rgba_img = cv2.imread( "rgba.png", cv2.IMREAD_UNCHANGED )   # アルファチャンネル付き画像
cv2.imshow( 'window_name', rgba_image )   # これは可能（しかしαは考慮されない)

import cv2
import numpy as np

IMAGE_PATH = "/path/to/the/rgba_image.png"

# 引数shapeと同じサイズのチェスボードを作成する関数
def chessboardGenerator( shape, cell_size=16, color1=(64,64,64), color2=(128,128,128) ):

    # 目標画像より少し大きい、cell_sizeで割り切れるサイズのshapeを作成
    chessboard_shape = ( (shape[0]//cell_size+1)*cell_size, (shape[1]//cell_size+1)*cell_size, 3 )

    # 1色目にて塗りつぶす
    chessboard = np.full( chessboard_shape, color1, dtype=np.uint8)

    # 2色目の1辺がcell_sizeの正方形
    cell = np.full( (cell_size, cell_size, 3), color2, dtype=np.uint8 )

    # チェスボード柄に　該当箇所を2色目の正方形で置き換え
    for cx in range( 0, chessboard_shape[1]//cell_size ):
        for cy in range( 0, chessboard_shape[0]//cell_size ):
            if (cx+cy) % 2: continue
            x, y = cx*cell_size, cy*cell_size
            chessboard[y:y+cell_size, x:x+cell_size, :] = cell

    # 少し大きく作った分を削る
    chessboard = chessboard[:shape[0], :shape[1], :]

    return chessboard

if __name__ == "__main__":
    rgba_image = cv2.imread( IMAGE_PATH, cv2.IMREAD_UNCHANGED )
    chessboard = chessboardGenerator( rgba_image.shape )
    cv2.namedWindow( "chessboard", cv2.WINDOW_NORMAL )
    cv2.imshow( "chessboard", rgba_image )
    cv2.waitKey(0)

import cv2
import numpy as np

IMAGE_PATH = "/path/to/the/rgba_image.png"

def chessboardGenerator( shape, cell_size=16, color1=(64,64,64), color2=(128,128,128) ):

    chessboard_shape = ( (shape[0]//cell_size+1)*cell_size, (shape[1]//cell_size+1)*cell_size, 3 )

    chessboard = np.full( chessboard_shape, color1, dtype=np.uint8)

    cell = np.full( (cell_size, cell_size, 3), color2, dtype=np.uint8 )

    for cx in range( 0, chessboard_shape[1]//cell_size ):
        for cy in range( 0, chessboard_shape[0]//cell_size ):
            if (cx+cy) % 2: continue
            x, y = cx*cell_size, cy*cell_size
            chessboard[y:y+cell_size, x:x+cell_size, :] = cell

    chessboard = chessboard[:shape[0], :shape[1], :]

    return chessboard


# 背景をブレンドするための関数
def chessboardBG( img ):

    # 入力画像がアルファチャンネルを含まない(4チャンネルでない)場合はそのまま
    if img.shape[2] != 4:
        return img

    # 先ほど作成した関数にてチェスボードを取得
    chessboard = chessboardGenerator( img.shape )

    # ここがメインのブレンドする箇所
    # アルファチャンネルの値がαの時、背景：メインを(1-α)；αにて混ぜ合わせる
    chessboard_bg_img = np.array( chessboard*(1-img[:,:,3:]/255) + img[:,:,:3]*(img[:,:,3:]/255), dtype=np.uint8 )

    return chessboard_bg_img


if __name__ == "__main__":
    # cv2.IMREAD_UNCHANGEDにてアルファチャンネルをそのまま読み込む
    rgba_image = cv2.imread( IMAGE_PATH, cv2.IMREAD_UNCHANGED )
    processed_image = chessboardBG( rgba_image )
    cv2.namedWindow( 'processed', cv2.WINDOW_NORMAL )
    cv2.imshow( 'processed', processed_image )
    cv2.waitKey(0)