How to combine two ImageSurface objects into one

Question

Based on this answer, I have two cairo.ImageSurface objects generated at runtime. Both are RGBA of equal dimensions. I'd like to combine / stack them before saving them to disk:

new_surface = surface1 + surface2 # pseudo-code

(How) does this work?

Answer 1

Going through PIL (pillow) actually gives pretty decent and relatively performant results. The following works for me:

import cairo
from PIL import Image

def surface_to_pil(surface):
    return Image.frombuffer(
        mode = 'RGBA',
        size = (surface.get_width(), surface.get_height()),
        data = surface.get_data(),
    )

def pil_to_surface(image):
    return cairo.ImageSurface.create_for_data(
        bytearray(image.tobytes('raw', 'BGRa')),
        cairo.FORMAT_ARGB32,
        image.width,
        image.height,
        )

def add_surfaces(a, b):
    assert a.get_width() == b.get_width() and a.get_height() == b.get_height()
    result_pil = Image.new(
        mode = 'RGBA',
        size = (a.get_width(), a.get_height()),
        color = (0.0, 0.0, 0.0, 0.0),
        )
    for surface in (a, b):
        surface_pil = surface_to_pil(surface)
        result_pil.paste(im = surface_pil, mask = surface_pil)
    return pil_to_surface(result_pil)

new_surface = add_surfaces(surface1, surface2)