 289 training images with two labels (road/not road) and 290 testing samples. The second one has 2975 training, 500 validation, and 1525 testing pictures taken while driving around large German cities. It has fine-grained annotations for 29 classes (including "unlabeled" and "dynamic"). The annotations are color-based and look like the picture below.

Picture Labels
Picture Labels

Even though I concentrated on those two datasets, both the training and the inference software is generic and can handle any pixel-labeled dataset. All you need to do is to create a new source_xxxxxx.py file defining your custom samples. The definition is a class that contains seven attributes:

  • image_size - self-evident, both horizontal and vertical dimensions need to be divisible by 32
  • num_classes - number of classes that the model is supposed to handle
  • label_colors - a dictionary mapping a class number to a color; used for blending of the classification results with input image
  • num_training - number of training samples
  • num_validation - number of validation samples
  • train_generator - a generator producing training batches
  • valid_generator - a generator producing validation batches

See source_kitti.py or source_cityscapes.py for a concrete example. The training script picks the source based on the value of the --data-source parameter.

Normalization

Typically, you would normalize the input dataset such that its mean is at zero and its standard deviation is at one. It significantly improves convergence of the gradient optimization. In the case of the VGG model, the authors just zeroed the mean without scaling the variance (see section 2.1 of the paper). Assuming that the model was trained on the ImageNet dataset, the mean values for each channel are muR = 123.68, muG = 116.779, muB = 103.939. The pre-trained model provided by Udacity already has a pre-processing layer handling these constants. Judging from the way it does it, it expects plain BGR scaled between 0 and 255 as input.

Label Validation

Since the network outputs softmaxed logits for each pixel, the training labels need to be one-hot encoded. According to the TensorFlow documentation, each row of labels needs to be a proper probability distribution. Otherwise, the gradient calculation will be incorrect and the whole model will diverge. So, you need to make sure that you're never in a situation where you have all zeros or multiple ones in your label vector. I have made this mistake so many time that I decided to write a checker script for my data source modules. It produces examples of training images blended with their pixel labels to check if the color maps have been defined correctly. It also checks every pixel in every sample to see if the label rows are indeed valid. See here for the source.

Initialization of variables

Initialization of variables is a bit of a pain in TensorFlow. You can use the global initializer if you create and train your model from scratch. However, in the case when you want to do transfer learning - load a pre-trained model and extend it - there seems to be no convenient way to initialize only the variables that you created. I ended up doing acrobatics like this:

1 uninit_vars    = []
2 uninit_tensors = []
3 for var in tf.global_variables():
4     uninit_vars.append(var)
5     uninit_tensors.append(tf.is_variable_initialized(var))
6 uninit_bools = sess.run(uninit_tensors)
7 uninit = zip(uninit_bools, uninit_vars)
8 uninit = [var for init, var in uninit if not init]
9 sess.run(tf.variables_initializer(uninit))

Training

For training purposes, I reshaped both labels and logits in such a way that I ended up with 2D tensors for both. I then used tf.nn.softmax_cross_entropy_with_logits as a measure of loss and used AdamOptimizer with a learning rate of 0.0001 to minimize it. The model trained on the KITTI dataset for 500 epochs - 14 seconds per epoch on my GeForce GTX 1080 Ti. The CityScapes dataset took 150 epochs to train - 9.5 minutes per epoch on my GeForce vs. 25 minutes per epoch on an AWS P2 instance. The model exhibited some overfitting. However, the visual results seemed tighter the more it trained. In the picture below the top row contains the ground truth, the bottom one contains the inference results (TensorBoard rocks! :).

CityScapes Validation Examples
CityScapes Validation Examples

CityScapes Validation Loss
CityScapes Validation Loss

CityScapes Training Loss
CityScapes Training Loss

Results

The inference (including image processing) takes 80 milliseconds per image on average for CityScapes and 27 milliseconds for KITTI. Here are some examples from both datasets. The model seems to be able to distinguish a pedestrian from a bike rider with some degree of accuracy, which is pretty impressive!

CityScapes Example #1
CityScapes Example #1

CityScapes Example #2
CityScapes Example #2

KITTI Example #1
KITTI Example #1

KITTI Example #2
KITTI Example #2

Go here for the full code.

The challenge

Here's another cool project I have done as a part of the Udacity's self-driving car program. There were two problems solve. The first one was to find the lane lines and compute some of their properties. The second one was to detect and draw bounding boxes around nearby vehicles. Here's the result I got:

Detecting lane lines and vehicules

Detecting lanes

The first thing I do after correcting for camera lens distortion is applying a combination of Sobel operators and color thresholding to get an image of edges. This operation makes lines more pronounced and therefore much easier to detect.

Edges
Edges

I then get a birds-eye view of the scene by applying a perspective transform and produce a histogram of all the white pixels located in the bottom half of the image. The peaks in this histogram indicate the presence of mostly vertical lines, which is what we're looking for. I detect all these lines by using a sliding window search. I start at the bottom of the image and move towards the top adjusting the horizontal position of each successive window to the average of the x coordinate of all the pixels contained in the previous one. Finally, I fit a parabola to all these pixels. Out of all the candidates detected this way, I select a pair that is the closest to being parallel and is roughly in the place where a lane line would be expected.

The orange area in the picture below visualizes the histogram, and the red boxes with blue numbers in them indicate the positions of the peaks found by the find_peaks_cwt function from scipy.

Bird's eye view - histogram search
Bird's eye view - histogram search

Once I have found the lanes in one video frame, locating them in the next one is much simpler - their position did not change by very much. I just take all the pixels from a close vicinity of the previous detection and fit a new polynomial to them. The green area in the image below denotes the search range, and the blue lines are the newly fitted polynomials.

Bird's eye view - vicinity search
Bird's eye view - vicinity search

I then use the equations of the parabolas to calculate the curvature. The program that produced the video above uses cross-frame averaging to make the lines smoother and to vet new detections in successive video frames.

Vehicle detection

I detect cars by dividing the image into a bunch of overlapping tiles of varying sizes and running each tile through a classifier to check if it contains a car or a fraction of a car. In this particular solution, I used a linear support vector machine (LinearSVC from sklearn). I also wrapped it in a CalibratedClassifierCV to get a measure of confidence. I rejected predictions of cars that were less than 85% certain. The classifier trained on data harvested from the GTI, KITTI, and Udacity datasets from which I collected around 25 times more background samples than cars to limit the occurrences of false-positive detections.

As far as image features are concerned, I use only Histograms of Oriented Gradients with parameters that are essentially the same as the ones presented in this paper dealing with detection of humans. I used OpenCV's HOGDescriptor to extract the HOGs. The reason for this is that it can compute the gradients taking into account all of the color channels. See here. It is the capability that other libraries typically lack limiting you to a form of grayscale. The training set consists of roughly 2M images of 64 by 64 pixels.

Tiles containing cars
Tiles containing cars

Since the samples the classifier trains on contain pictures of fractions of cars, the same car is usually detected multiple times in overlapping tiles. Also, the types of background differ quite a bit, and it's hard to find images of all the possible things that are not cars. Therefore false-positives are quite frequent. To combat these problems, I use heat maps that are averaged across five video frames. Every pixel that has less than three detections on average per frame is rejected as a false positive.

Heat map
Heat map

I then use OpenCV's connectedComponentsWithStats to find connected components and get centroids and bounding boxes for the detections. The centroids are used to track the objects across frames and smooth the bounding boxes by averaging them with 12 previous frames. To further reject false-positives, an object needs to be classified as a car in at least 6 out of 12 consecutive frames.

Conclusions

The topic is pretty fascinating and the results I got could be significantly improved by:

  • employing smarter sliding window algorithms (i.e., having momentum) to better detect dashed lines that are substantially curved
  • finding better ways to do perspective transforms
  • using a better classifier for cars (a deep neural network perhaps)
  • using techniques like YOLO
  • using something smarter than strongly connected components to distinguish overlapping detections of different vehicles - mean shift clustering comes to mind
  • making performance improvements here and there (use C++, parallelize video processing and so on)

I learned a lot of computer vision techniques and had plenty of fun doing this project. I also spent a lot of time reading the code of OpenCV. It has a lot of great tutorials, but its API documentation is lacking.

Video Link

Pretty interesting talk on how to prevent squirrels from stealing bird food using python and computer vision.

Steps to recognize a squirrel on a picture:

  • Subtract background.
    • Compute average value of each pixel over time to build a background profile.
  • Detect blobs.
  • Discriminate blobs. Distinguish between squirrels from other things. The author used support vector machines.
    • Blob size
    • Color histograms
    • Entropy detection (squirrel tail)

Other interesting stuff mentioned: