OpenCV line detection for 45 degree lines - image

Question

I have an image:
In this image, the OpenCV Hough transform can't detect the big -45 degree line using
minLineLength = 150
maxLineGap = 5
line_thr = 150
linesP = cv.HoughLinesP(dst, 1, np.pi / 180, line_thr, None, minLineLength, maxLineGap)
The only lines found are:
I tried playing with various thresholds also but I can't find the line here.
If I manually crop the image like this:
then I can clearly see the OpenCV Hough transform finding the right line:
I want to find this same line in the non cropped version. Any suggestions on the non-cropped version to find it?
Also there can be cases where there is no line at all or the line doesn't go all the way for X-axis length.
Examples

Answer 1

I implemented a slightly simpler algorithm than my other answer but in Python with OpenCV this time.
Basically, rather than taking the mean of vertical columns of pixels, it sums the pixels in the columns and chooses the column that is brightest. If I show the padded, rotated image with another image below representing the sums of the columns, you should see how it works:
#!/usr/bin/env python3
import cv2
import numpy as np
# Load image as greyscale
im = cv2.imread('45.jpg',cv2.IMREAD_GRAYSCALE)
# Pad with border so it isn't cropped when rotated
bw=300
bordered = cv2.copyMakeBorder(im, top=bw, bottom=bw, left=bw, right=bw, borderType= cv2.BORDER_CONSTANT)
# Rotate -45 degrees
w, h = bordered.shape
M = cv2.getRotationMatrix2D((h/2,w/2),-45,1)
paddedrotated = cv2.warpAffine(bordered,M,(h,w))
# DEBUG cv2.imwrite('1.tif',paddedrotated)
# Sum the elements of each column and find column with most white pixels
colsum = np.sum(paddedrotated,axis=0,dtype=np.float)
col = np.argmax(colsum)
# DEBUG cv2.imwrite('2.tif',colsum)
# Fill with black except for the line we have located which we make white
paddedrotated[:,:] = 0
paddedrotated[:,col] = 255
# Rotate back to straight
w, h = paddedrotated.shape
M = cv2.getRotationMatrix2D((h/2,w/2),45,1)
straight = cv2.warpAffine(paddedrotated,M,(h,w))
# Remove padding and save to disk
straight = straight[bw:-bw,bw:-bw]
cv2.imwrite('result.png',straight)
Note that you don't actually have to rotate the image back to straight and crop it back to its original size. You could actually stop after the first line that says:
col = np.argmax(colsum)
and use some elementary trigonometry to work out what that means in your original image.
Here is the output:
Keywords: line detection, detect line, rotate, pad, border, projection, project, image, image processing, Python, OpenCV, affine, Hough

Answer 2

I did this on the command-line in Terminal with ImageMagick but you can apply exactly the same technique with OpenCV.
Step 1
Take the image and rotate it 45 degrees introducing black pixels as background where required:
convert 45.jpg -background black -rotate 45 result.png
Step 2
Now, building on the previous command, set every pixel to the median of the box 1px wide and 250px tall centred on it:
convert 45.jpg -background black -rotate 45 -statistic median 1x250 result.png
Step 3
Now, again building on the previous command, rotate it back 45 degrees:
convert 45.jpg -background black -rotate 45 -statistic median 1x250 -rotate -45 result.png
So, in summary, the entire processing is:
convert input.jpg -background black -rotate 45 -statistic median 1x250 -rotate -45 result.png
Obviously then crop it back to the original size and append side-by-side with the original for checking:
convert 45.jpg -background black -rotate 45 -statistic median 5x250 -rotate -45 +repage -gravity center -crop 184x866+0+0 result.png
convert 45.jpg result.png +append result.png
You can also use mean statistic plus thresholding rather than median since it is quicker than sorting to find the median, however it tends to lead to smearing:
convert 45.jpg -background black -rotate 45 -statistic mean 1x250 result.png
Your newly-added image gets processed to this result:

Answer 3

The problem is clearly that the line you are searching for is not a line. It looks actually like a train of connected circles and boxes. Therefore, I recommend that you do the following:
Find all contours in the image using find contours
img = cv.imread('image.jpg')
img_gray = cv.cvtColor(img, cv.COLOR_BGR2GRAY)
ret, thresh = cv.threshold(img_gray, 127, 255, 0)
img2, contours, hierarchy = cv.findContours(thresh, CHAIN_APPROX_SIMPLE ,cv.RETR_EXTERNAL)
This will return many many contours, so use a loop to save only long enough contours. Since the image size is 814x1041 pixels, I assume the contour long if it is at least 10% of the image width which is almost 100 (you must apparently optimize this value)
long_contours = []
for contour in contours[i]:
perimeter = cv2.arcLength(contour,True)
if (perimeter > 0.1 * 1018) # 10% of the image width
long_contours.append(contour)
Now draw a rotated bounding rectangle around those long contours that might be a line as well. The long contour is considered a line if its width is much longer than its height, or its aspect ratio is large (such as 8, and you need also to optimize this value)
for long_contour in long_contours:
rect = cv2.minAreaRect(long_contour)
aspec_ratio = rect.width / rect.height
if aspec_ratio > 8 :
box = cv2.boxPoints(rect)
box = np.int0(box)
cv2.drawContours(img,[box],0,(255,255,255),cv.FILLED)
Finally you should get something like that. Please note the code here is for guidance only.

Answer 4

Your original code is fine as a whistle. The only problem is that your image contains too many information which mess up the accumulator scores. Everything will work out if you increase the line threshold to 255.
minLineLength = 150
maxLineGap = 5
line_thr = 255
linesP = cv2.HoughLinesP(dst, 1, np.pi / 180.0, line_thr, None, minLineLength, maxLineGap)
Here are the results using that value.
3 lines are detected here due to the large white pixel size.
[ 1 41 286 326]
[ 0 42 208 250]
[ 1 42 286 327]
5 lines are detected around the same area due to the same reason as above. Reducing the pixel sizes using morphological operation or distance transform should fix this.
[110 392 121 598]
[112 393 119 544]
[141 567 147 416]
[ 29 263 29 112]
[ 0 93 179 272]
No line found here.