How do I convert an OpenCV image (BGR and BGRA) to a pygame.Surface object?

Question

I created images from OpenCV/opencv-python (numpy.array) and I want to convert them to a pygame.Surface object:

def cvImageToSurface(cv2Image):

    pygameSurface = # ? create from "cvImage"

    return pygameSurface 

surface = cvImageToSurface(cv2Image)

Some of the images have three channels (BGR) and some of the images also have an alpha channel (BGRA). What do I have to do in cvImageToSurface to convert images with one of the formats into a pygame.Surface object?

Answer 1

The shape attribute of a numpy.array is the number of elements in each dimension. The first element is the height, the second the width and the third the number of channels. A pygame.Surface can be generated by pygame.image.frombuffer. The 1st argument can be a numpy.array and the second argument is the format (RGB or RGBA).

Get the size (width, height) for the pygame.Surface object by slicing:

size = cvImage.shape[1::-1]

Determine the target format for the pygame.Surface object, depending on the third channel:

format = 'RGBA' if cvImage.shape[2] == 4 else 'RGB'

Since the source format is BGR or BGRA, but the target format is RGB or RGBA, the red and blue channels have to be swapped:

cvImage[:, :, [0, 2]] = cvImage[:, :, [2, 0]]

In the case of a grayscale image, the shape of the array must be changed using numpy.reshape and the gray channel must be expanded to a red-green and blue color channel using numpy.repeat:

cvImage = np.repeat(cvImage.reshape(size[1], size[0], 1), 3, axis = 2)

With his data the pygame.Surface object can be generated by pygame.image.frombuffer:

surface = pygame.image.frombuffer(cvImage.flatten(), size, format)

To ensure that the image has the same pixel format as the display Surface and for optimal performance, the Surface should be converted with either convert or convert_alpha:

surface = surface.convert_alpha() if format == 'RGBA' else surface.convert()

Complete function cvImageToSurface:

def c2ImageToSurface(cvImage):
    if cvImage.dtype.name == 'uint16':
        cvImage = (cvImage / 256).astype('uint8')
    size = cvImage.shape[1::-1]
    if len(cvImage.shape) == 2:
        cvImage = np.repeat(cvImage.reshape(size[1], size[0], 1), 3, axis = 2)
        format = 'RGB'
    else:
        format = 'RGBA' if cvImage.shape[2] == 4 else 'RGB'
        cvImage[:, :, [0, 2]] = cvImage[:, :, [2, 0]]
    surface = pygame.image.frombuffer(cvImage.flatten(), size, format)
    return surface.convert_alpha() if format == 'RGBA' else surface.convert()

Minimal example:

import os
import pygame
import cv2 as cv
import numpy as np

def cvImageToSurface(cvImage):
    if cvImage.dtype.name == 'uint16':
        cvImage = (cvImage / 256).astype('uint8')
    size = cvImage.shape[1::-1]
    if len(cvImage.shape) == 2:
        cvImage = np.repeat(cvImage.reshape(size[1], size[0], 1), 3, axis = 2)
        format = 'RGB'
    else:
        format = 'RGBA' if cvImage.shape[2] == 4 else 'RGB'
        cvImage[:, :, [0, 2]] = cvImage[:, :, [2, 0]]
    surface = pygame.image.frombuffer(cvImage.flatten(), size, format)
    return surface.convert_alpha() if format == 'RGBA' else surface.convert()

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

cvImage1 = cv.imread('woodtiles.jpg', cv.IMREAD_GRAYSCALE)
cvImage2 = cv.imread('woodtiles.jpg', cv.IMREAD_UNCHANGED)
cvImage3 = cv.imread('Apple1-256.png', cv.IMREAD_UNCHANGED)
pygameSurface1 = cvImageToSurface(cvImage1)
pygameSurface2 = cvImageToSurface(cvImage2)
pygameSurface3 = cvImageToSurface(cvImage3)

run = True
while run:
    clock.tick(60)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

    window.fill(0)
    window.blit(pygameSurface1, pygameSurface1.get_rect(topleft = window.get_rect().inflate(-10, -10).topleft))
    window.blit(pygameSurface2, pygameSurface2.get_rect(center = window.get_rect().center))
    window.blit(pygameSurface3, pygameSurface3.get_rect(bottomright = window.get_rect().inflate(-10, -10).bottomright))
    pygame.display.flip()

pygame.quit()