Pygame surface to opencv image object (not file saving)

Question

I have a rectangle area on a pygame surface. I want to convert that area into an opencv image object (without saving to file) for further image processing on that rectangle area only. How can I do that?

selected_area =  mysurface.subsurface((x,y,w,h))
img_array=numpy.array(pygame.surfarray.array2d(selected_area))

#image_object = ..... ?

gray = cv2.cvtColor(image_object, cv2.COLOR_BGR2GRAY)
cv2.imshow("test",gray)

Answer 1

Actually, a cv2 image is just a three-dimensional numpy array. the 1st dimension is the height, the 2nd the width and the 3rd the number of channels in the order blue, green, red.
Use pygame.surfarray.pixels3d to reference pixels into a 3d array:

img_array = numpy.array(pygame.surfarray.pixels3d(selected_area))

Unfortunately, the pixels are stored in row mayor order. The color channels are in the order red, green, blue. So you need to reformat this array. transpose the array:

image_object = numpy.transpose(img_array, (1, 0, 2))

Finally swap the red and blue color channel with either

image_object[:, :, [0, 2]] = image_object[:, :, [2, 0]]

or

image_object = cv2.cvtColor(image_object, cv2.COLOR_RGB2BGR)

Convert function:

def pygameSurfaceToCv2Image(mysurface, x, y, w, h):
    selected_area =  mysurface.subsurface((x, y, w, h))
    img_array = numpy.array(pygame.surfarray.pixels3d(selected_area))
    image_object = numpy.transpose(img_array, (1, 0, 2))
    image_object[:, :, [0, 2]] = image_object[:, :, [2, 0]]
    return image_object

---

Minimal example:

import pygame
import cv2, numpy

pygame.init()
window = pygame.display.set_mode((400, 400))
clock = pygame.time.Clock()

def pygameSurfaceToCv2Image(mysurface, x, y, w, h):
    selected_area =  mysurface.subsurface((x, y, w, h))
    img_array = numpy.array(pygame.surfarray.pixels3d(selected_area))
    image_object = numpy.transpose(img_array, (1, 0, 2))
    #image_object[:, :, [0, 2]] = image_object[:, :, [2, 0]]
    image_object = cv2.cvtColor(image_object, cv2.COLOR_RGB2BGR)
    return image_object

apple_surf = pygame.image.load("Apple1.bmp")
image_object = pygameSurfaceToCv2Image(apple_surf, 20, 20, 200, 120)
gray = cv2.cvtColor(image_object, cv2.COLOR_BGR2GRAY)
cv2.imshow("apple subsurface", image_object)
cv2.imshow("gray apple subsurface", gray)

run = True
while run:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False 

    window.fill(0)
    window.blit(image, image.get_rect(center = window.get_rect().center))
    pygame.display.flip()
    clock.tick(60)

pygame.quit()
exit()