# Road Lane Line Detection using Computer Vision models

A tutorial on how to implement a computer vision data pipeline for road lane detection used by self-driving cars.

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*Fig. The hough transform identifies the lane lines in the image*

**Step 5: Extrapolate the individual, small lines and construct the left and right lane lines**

The hough transform gives us small lines based on the intersections in hough space. Now, we can take this information and construct a global left lane line and a right lane line.

We do this by separating the small lines into two groups, one with a positive gradient and the other with a negative gradient. Due to the depth of the camera’s view, the lane lines slant towards each other as we go further in depth, so that should have the opposite gradients.

We then take the average gradient and intercept values for each group and construct our global lane lines from there. The lane lines are then extrapolated to the edge detected pixel with the minimum y-axis coordinate and the pixel with the maximum y-axis coordinate.

To implement this algorithm, we simply change the draw_lines function stated before to extrapolate the lines.

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def draw_lines(img, lines, color=[255, 0, 0], thickness=2): """ This function draws `lines` with `color` and `thickness`. """ imshape = img.shape # these variables represent the y-axis coordinates to which # the line will be extrapolated to ymin_global = img.shape[0] ymax_global = img.shape[0] # left lane line variables all_left_grad = [] all_left_y = [] all_left_x = [] # right lane line variables all_right_grad = [] all_right_y = [] all_right_x = [] for line in lines: for x1,y1,x2,y2 in line: gradient, intercept = np.polyfit((x1,x2), (y1,y2), 1) ymin_global = min(min(y1, y2), ymin_global) if (gradient > 0): all_left_grad += [gradient] all_left_y += [y1, y2] all_left_x += [x1, x2] else: all_right_grad += [gradient] all_right_y += [y1, y2] all_right_x += [x1, x2] left_mean_grad = np.mean(all_left_grad) left_y_mean = np.mean(all_left_y) left_x_mean = np.mean(all_left_x) left_intercept = left_y_mean - (left_mean_grad * left_x_mean) right_mean_grad = np.mean(all_right_grad) right_y_mean = np.mean(all_right_y) right_x_mean = np.mean(all_right_x) right_intercept = right_y_mean - (right_mean_grad * right_x_mean) # Make sure we have some points in each lane line category if ((len(all_left_grad) > 0) and (len(all_right_grad) > 0)): upper_left_x = int((ymin_global - left_intercept) / left_mean_grad) lower_left_x = int((ymax_global - left_intercept) / left_mean_grad) upper_right_x = int((ymin_global - right_intercept) / right_mean_grad) lower_right_x = int((ymax_global - right_intercept) / right_mean_grad) cv2.line(img, (upper_left_x, ymin_global), (lower_left_x, ymax_global), color, thickness) cv2.line(img, (upper_right_x, ymin_global), (lower_right_x, ymax_global), color, thickness)

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*Fig. Extrapolated lane lines*

**Step 6: Add the extrapolated lines to the input image**

We then overlay the extrapolated lines to the input image. We do this by adding a weight value to the original image based on the detected lane line coordinate.

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def weighted_img(img, initial_img, α=0.8, β=1., λ=0.): """ `img` is the output of the hough_lines(), An image with lines drawn on it. Should be a blank image (all black) with lines drawn on it. `initial_img` should be the image before any processing. The result image is computed as follows: initial_img * α + img * β + λ NOTE: initial_img and img must be the same shape! """ return cv2.addWeighted(initial_img, α, img, β, λ) # outline the input image colored_image = weighted_img(houged, image)

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*Fig. Line lanes overlaid on the input image*

**Step 7: Add pipeline to video**

Once we have designed and implemented the entire car lane detection pipeline, we apply the transformation frame by frame. We need a video of car driving on lane lines to do this.

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from moviepy.editor import VideoFileClip from IPython.display import HTML def process_image(image): # grayscale the image grayscaled = grayscale(image) # apply gaussian blur kernelSize = 5 gaussianBlur = gaussian_blur(grayscaled, kernelSize) # canny minThreshold = 100 maxThreshold = 200 edgeDetectedImage = canny(gaussianBlur, minThreshold, maxThreshold) # apply mask lowerLeftPoint = [130, 540] upperLeftPoint = [410, 350] upperRightPoint = [570, 350] lowerRightPoint = [915, 540] pts = np.array([[lowerLeftPoint, upperLeftPoint, upperRightPoint, lowerRightPoint]], dtype=np.int32) masked_image = region_of_interest(edgeDetectedImage, pts) # hough lines rho = 1 theta = np.pi/180 threshold = 30 min_line_len = 20 max_line_gap = 20 houged = hough_lines(masked_image, rho, theta, threshold, min_line_len, max_line_gap) # outline the input image colored_image = weighted_img(houged, image) return colored_image output = 'car_lane_detection.mp4' clip1 = VideoFileClip("insert_car_lane_video.mp4") white_clip = clip1.fl_image(process_image) %time white_clip.write_videofile(output, audio=False)

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And there you have it: an end to end implementation of car lane detection in python!

Original. Reposted with permission.

**Bio: **Vijay Ramakrishnan is a Machine Learning engineer at Mindmeld Inc, a conversational AI company that Cisco acquired in 2017. Former Project Manager at Qualcomm and Data engineer at Google. Loves to scale machine learning systems, develop self-driving technologies and conversational AI products.

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