alpha_bounding.py

path = "path/to/your/image"
img = cv2.imread(path, cv2.IMREAD_UNCHANGED)

x1 = 0
x2 = 0
y1 = 0
y2 = 0

fx1 = False
fx2 = False
fy1 = False
fy2 = False

cursorY = 0
matchesY = False

for matrix in img:
    cursorX = 0
    matchesX = False
    for rgb in matrix:
        if rgb[3] > 0:
            matchesX = True
            if fx1 == False and fy1 == False:
                x1 = cursorX
                y1 = cursorY
                fx1 = True
                fy1 = True
            if fy1 == False:
                y1 = cursorY
                fy1 = True
        elif rgb[3] == 0 and matchesX == True:
            if fx2 == False:
                x2 = cursorX - 1
                fx2 = True

        cursorX = cursorX + 1

    if matchesY == False and matchesX == False and fx1 == True:
        if fy2 == False:
            y2 = cursorY - 1
            fy2 = True

    cursorY = cursorY + 1

img = img[ y1 : y1 + y2, x1 : x1 + x2 ]