Python Pillow: Make gradient for transparency

Question

I have code which add gradient on image.

def st(path, gradient_magnitude=2.):
    im = Image.open(path)
    if im.mode != 'RGBA':
        im = im.convert('RGBA')
    width, height = im.size
    gradient = Image.new('L', (width, 1), color=0xFF)
    for x in range(width):
        gradient.putpixel((x, 0), int(255 * (1 - gradient_magnitude * float(x) / width)))
    alpha = gradient.resize(im.size)
    black_im = Image.new('RGBA', (width, height), color=0x000000)
    black_im.putalpha(alpha)
    gradient_im = Image.alpha_composite(im, black_im)
    gradient_im.save('out.jpg', 'JPEG')

After run I get this image

How can I make gradient more transparency?

Answer 1

Try this. Initial opacity values of 0.9 or 0.95 should give you what you want. I also restructured the code a bit.

from PIL import Image

def apply_black_gradient(path_in, path_out='out.png',
                         gradient=1., initial_opacity=1.):
    """
    Applies a black gradient to the image, going from left to right.

    Arguments:
    ---------
        path_in: string
            path to image to apply gradient to
        path_out: string (default 'out.png')
            path to save result to
        gradient: float (default 1.)
            gradient of the gradient; should be non-negative;
            if gradient = 0., the image is black;
            if gradient = 1., the gradient smoothly varies over the full width;
            if gradient > 1., the gradient terminates before the end of the width;
        initial_opacity: float (default 1.)
            scales the initial opacity of the gradient (i.e. on the far left of the image);
            should be between 0. and 1.; values between 0.9-1. give good results
    """

    # get image to operate on
    input_im = Image.open(path_in)
    if input_im.mode != 'RGBA':
        input_im = input_im.convert('RGBA')
    width, height = input_im.size

    # create a gradient that
    # starts at full opacity * initial_value
    # decrements opacity by gradient * x / width
    alpha_gradient = Image.new('L', (width, 1), color=0xFF)
    for x in range(width):
        a = int((initial_opacity * 255.) * (1. - gradient * float(x)/width))
        if a > 0:
            alpha_gradient.putpixel((x, 0), a)
        else:
            alpha_gradient.putpixel((x, 0), 0)
        # print '{}, {:.2f}, {}'.format(x, float(x) / width, a)
    alpha = alpha_gradient.resize(input_im.size)

    # create black image, apply gradient
    black_im = Image.new('RGBA', (width, height), color=0) # i.e. black
    black_im.putalpha(alpha)

    # make composite with original image
    output_im = Image.alpha_composite(input_im, black_im)
    output_im.save(path_out, 'PNG')

    return