5. OpenCV Tutorial - Learn Classic Computer Vision & Face Detection (OPTIONAL)/30. Line Detection - Detect Straight Lines E.g. The Lines On A Sudoku Game.srt

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So let's talk a bit about line detection. Line detection is a very important completive vision method.
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And you can imagine it being used in applications such as lane detection for self-driving cars or perhaps
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for something that actually detects lines on a chessboard like this example here.
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So the two algorithms that are available in OpenCV are Hough lines and probabilistic Hough lines.
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Now there are some differences between them which illustrate when we run the code.
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But for now let's talk about how Hough lines represents lines quickly.
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I won't go into too much detail with the Math but you may have remembered this from high school mathematics
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that you can read up represent a straight line by this equation y = mx+c,  M being a gradient
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or slope of two lane and C being the y intercept.
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However in Hough lines we represent lines a little bit differently.
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So remember OpenCV uses here as this reference point.
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Top left corner.
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So what we do here in this representation we have P which is the perpendicular distance of the lane from
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the origin.
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In this case it's pixel's and theta being the angle of this line here, the normal of the lane meeting
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this origin point here and theta is measured in radians in OpenCV
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So if you actually do open this theta value in openCV or access it you'll see it's in radians.
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So these are some details here that I won't go into until we reach into the code but this is basically
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the cv2 Hough Lines function and cv2 to Probabilistc Hough Lines function
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So let's actually use these methods now in our code.
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OK.
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So let's open our line detection code here and select 4.6 OK.
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So here we go.
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So firstly let's run this could give you an idea what's going on.
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So this was a blank Soduko board here where we had lines here.
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So as we can see we've actually identified all the major lines here.
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However we've actually left with one line here surprisingly and we've identified multiple lanes in a
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few locations.
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You can see there's maybe two lines here two lines here.
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This one looks like three cause there's.
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A thicker point (line) here.
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So you can see it's not perfect.
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And what researchers do, let's bring it up again.
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They sometimes merge these lines together.
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You find the average of the tree closest points and merge them together.
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So let's see it take a look and see what's going on here.
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So similarly in the findcontours function we've grey scaled our image we found canny edges because canny edges
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helps quite a bit especially in off lane transforms not transform sorry as HOV lanes.
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And these are the parameters here.
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So these parameters here in this Hough lines function takes it, it takes the input image here and then we
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set some accuracy parameters here.
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So the first parameter we set rho or P value here.
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We typically said this that one pixel but you can experiment with larger values if you wish.
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And then we said the theta accuracy a pie over 180 using numpy dot pie or np.pie to generate our pie value
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and then this Value here
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This here is a key parameter.
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This is a threshold for determining if something is a line.
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So if we set this at a lower value let's say at 150 we should get a lot more lines, way too much lines
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here!
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And if we said this at a higher value let's say 300
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We get only some few lines.
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So 240 was a good parameter to use for this.
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We only miss, actually we missed two lines here miscounted this one here and that's the first part of
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the function there.
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So the next part of the code looks a bit confusing.
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But what's important to remember here is that cv2.HoughLines generates this object here, lines and lines
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has rho and theta values inside of it.
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So what's going on here is that we're actually looping through all the values inside of lines here and
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converting it into a format where we can actually get the endpoints of lines.
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That's what we use to do that line which we went through in section 2 I believe and we just plot all
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the lines that this cv2.HoughLines function generates.
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So now let's take a look at probabilistic Hough Lines which is a little bit different.
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And actually the code looks a lot shorter and that's mainly because we don't need to convert it into
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a format the line format into into something that cv2.line can use.
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It actually generates a start and end points of the lines automatically here and also has similar parameters
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here where we have pixel accuracy theta accuracy but it also has some new thresholds here.
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There's a minimum line length and a maximum line gap as well as the threshold value.
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And similarly we do have grayscale and use Canny Edges again
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So now let's run the probabilistic hough lines function which exists right here.
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And it looks a lot different you actually see what looks like small lines like worms almost on image
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and we can actually play with parameters to actually get straight lines if you want.
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But atually let's bring that up one more time what's key to note here is that because of these new parameters
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we have maximum length gaps maximum lane gaps actually eliminate the need.
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Well the problem where we have multiple lanes drawn over each other in the same lane here.
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If we set a maximum lane gap and see five or 10 there's no there's not going to be any lanes running
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over the existing line that are similar as well as we have a minimum length of five for lanes five pixels is
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quite small.
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But generally we can start with that if you want to get many lines here.
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Lines and a character perhaps.
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And let's actually play with the threshold value and see what happens now.
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So let's set this 50.
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There we go.
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I'm seeing slightly more lines here.
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It looks generally similar.
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So let's actually set this 10 actually less line sorry.
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So maybe we need to set this at a higher value.
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So that gives you a feel of these parameters here.
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Hopefully you would you would probably find whichever one more useful.
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Generally I tend to use Houghlines more.
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But they are definitely use cases for probabilistic.
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And if you'd like to learn more about probabilistic hough lines I've actually put a link at the bottom
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here.
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This is a link to the research paper where probabilistic Hough Lines with first proposed.
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So hope you enjoyed this lesson on line detection.
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Thanks.