# Detect diagonal straight lines

**URL:** <https://forum.opencv.org/t/detect-diagonal-straight-lines/12779>\
**Category:** Python\
**Tags:** houghlines\
**Created:** [April 12, 2023, 7:59am UTC](https://forum.opencv.org/t/detect-diagonal-straight-lines/12779 "2023-04-12T07:59:46Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![pdw](https://avatars.discourse-cdn.com/v4/letter/p/b19c9b/32.png) [@pdw](https://forum.opencv.org/u/pdw)\
**Post date:** [April 12, 2023, 7:59am UTC](https://forum.opencv.org/t/detect-diagonal-straight-lines/12779/1 "2023-04-12T07:59:46Z")

</div>

My script can detect horizonal and vertical lines but I can’t figure out why it fails on diagonal lines. As an example I’m creating simple black and white images of letters with this code:

```auto
###### CODE TO CREATE THE IMAGES

from PIL import Image, ImageDraw, ImageFont
import numpy as np
import cv2 as cv
from matplotlib import pyplot as plt

def create_image(size, bgColor, message, font, fontColor):

    W, H = size
    image = Image.new('1', size, bgColor)
    draw = ImageDraw.Draw(image)
    _, _, w, h = draw.textbbox((0, 13), message, font=font)
    draw.text(((W - w) / 2, (H - h) / 2), message, font=font, fill=fontColor)
    return image

myMessage = 'l'
myFont = ImageFont.truetype("data/arial.ttf", 1714)
myImage = create_image((5142, 2000), 'white', myMessage, myFont, 'black')
myImage.save('hello_world.png', "PNG")
Image.open("hello_world.png").show()
img = cv.imread("hello_world.png")

```

And here is the code to detect the straight lines in the image:

```auto
def findStraightLines(img, rho, theta, threshold, min_line_length, max_line_gap):

    gray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)

    kernel_size = 5
    blur_gray = cv.GaussianBlur(gray, (kernel_size, kernel_size), 0)

    low_threshold = 50
    high_threshold = 150
    edges = cv.Canny(blur_gray, low_threshold, high_threshold)
    line_image = np.copy(img) * 0 # creating a blank to draw lines on

    # Run Hough on edge detected image
    # Output "lines" is an array containing endpoints of detected line segments
    lines = cv.HoughLinesP(edges, rho, theta, threshold, np.array([]),
                            min_line_length, max_line_gap)

    if lines != None:

        for line in lines:
            for x1, y1, x2, y2 in line:
                cv.line(line_image, (x1, y1), (x2, y2), (255, 0, 0), 5)

        # Draw the lines on the image
        lines_edges = cv.addWeighted(img, 0.8, line_image, 1, 0)

    else:

        lines_edges = img.copy()
    
        return lines_edges, lines

```

And here I run the functions and plot the images with the straight lines that are detected outlined in red:

```auto
lines_edges, lines = findStraightLines(img, rho=1, theta=np.pi / 180,
                                       threshold=20, min_line_length=50, max_line_gap=0)

plt.imshow(lines_edges)

```

f you run this minimally reproducible example you will see that with a lower case `l` as an image, the code detects all of the straight edges, and with a lower case `o` there are no straight edges detected. This is what I hoped to see. However with a lower case `w` the code is only detecting the horizontal aspects, and is missing the diagonal lines.

How can I adjust this to make it detect diagonal lines at different orientations, as well as horizonal and vertical lines? Thanks in advance!

---

<div class="post-metadata">

**Author:** ![crackwitz](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.opencv.org/crackwitz/32/14_2.png) [@crackwitz](https://forum.opencv.org/u/crackwitz)\
**Post date:** [April 12, 2023, 12:22pm UTC](https://forum.opencv.org/t/detect-diagonal-straight-lines/12779/2 "2023-04-12T12:22:19Z")

</div>

[crosspost](https://meta.stackoverflow.com/a/266159):

> <https://stackoverflow.com/questions/75989232/python-opencv-detect-diagonal-lines>