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  1. PO
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    <p>Your problem have few solutions but all have one very important issue which you seem to neglect. Note: When you are trying to make geometrical calculation in the image, the points you use must be as far as possible one from the other. You are taking 2 points inside a single square. Those points are very close one to another, so a slight error in pixel location of of the points leads to a large error in the angle. Why do you use only a single square, when you have many squares in the image? Here are few solutions:</p> <ol> <li>Find the line angle of every square. You have at least 9 squares in the image, 4 lines in each square which give you total of 36 angles (18 will be roughly at 3[deg] and 18 will be ~93[deg]). Remove the 90[degrees] and you get 36 different measurements of the angle. Sort them and take the average of the middle 30 (disregarding the lower 3 and higher 3 measurements). This will give you an accurate result</li> <li>Second solution, find the left extreme point of the leftmost square and the right extreme point of the rightmost square. Now calculate the angle between them. The result will be much more accurate because the points are far away.</li> <li>A third algorithm which will give you accurate results because it doesn't involve finding any points and no need for thresholding. Just smooth the image, calculate gradients in X and Y directions <code>(gx,gy)</code>, calculate the angle of the gradient in each pixel <code>atan(gy,gx)</code> and make histogram of the angles. You will have 2 significant peaks near the 3[deg] and 93[deg]. Just find the peaks by searching the maximum in the histogram. This will work even if you have a lot of noise in the image, even with antialising and jpg artifacts, and even if you have other drawings on the image. But remember, you must smooth the image a lot before calculating the derivatives. </li> </ol>
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    1. POEdge detection / angle
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    1. USDanielHsH
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    1. COI'm well aware of the issue of close measurement points resulting in angle error, what about allowing for optical distortion? If you calculated a global rotation, all the angles will be slightly off.
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      1. USillumi
    2. COIf you have a global distortion like fish eye, than you cannot use global calculations. But the smoothing and taking histogram of orientations will work locally.
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