Object orientation (horizontal/vertical)

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OpenCV has a useful tool called PCA (principal component analysis) https://robospace.wordpress.com/2013/... Here is how I used it:

• resized the image by 1/4 as we don't really need such high resolution here
• extract the red channel so that the blue part of the pen is darker than the surroundings
• apply a threshold using binary_inverse with treshold value about 90
• do some morphology to clean up the threshold image: one iteration of erosion with 3x3 rect and 5 iterations of dilation with 3x3 rect
• do some contour filtering to only include the contour whose center is nearest the center of the image
• now do PCA on this remaining contour to give 2 eigenvectors that tell us the orientation

From here, you can apply some test to tell if it is in some desired orientation.