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You can also look the hierarchy of the contours.

If one contour is inside of another: 1. It could be an empty circle 2. It could an empty circle

If it isn't inside: 1. It could be a circle 2. It could a cross

Then you can also use approxPolyDP (approximate a curve or a polygon with another curve/polygon with less vertices) to distinguish between the two options.

You can also look the hierarchy of the contours.

If one contour is inside of another: 1. It could be an empty circle 2. It could an empty circletriangle

If it isn't inside: 1. It could be a circle 2. It could a cross

Then you can also use approxPolyDP (approximate a curve or a polygon with another curve/polygon with less vertices) to distinguish between the two options.

You can also look the hierarchy of the contours.

If one contour is inside of another: 1. another:

1. It could be an empty circle 2. circle
2. It could be an empty triangle

It could an empty triangle

If it isn't inside: 1. inside:

1. It could be a circle 2. circle
2. It could a cross

Then you can also use approxPolyDP (approximate a curve or a polygon with another curve/polygon with less vertices) to distinguish between the two options.

You can also look the hierarchy of the contours.

If one contour is inside of another:

1. It could be an empty circle
2. It could be an empty triangle

If it isn't inside:

1. It could be a circle
2. It could a cross

Then you can also use approxPolyDP (approximate a curve or a polygon with another curve/polygon with less vertices) to distinguish between the two options.