Project 02: Advanced Lane Finding

Overview of the Project

In this project, a software pipeline was constructed to identify the lane boundaries in a video. Numerical estimation of lane curvature and vehicle position can also be implemented using this pipeline. The main steps of this pipeline are the following:

• Compute the camera calibration matrix and distortion coefficients given a set of chessboard images.
• Apply a distortion correction to raw images.
• Use color transforms, gradients, etc., to create a thresholded binary image.
• Apply a perspective transform to rectify binary image ("birds-eye view").
• Detect lane pixels and fit to find the lane boundary.
• Determine the curvature of the lane and vehicle position with respect to center.
• Warp the detected lane boundaries back onto the original image.
• Output visual display of the lane boundaries and numerical estimation of lane curvature and vehicle position.

Camera Calibration

• Compute the camera calibration matrix and distortion coefficients given a set of chessboard images.

  In this step, images stored in the folder called cameral_cal were loaded and converted to grayscale first. As can be found, there are nine and six interior corners in the x direction(ny=9) and y direction(ny=6) in these calibration images, respctively. The function cv2.findChessboardCorners() was used to find these corner points in images. Then, the chessboard indices and corresponding pixel coordinates of those successfully found point were appended into array objpoints and imgpoints, respectively. The function cv2.calibrateCamera() was then utilized to calculate the calibration and distortion coefficients. The function cv2.undistort() was finnaly performed based on aboved calculated coefficients to undistort the images(see following image calibration1.jpg).

  

Pipeline Construction

The pipeline to identify the lane boundaries in single images is implemented by following steps:

Image undistortion

• Apply a distortion correction to raw images. See undistortion()

  The function cv2.undistort() was performed based on cameral calibration coefficients to undistort the raw images(See Undistortion subplot of images in Test on Images part)

Binary Image

• Use color transforms, gradients, etc., to create a thresholded binary image. See abs_sobel_thresh(), mag_thresh(), dir_threshold(), hls_select(), and apply_thresh() for details.

  A combination of gradient and color thresholds was used to generate a binary image. More specifically, gradient in x direction, magnitude gradient threshold, direction threshold and S channel(from HLS) threshold.

  Gradient in x direction: [20, 100]
  Gradient magnitude: [30, 170]
  Direction: [0, np.pi/2]
  S channel in HLS space: [90, 255]
  L channel in LUV space: [215, 255]
  B channel in Lab space: [145, 200]
  

  Combined in following form:

  [((gradx == 1) & (mag_binary == 1) & (dir_binary == 1)) | ((color_binary_S == 1) & (color_binary_luvL == 1)) | (color_binary_labB == 1)] = 1 

  See Binary subplot of images in Test on Images part

Region of Interest(ROI)

• Selection of region of interest. See ROI() for details.

  Region selected in current pipeline:

   vertices = np.array([[(100,720),(545,470),(755,470),(1290,720)]],dtype=np.int32)
  

  See ROI subplot of images in Test on Images part

Perspective transform

• Apply a perspective transform to rectify binary image ("birds-eye view"). See bird_view()

  The code for my perspective transform includes a function called warp(). The warp() function takes as inputs an image, and runs cv2.warpPerspective() using the follwing source (src) and destination (dst) points. I chose the hardcode the source and destination points in the following manner:

  src = np.float32(
  [[(img_size[0] / 2) - 62, img_size[1] / 2 + 100],
  [((img_size[0] / 6) - 10), img_size[1]],
  [(img_size[0] * 5 / 6) + 60, img_size[1]],
  [(img_size[0] / 2 + 62), img_size[1] / 2 + 100]])
  
  dst = np.float32(
  [[(img_size[0] / 4), 0],
  [(img_size[0] / 4), img_size[1]],
  [(img_size[0] * 3 / 4), img_size[1]],
  [(img_size[0] * 3 / 4), 0]])

  This resulted in the following source and destination points:

  Source	Destination
  578, 460	320, 0
  203, 720	320, 720
  1127, 720	960, 720
  702, 460	960, 0

  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. (See Bird_view subplot of images in Test on Images part)

Lane boundary detection and fitting

• The function find_lane_pixels() uses a slide window to find the lane line in pixles. After get the lane line pixels, these pixels was grouped into left and right set.
• The function search_around_poly() searches around previouly detected lane line because consecutive frames are likely to have lane lines in roughly similar positions. We search around a margin of 80 pixels of the previously detected lane lines.
• Fit polynomials(degree=2) to both left and right set of points found in the previous step.

Lane curvature determination

• Determine the curvature of the lane and vehicle position with respect to center. See measure_curvature_real() and measure_vehicle_offset.

  The offset of the vehicle was calculated by fining the center_of_lane values of the left and right fitted lines and then took the difference from the center subplot of the image center_of_image, converted to meters: offset = (center_of_image - center_of_lane)*xm_per_pix. If the offset value is negative, the vehicle is right of the lane center, else left (See Final subplot of images in Test on Images part).

Output visual display of the lane boundaries and information

• Warp the detected lane boundaries back onto the original image. See project_fit_lane(). (See Line subplot of images in Test on Images part).
• Output visual display of the lane boundaries and numerical estimation of lane curvature and vehicle position. See project_fit_lane_info(). (See Final subplot of images in Test on Images part).

Test on Images

straight_lines1_undis.jpg straight_lines1.jpg straight_lines2.jpg test1.jpg test2.jpg test3.jpg test4.jpg test5.jpg ctest6.jpg

Test on Videos

project_video.mp4 link to result

Discussion

The approach adopted in current pipeline worked pretty well for project_video.mp4. However, it fails to detect the lane boundary correctly in challenge_video.mp4 and harder_challenge_video.mp4. In challenge_video.mp4, the pipeline had some problems in case the surface of the roads changes, there is much shadow on the road, lane lines are missing, etc. Besides, the current pipeline might struggle with lines in the middle of the lane between the light and dark asphalt surface. In harder_challenge_video.mp4, the pipeline fails to follow the lane boundary since the curature and slope of the road changes fast.

Further Work

However, there are still room for improment on this project. Following aspects can be taken into consideration in near future:

• There might be better values for ROI and thresholds during image pre-preocssing. More finetuning might produce even better results.
• Consider provious polynoms that were fitted onto the lane lines in order to smoothen lane line detection. Average over a number of frames to get better results since the shape and info of lanes changes quite fast.
• Add sanity checks that kick out values that do nor make sense. In that case the previous polynoms could be reused.