import cairo
CAIRO_OPERATOR_HSL_LUMINOSITY = 28  # my py2cairo seems outdated

def convert_to_grayscale(img_in):
    img_out = img_in.create_similar(
        cairo.CONTENT_COLOR_ALPHA, img_in.get_width(), img_in.get_height())
    cr = cairo.Context(img_out)
    cr.set_source_rgba(1, 1, 1, 1)
    cr.paint()
    cr.set_source_surface(img_in)
    cr.set_operator(CAIRO_OPERATOR_HSL_LUMINOSITY)
    cr.paint()

    return img_out

import cairo
import numpy
import sys


def convert_to_grayscale(img_in):
    """Convert an image to grayscale.

    Arguments:
        img_in: (cairo.ImageSurface) input image.

    Return:
        (cairo.ImageSurface) image in grayscale, in ARGB32 mode.

    Timing:
        ~100ms to convert an image of 800x800

    Examples:
        # returns a B&W image
        >>> convert_to_grayscale(cairo.ImageSurface.create_from_png('test.png'))
    """
    a = numpy.frombuffer(img_in.get_data(), numpy.uint8)
    w, h = img_in.get_width(), img_in.get_height()
    a.shape = (h, h, 4)

    assert sys.byteorder == 'little', (
        'The luminosity vector needs to be switched if we're in a big endian architecture. '
        'The alpha channel will be at position 0 instead of 3.')
    alpha = a[:, :, 3]
    alpha.shape = (w, h, 1)

    luminosity_float = numpy.sum(a * numpy.array([.114, .587, .299, 0]), axis=2)
    luminosity_int = numpy.array(luminosity_float, dtype=numpy.uint8)
    luminosity_int.shape = (w, h, 1)
    grayscale_gbra = numpy.concatenate((luminosity_int, luminosity_int, luminosity_int, alpha),
                                       axis=2)
    stride = cairo.ImageSurface.format_stride_for_width(cairo.FORMAT_ARGB32, w)
    assert stride == 4 * w, 'We need to modify the numpy code if the stride is different'
    img_out = cairo.ImageSurface.create_for_data(grayscale_gbra, cairo.FORMAT_ARGB32, w, h, stride)

    return img_out