Triangle Filling in opencv

You can draw a polygon and then fill it, like @ZdaR said.

# draw a triangle
vertices = np.array([[480, 400], [250, 650], [600, 650]], np.int32)
pts = vertices.reshape((-1, 1, 2))
cv2.polylines(img_rgb, [pts], isClosed=True, color=(0, 0, 255), thickness=20)

# fill it
cv2.fillPoly(img_rgb, [pts], color=(0, 0, 255))
# show it
plt.imshow(img_rgb)

The simplest solution of filling a triangle shape is using draw contour function in OpenCV. Assuming we know the three points of the triangle as "pt1", "pt2" and "pt3":

import cv2
import numpy as np

image = np.ones((300, 300, 3), np.uint8) * 255

pt1 = (150, 100)
pt2 = (100, 200)
pt3 = (200, 200)

cv2.circle(image, pt1, 2, (0,0,255), -1)
cv2.circle(image, pt2, 2, (0,0,255), -1)
cv2.circle(image, pt3, 2, (0,0,255), -1)

We can put the three points into an array and draw as a contour:

triangle_cnt = np.array( [pt1, pt2, pt3] )

cv2.drawContours(image, [triangle_cnt], 0, (0,255,0), -1)

cv2.imshow("image", image)
cv2.waitKey()

Here is the output image. Cheers.