I trying to find all triangle in image with that code without success
import numpy as np
import cv2
img = cv2.imread('2.jpg')
for gray in cv2.split(img):
canny = cv2.Canny(gray,50,200)
contours,hier = cv2.findContours(canny,1,2)
for cnt in contours:
approx = cv2.approxPolyDP(cnt,0.02*cv2.arcLength(cnt,True),True)
if len(approx)==3:
cv2.drawContours(img,[cnt],0,(0,255,0),2)
tri = approx
for vertex in tri:
cv2.circle(img,(vertex[0][0],vertex[0][1]),5,255,-1)
cv2.imshow('img',img)
cv2.waitKey(0)
cv2.destroyAllWindows()
So from this picture

I want get this ( Look at licence plate, I filled with red lines triangles)

That what I get now

If the triangles are always the same colour you can preprocess the image to only display that colour, then use the code you have already written to find those triangles.
This link should get you started:
http://opencv-python-tutroals.readthedocs.org/en/latest/py_tutorials/py_imgproc/py_colorspaces/py_colorspaces.html
Hope this helps
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