How to merge lines after HoughLinesP?

My task is to find coordinates of lines (startX, startY, endX, endY) and rectangles (4 lines). Here is input file:

I use the next code:

img = cv.imread(image_src)
gray = cv.cvtColor(img,cv.COLOR_BGR2GRAY)
ret, thresh1 = cv.threshold(gray,127,255,cv.THRESH_BINARY)

edges = cv.Canny(thresh1,50,150,apertureSize = 3)

minLineLength = 100
maxLineGap = 10
lines = cv.HoughLinesP(edges,1,np.pi/180,10,minLineLength,maxLineGap)
print(len(lines))
for line in lines:
    cv.line(img,(line[0][0],line[0][1]),(line[0][2],line[0][3]),(0,0,255),6)

I get the next results:

From the last image you can see big amount of small red lines.

Questions:

1. What is the best way to merge small lines?
2. Why there are a lot of small portions that are not detected by HoughLinesP?

Answer

I have finally completed the pipeline:

1. fixed incorrect parameters (as were suggested by Dan)
2. developed my own ‘merging line segments’ algorithm. I had bad results when I implemented TAVARES and PADILHA algorithm (as were suggested by Andrew).
3. I have skipped Canny and got better results (as were suggested by Alexander)

Please find the code and results:

def get_lines(lines_in):
    if cv.__version__ < '3.0':
        return lines_in[0]
    return [l[0] for l in lines_in]


def process_lines(image_src):
    img = mpimg.imread(image_src)
    gray = cv.cvtColor(img,cv.COLOR_BGR2GRAY)
    
    ret, thresh1 = cv.threshold(gray,127,255,cv.THRESH_BINARY)
    
    thresh1 = cv.bitwise_not(thresh1)
    
    edges = cv.Canny(thresh1, threshold1=50, threshold2=200, apertureSize = 3)

    lines = cv.HoughLinesP(thresh1, rho=1, theta=np.pi/180, threshold=50,
                            minLineLength=50, maxLineGap=30)

    # l[0] - line; l[1] - angle
    for line in get_lines(lines):
        leftx, boty, rightx, topy = line
        cv.line(img, (leftx, boty), (rightx,topy), (0,0,255), 6) 
        
    # merge lines
        
    #------------------
    # prepare
    _lines = []
    for _line in get_lines(lines):
        _lines.append([(_line[0], _line[1]),(_line[2], _line[3])])
        
    # sort
    _lines_x = []
    _lines_y = []
    for line_i in _lines:
        orientation_i = math.atan2((line_i[0][1]-line_i[1][1]),(line_i[0][0]-line_i[1][0]))
        if (abs(math.degrees(orientation_i)) > 45) and abs(math.degrees(orientation_i)) < (90+45):
            _lines_y.append(line_i)
        else:
            _lines_x.append(line_i)
            
    _lines_x = sorted(_lines_x, key=lambda _line: _line[0][0])
    _lines_y = sorted(_lines_y, key=lambda _line: _line[0][1])
        
    merged_lines_x = merge_lines_pipeline_2(_lines_x)
    merged_lines_y = merge_lines_pipeline_2(_lines_y)
    
    merged_lines_all = []
    merged_lines_all.extend(merged_lines_x)
    merged_lines_all.extend(merged_lines_y)
    print("process groups lines", len(_lines), len(merged_lines_all))
    img_merged_lines = mpimg.imread(image_src)
    for line in merged_lines_all:
        cv.line(img_merged_lines, (line[0][0], line[0][1]), (line[1][0],line[1][1]), (0,0,255), 6)

    
    cv.imwrite('prediction/lines_gray.jpg',gray)
    cv.imwrite('prediction/lines_thresh.jpg',thresh1)
    cv.imwrite('prediction/lines_edges.jpg',edges)
    cv.imwrite('prediction/lines_lines.jpg',img)
    cv.imwrite('prediction/merged_lines.jpg',img_merged_lines)
    
    return merged_lines_all

def merge_lines_pipeline_2(lines):
    super_lines_final = []
    super_lines = []
    min_distance_to_merge = 30
    min_angle_to_merge = 30
    
    for line in lines:
        create_new_group = True
        group_updated = False

        for group in super_lines:
            for line2 in group:
                if get_distance(line2, line) < min_distance_to_merge:
                    # check the angle between lines       
                    orientation_i = math.atan2((line[0][1]-line[1][1]),(line[0][0]-line[1][0]))
                    orientation_j = math.atan2((line2[0][1]-line2[1][1]),(line2[0][0]-line2[1][0]))

                    if int(abs(abs(math.degrees(orientation_i)) - abs(math.degrees(orientation_j)))) < min_angle_to_merge: 
                        #print("angles", orientation_i, orientation_j)
                        #print(int(abs(orientation_i - orientation_j)))
                        group.append(line)

                        create_new_group = False
                        group_updated = True
                        break
            
            if group_updated:
                break

        if (create_new_group):
            new_group = []
            new_group.append(line)

            for idx, line2 in enumerate(lines):
                # check the distance between lines
                if get_distance(line2, line) < min_distance_to_merge:
                    # check the angle between lines       
                    orientation_i = math.atan2((line[0][1]-line[1][1]),(line[0][0]-line[1][0]))
                    orientation_j = math.atan2((line2[0][1]-line2[1][1]),(line2[0][0]-line2[1][0]))

                    if int(abs(abs(math.degrees(orientation_i)) - abs(math.degrees(orientation_j)))) < min_angle_to_merge: 
                        #print("angles", orientation_i, orientation_j)
                        #print(int(abs(orientation_i - orientation_j)))

                        new_group.append(line2)

                        # remove line from lines list
                        #lines[idx] = False
            # append new group
            super_lines.append(new_group)
        
    
    for group in super_lines:
        super_lines_final.append(merge_lines_segments1(group))
    
    return super_lines_final

def merge_lines_segments1(lines, use_log=False):
    if(len(lines) == 1):
        return lines[0]
    
    line_i = lines[0]
    
    # orientation
    orientation_i = math.atan2((line_i[0][1]-line_i[1][1]),(line_i[0][0]-line_i[1][0]))
    
    points = []
    for line in lines:
        points.append(line[0])
        points.append(line[1])
        
    if (abs(math.degrees(orientation_i)) > 45) and abs(math.degrees(orientation_i)) < (90+45):
        
        #sort by y
        points = sorted(points, key=lambda point: point[1])
        
        if use_log:
            print("use y")
    else:
        
        #sort by x
        points = sorted(points, key=lambda point: point[0])
        
        if use_log:
            print("use x")
    
    return [points[0], points[len(points)-1]]

# https://docs.scipy.org/doc/scipy/reference/generated/scipy.spatial.distance.cdist.html
# https://stackoverflow.com/questions/32702075/what-would-be-the-fastest-way-to-find-the-maximum-of-all-possible-distances-betw
def lines_close(line1, line2):
    dist1 = math.hypot(line1[0][0] - line2[0][0], line1[0][0] - line2[0][1])
    dist2 = math.hypot(line1[0][2] - line2[0][0], line1[0][3] - line2[0][1])
    dist3 = math.hypot(line1[0][0] - line2[0][2], line1[0][0] - line2[0][3])
    dist4 = math.hypot(line1[0][2] - line2[0][2], line1[0][3] - line2[0][3])
    
    if (min(dist1,dist2,dist3,dist4) < 100):
        return True
    else:
        return False
    
def lineMagnitude (x1, y1, x2, y2):
    lineMagnitude = math.sqrt(math.pow((x2 - x1), 2)+ math.pow((y2 - y1), 2))
    return lineMagnitude
 
#Calc minimum distance from a point and a line segment (i.e. consecutive vertices in a polyline).
# https://nodedangles.wordpress.com/2010/05/16/measuring-distance-from-a-point-to-a-line-segment/
# http://paulbourke.net/geometry/pointlineplane/
def DistancePointLine(px, py, x1, y1, x2, y2):
    #http://local.wasp.uwa.edu.au/~pbourke/geometry/pointline/source.vba
    LineMag = lineMagnitude(x1, y1, x2, y2)
 
    if LineMag < 0.00000001:
        DistancePointLine = 9999
        return DistancePointLine
 
    u1 = (((px - x1) * (x2 - x1)) + ((py - y1) * (y2 - y1)))
    u = u1 / (LineMag * LineMag)
 
    if (u < 0.00001) or (u > 1):
        #// closest point does not fall within the line segment, take the shorter distance
        #// to an endpoint
        ix = lineMagnitude(px, py, x1, y1)
        iy = lineMagnitude(px, py, x2, y2)
        if ix > iy:
            DistancePointLine = iy
        else:
            DistancePointLine = ix
    else:
        # Intersecting point is on the line, use the formula
        ix = x1 + u * (x2 - x1)
        iy = y1 + u * (y2 - y1)
        DistancePointLine = lineMagnitude(px, py, ix, iy)
 
    return DistancePointLine

def get_distance(line1, line2):
    dist1 = DistancePointLine(line1[0][0], line1[0][1], 
                              line2[0][0], line2[0][1], line2[1][0], line2[1][1])
    dist2 = DistancePointLine(line1[1][0], line1[1][1], 
                              line2[0][0], line2[0][1], line2[1][0], line2[1][1])
    dist3 = DistancePointLine(line2[0][0], line2[0][1], 
                              line1[0][0], line1[0][1], line1[1][0], line1[1][1])
    dist4 = DistancePointLine(line2[1][0], line2[1][1], 
                              line1[0][0], line1[0][1], line1[1][0], line1[1][1])
    
    
    return min(dist1,dist2,dist3,dist4)

There are still 572 lines. After my “merging line segments” we have only 89 lines