In this post I am going to solve this problem, how to find simple shapes like triangular and square ( or rectangular) in the image. For simple shape like square or triangular I normally use this procedure:

1. Find Contours in the image ( image should be binary)

2. Approximate each contour using approxPolyDP function.

3. Check number of elements in the approximated contours of all the shapes to recognize the shape. For eg, triangular will have 3; for square or rectangle, it has to meet the following conditions:

* It is convex.

* It has 4 vertices.

* All angles are ~90 degree.

4. Assign the color, run the code for your test image, check its number, fill it with corresponding colors.

**Assumptions:** Shapes don’t overlap, both of them solid (meaning, there is no white pixels inside the shape (all shapes are black). There can be multiple shapes in the image and they can be rotated any arbitrary number of degrees, and they can be of any size. Important: Triangles are non-obtuse!