Self Driving Cars: Lane Finding

The goal of this project is to write a software pipeline to identify lane boundaries. As input we take a video stream from a forward-facing camera mounted on the front of a car, and we will output a annotated video.

Step 1: Camera Calibration and Distortion Correction

The first step in the project is to remove any distortion from the images by calculating the camera calibration matrix and distortion coefficients using a series of images of a chessboard.

I start by preparing “object points”, which will be the (x, y, z) coordinates of the chessboard corners in the world. Here I am assuming the chessboard is fixed on the (x, y) plane at z=0, such that the object points are the same for each calibration image. Thus, objp is just a replicated array of coordinates, and objpoints will be appended with a copy of it every time I successfully detect all chessboard corners in a test image. imgpoints will be appended with the (x, y) pixel position of each of the corners in the image plane with each successful chessboard detection.

I then used the output objpoints and imgpoints to compute the camera calibration and distortion coefficients using the cv2.calibrateCamera() function. I applied this distortion correction to the test image using the cv2.undistort() function and obtained this result:

Focus on the edges of the images to see the effect of the distortion correction.

Step 2: Perspective Transform

In this step a perspective transform to convert the binary image into “birds-eye view” is applied.

The perspective_transform() function takes as inputs an image (img) and converts it tos bird view. We can invert this transformation by setting the inverse parameter to True. To make the perspective transform work I had to hardcode source (src_coords) and destination (dst_coords) points. I chose to hard code the source and destination points in the following manner:

src_coords = np.float32([[490, 482],[810, 482], [1250, 720],[40, 720]]) dst_coords = np.float32([[40, 0], [1200, 0], [1200, 720],[40, 720]])

I verified that my perspective transform was working as expected by drawing the src and dst points onto a test image and its warped counterpart to verify that the lines appear parallel in the warped image.

Step 3: Apply Color and Gradient Thresholds

I used a combination of color and gradient thresholds to generate a binary image.

First I made use of the Sobel operator to detect edges. The Sobel operator is at the heart of the Canny edge detection algorithm. Applying the Sobel operator to an image is a way of taking the derivative of the image in the x or y direction. For our lane detection algorithm I took the gradient in the x direction, because it emphasizes edges closer to vertical. Lane lines tend to be vertical. This was implemented with OpenCV’s cv2.Sobel function, which works with a single color channel. So I had to convert the images to grayscale beforehand.

After that I applied a color threshold. Therefore I converted the image data to HLS color space and separate the S channel. In HLS color space we can represent color independent of any change in brightness. In our test images the S channel (Saturation) did best at picking up lanes. I did not have the time to further investigate this. But better results could be achieved by combining multiple color channels.

As output I created a binary images that combines the color and sobel threshold. Here is in example output:

Step 4: Identify Lane Pixels and Fit Polynomial

In this step we will try to extract the actual lane pixels.

After all our preprocessing we should have a binary image where the lane lines stand out clearly. However, we still need to decide explicitly which pixels are part of the lines and which belong to the left line and which belong to the right line.

I first take a histogram along all the columns in the lower half of the image. The result looks like this:

With this histogram I am adding up the pixel values along each column in the image. In my thresholded binary image, pixels are either 0 or 1, so the two most prominent peaks in this histogram will be good indicators of the x-position of the base of the lane lines. I can use that as a starting point for where to search for the lines. From that point, I can use a sliding window, placed around the line centers, to find and follow the lines up to the top of the frame.

Once I have identified pixels for left and right lanes I can use the Numpy polyfit() function to fit a polynomial equation to each lane.

left_fit = np.polyfit(lefty, leftx, 2) right_fit = np.polyfit(righty, rightx, 2)

Step 5: Radius of Curvature and Vehicle Position

Here I calculate the radius of curvature for each lane and the vehicle position with respect to the center of the lane

I created a helper function radius_of_curvature(xvals, yvals) to calculate the radius of curvature. Inside the function first I fit a circle to the my lane pixel values using np.polyfit after that I use this formula to calculate the radius

curverad = ((1 + (2*fit_cr[0]*np.max(yvals) + fit_cr[1])**2)**1.5) / np.absolute(2*fit_cr[0])

Take a closer look at this awesome tutorial to get some intuition for the formula. We calculate the radius of curvature for both lanes and take the average as result.

After that I checked weather my result makes sense. Therfore I compared my result with the U.S. government specifications for highway curvature. Effectively, this checks to see that the calculated lane is not turning faster than we would expect.

Next I calculate where the lane intercects the bottom of the image. Check the get_interceptss(polynomial) helper function for this. We already have the fitted a polynomial to our lane lines so all we have to to is fill in max/min y values to get the bottom/top intercepts. With the intercepts we can calulate the postion of the vehicle. We assume the camera is mounted on the center of the car and does not move. So we can simply add the intercept x values and devide by 2. After converting from pixel space to meter space we have our final result.

When working with the video stream I average the intercept values to get smoother lane detection.

I fill the space between the left and right lane line with cv2.fillPoly to get a better visualisation. Here is an example result:

Discussion

My pipeline performs pretty well on the test data but does fail on the challenge data. There is room for improvement in the color thresholding step. I could combine information from several color channels to create a better filter. This way my algorithm would be more robust to shadows and other dark lines on the road